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Alice and Behringer Sitting In A Tree – Part 1

Posted by Tom Benedict on 03/11/2016

The field tests on my BM-800 Alice conversion will have to wait. Late last week I handed it over to a friend for tests I’m not equipped to make, including several mic comparisons. I’m eager to see (and hear!) his results.

Meanwhile my Behringer C-2 mics showed up. These are the ones I placed a bid on over at Ebay before I realized they were coming from Haifa, Israel. Despite the distance the shipping was actually less than FedEx charges to ship a letter-shaped package from the mainland US to Hawaii. (Go figure.) It still ramped the price of the mics up almost to market value, which on Amazon with its super saver free shipping basically means I could’ve ordered them new and had them weeks ago.

But they’re here. And they’re mine. And… to be honest they’re in pretty ratty shape. One of them had something loose in the capsule. If you pointed the mic up and shook it, it made a hellish noise and clipped constantly. Turn it upside down and shake, and you hear something rattling around. The other mic has something massively wrong with its circuit board. It sounds for all the world like a Huey is hovering overhead. Bup bup bup bup bup bup bup… It never ends.

So long story short, I don’t mind gutting these things and building something new. In the short term I put the good capsule on the good mic body so I have one working mic to play with. The other one I started taking apart.

Behringer C-2 Capsule Removed

These have interchangeable capsules, though I don’t know if Behringer (or anyone else) makes any other capsules for it. The one that came with my mics is a hypercardioid. (At least that’s what the icon on the side of the capsule looks like.) Given the size of the vents at the back, I can believe it.

Underneath the capsule is a white plastic plug with a pogo pin centered in it. Not much to look at. And no real clue how to open things up past there.

To gain access to the innards of the mic, peel back the the “Behringer Condenser Microphone” name tape at the base. This reveals a small set screw that should be familiar to anyone with Switchcraft XLR connectors. Screw the set screw all the way in. This releases the XLR connector from the body. Next, center the pad/high-pass filter switch and pull the switch button out with needle nose pliers. Finally, push on the white plastic plug to expose the circuit board.

Behringer C-2 Stereo Pair - Partially Disassembled

Here’s what’s inside:

Behringer C-2 PCB Top

Since one of my mics has a damaged board, rather than figure out how to tweak what’s already here, I went ahead and tried to figure out how to pack a Pimped Alice into the same board space. I started by taking measurements.

The board is 15.5mm wide x 52.35 mm long, and is 1mm thick. The thickness is important because the board slots into the white plastic insert. One nice thing about this method of mounting the board is that there are no screw holes, and except for the humongous XLR pins and the 2mm area that slots into the plastic insert the rest of the real estate on the board is free. The board is mounted just below the centerline of the mic, so there’s vertical room as well. Up to a point, anyway. Those capacitors are 6.5mm diameter x 8mm tall. Nothing bigger than that will fit, even centered on the board.

There’s really not enough room to use through-hole components everywhere, so I converted most of the Pimped Alice circuit to 0805 SMT components. The exceptions are the filter capacitors, the 1Gohm resistor, and the FET.

There seems to be some resistance to using surface mount technology for DIY mics, but SMT has been used for over a decade for DIY robotics and electronics. I’ve built AVR processor boards using SMT components, and figured this wouldn’t be much different. With the exception of the big filter caps and the FET, that’s how Behringer built the original board for the C-2, so I figured it was a safe way to go. As soon as I have a new PCB layout, I’ll send it out for fab.

Meanwhile I started taking apart the capsule. Just looking through the grille, it seems like the C-2 uses a Transsound capsule similar to the TSB-165A Scott Helmke used in the original Alice.

Behringer C-2 Capsule Front

I started by removing the back plate. This is just pressed into place, but it’s a bear to get out. I eventually removed it by gripping it by an inside edge with needle nose pliers (pushing outward), and spinning it out. It took a couple of attempts, but it came apart.

The rear side of the capsule has an open cell foam washer in it, presumably to provide wind protection and to act as a delay plate to shape the hypercardioid pickup pattern. With the washer removed, the back side of the capsule is visible, held in place by a brass retaining ring

Behringer C-2 Capsule Back - Baffle Removed

The holes in the ring are really tiny. My existing pin wrench didn’t work, so I used an old divider with dull points as a pin wrench. There’s a bit of red enamel to prevent the ring from backing out, which took a little force to crack. Once that was done, though, the ring backed out easily. (I’ll need to be sure to apply a fresh bit of enamel when I get the new capsule installed.)

Behringer C-2 Capsule Disassembled Back

Behringer C-2 Capsule Disassembled Front

I was hoping the capsule was a Transsound TSB-165A, the same one Scott Helmke used in his original Alice microphone. Unfortunately it’s not. The capsule in the C-2 is 16mm in diameter x 6mm thick. The TSB-165A is 16.5mm x 8mm. But after some poking around on the JLI Electronics web site I think I found a match: the TSB-160A. The specs are almost identical to the TSB-165A, so it should play nicely with the Alice circuit (yay!), but the form factor matches what’s in the C-2. I’ll order a pair of these when I place the order for the 165A capsules for my MS Alice.

Behringer C-2 Capsule Mesh Outside

Another concern with the C-2 capsule holder is how the capsule is recessed, and how close the edges of the holder come to the input ports on the capsule. From my experiences with my first rev of mic bodies, I know that can color the sound enough to hear it. I’d like to open this up, if possible. It’s a simple enough job on a lathe as long as I can get the grill out.

Behringer C-2 Capsule Mesh Inside

The grill looks like it’s a two-layer mesh that’s either glued or soldered into the capsule holder. That should be easy enough to remove with heat, one way or the other. I might even be able to re-use it if I’m not too rough getting it out.

The grill serves two purposes. First and foremost, it’s an RF shield to keep stray electromagnetic radiation from getting into the signal path. Second, it helps to keep the capsule free of debris. Third, some manufacturers will stick enough mesh in front of the capsule to act as a rudimentary pop filter, and at least reduce the effect of wind. The problem with that third purpose is that you need a lot of tight mesh to pull that off. Enough so that it colors the sound of the mic. Not surprisingly, one of the more obvious mic mods is to remove a layer of mesh from the capsule housing.

But given how open the outer mesh is, I’m afraid it will make the mic prone to RF interference. For now I’ll leave it alone.

The next steps are to finalize the design of the new board, send it out for fab, and source all the components and capsules. But before I can finalize the board design I want to see if I can add in one of the features of the C-2: The switch on the side of the mic lets you select a high pass filter or a -10dB pad. If I can find the real estate on the board to accommodate the switch and the components necessary to add these into the Alice circuit, I will.


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BM-800 Microphone Conversion Part 2

Posted by Tom Benedict on 22/10/2016

This is the second half of a two-part article describing my conversion of a BM-800 microphone to an Alice microphone using a Transsound TSB-2555B cardioid capsule. All of this is based off of a pair of Instructables written by Jules Ryckebusch: Modify a cheap LDC Condenser microphone and Build the MS Alice Stereo Microphone.

Part 1 of this article showed pretty pictures of the donor mic (a Neewer NW-800 with an excess of bling), a description of the cable that came with the mic (which I don’t intend to use), photos of the mic in various stages of disassembly, and a CAD drawing of the salient features inside the microphone to help others lay out circuit boards for their own conversions.

Since writing part 1, all of the bits and pieces I ordered to do the conversion arrived: enough electronics to build three Alice boards, and a TSB-2555B capsule to put in the first one.

Everything for an Alice Conversion

Before populating the boards I did a test fit to make sure they would actually fit. I was pleased to see how well the screw holes lined up, and I came pretty close with the taper.

NW-800 With Alice PCB

The next step was to populate the boards. Opinions differ on how to wire the high-impedance (high-Z) end of the board, so I started with all of the low-Z components.

The circuit used in Jules’s first article had zener diodes on the output stage to protect it against over-voltage on the XLR pins. The circuit as-built in his second article omits the zeners since the 2N5087 transistors are rated for more than the 48V likely to be seen on an XLR connector. I ordered the zeners, but left them out for now.

Alice Trio with Low-Z Components

After I’d already wired all the boards I installed one in the mic and ran into my first problem: With the board installed right-side-up, the 47uF capacitor pokes up high enough that it interferes with the body tube. For my first mic I’m planning to install the board up-side-down to give the capacitor more room. But if I wind up building the MS mic from Jules’s second Instructable, I’ll need to install new capacitors that lay flat against the circuit board.

BM-800 Alice Board Placement

The reason for the difference of opinions on the high-Z end of the circuit is that it’s sensitive to contamination: leftover solder flux, dirt, dirt combined with humidity, oxidization, etc. on the high-Z end can all cause unwanted noise in the mic. Jules soldered his components to the board without issue. Others have used Teflon standoffs to float that part of the circuit above the PCB. Homero Leal built his Charis mic by point-to-point soldering the high-Z components, letting them float above the board without standoffs. Scott Helmke, the original designer of the Alice circuit, solders the high-Z components directly to the back of the mic capsule. For my first pass at this I soldered the low-Z legs of the FET to the board, but floated the high-Z circuit without stand-offs, similar to Homero’s Charis mic. I can always change my mind later and re-wire them.

High-Z Components Air-Floated

With the board built, the next step was to add 22nF capacitors between pins 1 and 3 and pins 1 and 2 on the XLR connector to provide additional RF noise filtering. After that I installed the modified connector and the board in the mic body.

Alice Board and XLR with RF Filter Caps

The rest of the action takes place inside the headbasket.

It’s possible to cut away the original mic capsule to leave a saddle for mounting the TSB-2555B, but I wanted to make an entirely new saddle. Chalk some of this up to not wanting to make a modification I can’t back out. Chalk some of it up to my wanting a machining project to go along with the electronics project. Either way it needlessly complicates an otherwise pretty simple project.

Space inside the headbasket is tight, so rather than run into more interference issues I fleshed out the 2D CAD drawing and turned it into a 3D model. The space constraints almost entirely dictated the shape of the new saddle and post. The mic frame is drilled and tapped for M2.5 screws on a 10mmx15mm rectangular pattern, only two of which are used on the original saddle. I chose to use all four. The mic wires pass through holes spaced 20mm apart, centered on the long axis of the bolt pattern. In the CAD model I indicated these with 3.13mm holes, but in the final part I cut them as slots to make installing and removing the capsule easier.

Mic Saddle - CAD vs. As-Built

I attached the TSB-2555B capsule to the saddle with E-6000 silicone adhesive. A better method for the saddle shape I used would’ve been a polyurethane adhesive like Gorilla Glue, but I wanted to be able to remove the capsule in case I decide to add shock isolation inside the mic to cut down on handling noise. As-built the capsule can be removed by passing a fine wire between the capsule and the saddle, cutting the silicone bond.

EDIT: The first time through, I missed an important step: One of the charms of the Pimped Alice circuit is the potentiometer next to the 1Gohm resistor. It allows you to bias the FET properly, regardless of which FET you use. The catch is that by definition, if you don’t do anything with the potentiometer it will not be properly biased! In all ignorance I soldered everything up, closed up the mic, and went testing. Even with an improperly biased FET it still performed beautifully. I did go back and do a proper job of it, though.

In Jules’s first Instructable, toward the end, there’s a nice write-up for how to bias the FET. The catch is that this step must be done before the capsule is soldered to the board.

With the FET properly biased and the capsule attached to the saddle and post, all that was left was to put it all together and close it up.

Finished BM-800 / TSB-2555B Alice

I did a quick side-by-side against one of my Primo-EM184 cardioid mics. The Alice runs a little hotter, but not by too much. I’m reserving further judgement on the new mic until I have a chance to get it out in the field and try it on some quiet sources.


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BM-800 Microphone Conversion Part 1

Posted by Tom Benedict on 09/10/2016

Several posts ago I mentioned a plan to build an MS mic by following an Instructable written by Jules Ryckebusch. Jules used a BM-800 microphone as a donor mic and replaced its guts with two Pimped Alice circuits and three cardioid capsules. After several Ebay vendors whose listings indicated they would ship to Hawaii later changed their story and said they wouldn’t, I finally picked up a BM-800 microphone off of Amazon. The one I got is a Neewer NW-800. (I liked its shock mount better than the other one I found.) It arrived, and I started poking and prodding at it.

Along the way I discovered another reason to use a windscreen on a microphone. This thing is bling central. To be fair some of the other BM-800 mics I found on Ebay didn’t have nearly as much… presence… but this is the one I could get.

Neewer NW-800 Bling

Unless I’m recording birds that are drawn to shiny objects the windscreen will probably become a permanent fixture on this mic, just to keep me from going blind.

Neewer NW-800 Windscreen

Before tearing into the thing I decided to try it as-is. On the face of it it’s a phantom powered cardioid condenser mic. This means plugging it into a device that doesn’t provide some kind of power (aka my laptop, my phone, even my kids’ desktop computer) won’t work.

The mic has a male XLR jack at the back, and came with an XLR-to-3.5mm cable. 3.5mm inputs that provide power typically provide plug-in-power (2.3V to 5V, depending on the device). XLR inputs provide phantom power (typically 12V, 24V, or 48V). That discrepancy made me a little leery of just plugging this into whatever and cranking volts through it. I started by ringing out the cable to see what it was actually doing.

XLR to 3.5mm Cable

Up to this point all the XLR plugs I’ve dealt with have been for balanced signals. That is to say that one pin of the 3-pin XLR is ground (pin 1), another is the positive signal (pin 2), and the third is the inverse or negative signal (pin 3).

3.5mm inputs typically use a TRS connector and unbalanced signals. In the case of the 3.5mm stereo input on my recorders the tip is the left positive channel, the ring is the right positive channel, and the sleeve is ground.

The cable supplied with the BM-800 ties XLR pins 1 and 3 together and routes them to the sleeve of the 3.5mm plug, and routes XLR pin 2 to both the tip and ring of the 3.5mm plug. This effectively turns the balanced output of the mic into two channel mono unbalanced output on the 3.5mm plug, meaning it should be able to be plugged into any 3.5mm stereo input and drive both left and right channels with the same signal. Neat!

What this also means is that as long as the mic can run on a wide range of voltages, the plug-in-power on any recorder should be able to drive this thing. So should the battery box I got from Church Audio. Or by removing the XLR-to-3.5mm cable and plugging in an XLR-to-XLR cable, I should be able to power it with phantom power (12V or 48v – the only two options on my recorder) and use it as a single channel balanced input.

Still leery of running such a wide range of voltages through it, I tried all three configurations anyway. I’m planning to gut this mic, after all, so if I burned it out the loss would be minimal. To my surprise all three worked! The plug-in-power on my DR-70D puts out a little under 3V, and my battery box from Church Audio puts out a little over 9V with a fresh battery. The 48V phantom power on the XLR inputs on the DR-70D put out right around 48V. I noticed a gain difference between the PiP and battery box, but because of the different gains on the XLR vs. 3.5mm inputs on the DR-70D I wasn’t able to tell if the additional voltage was doing anything to the mic itself. (My guess is it doesn’t. To survive that wide a range of voltages I’m guessing the mic has a voltage regulator on board. Past a certain point it’s just dissipating as heat.)

So how does it sound?


How to put this…

I’ve seen the shock mount it came with listed for more than what I paid for the mic. I don’t think this is too far out of line with how it sounds. It’s not bad, mind you. It’s just not anything I’d write home about. A little creative EQing would probably make it a decent podcast microphone. But as for making ambient nature recordings? Mmmm… no.

So without further ado I tore into it to see what I was going to have to deal with.

Neewer NW-800 Disassembly: Assembled

The first step in disassembling the microphone is to unscrew the butt cap. This also releases the shell, which simply slips off to expose the circuit board. The shell is keyed to a tab just under the headbasket which fixes the orientation of the logo on the mic. This is important since the mic is a side-entry rather than end-entry, meaning sound must enter from the side and not the end. Added to that, it’s a directional microphone so it’s only sensitive on one side. Can you guess which side? (Answer: The one with the logo.)

Neewer NW-800 Disassembly: Shell Removed

Some nice features on the inside of the thing: First, there’s a ton of room. Second, there’s a nice frame with mounting holes tapped for M2.5 screws. (More about those in a sec.) The only weird part is the taper on the frame and the circuit board. I like the look of the tapered board, so I decided to taper the boards for my Alice conversion, too, and put mounting holes in the boards to make use of the holes in the NW-800 frame.

Neewer NW-800 Disassembly: Headbasket Removed

Two M2.5 flat head Phillips screws hold the headbasket in place. They’re located just under the headbasket, above the circuit board. Once the screws are removed the headbasket lifts off, exposing the capsule.

Despite the appearance, the capsule in this mic is the same size as the EM-172 and the EM-184 capsules from Primo: 10mm diameter. At this point I was sorely tempted to gut the mic, drop an EM-184 capsule in the mic saddle, and call it quits. But the whole purpose of this exercise is to move beyond Primo all-in-one capsules and try my hand at building more complicated (and better performing!) microphones.

Neewer NW-800 Disassembly: Circuit Board Closeup

All of this starts with the circuit board.

Simple stuff first: The screws are M2.5, spaced 30mm apart. They’re biased a couple of millimeters above the centerline of the cavity. If you’re planning to make a rectangular circuit board to fit inside this mic, that’s probably all you’ll need. (The tube with the logo has vertical walls, so a rectangular board will fit fine.)

Since I wanted to make a tapered board I measured the whole cavity and threw it into CAD. At some point I’ll draw it in 3D, but for now a 2D representation is plenty for me to design the new board outlines. I’m building the Alice boards using through-hole components, so I needed a little more real estate than the original board provided. The 2D drawing of the cavity and the new board outline looks like this:

2D CAD - NW-800 Cavity and Board Outline

I sent the boards out for fab and ordered enough components from Mouser to build out three of them. One is destined to receive the TSB-2555B capsule I ordered from JLI. The other two will eventually be used to build a copy of Jules’s MS mic using three TSB-165A capsules, but that’s a project for another time. Once all the bits arrive I’ll write the second half of this article, which will cover the construction of the TSB-2555B mic.


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DIY Microphone: EM172 Capsule and XLR Plug

Posted by Tom Benedict on 05/03/2016

This is the last in a four part series about powering the Primo EM172 microphone capsule. Part 1 outlined the problem of how to provide 5-10v to the capsule and predicted some results. Part 2 shared some results and pointed out that the gain differences between inputs on my recorder invalidated my predictions. Part 3 discussed my reasons for going with XLR connectors on all my microphones, and some of the details of that. This last part puts it all together into a step-by-step DIY for building microphones with Primo EM172 capsules, powered by 48v phantom power on an XLR plug.

If you need to build a microphone based around the EM172 capsule that plugs into the 1/8″ mic jack on your recorder, or a laptop, tablet, whatever, there are already several excellent tutorials out there. Rather than adapt this one to your needs, refer to one of the existing tutorials. The two I used when I first started building EM172 microphones were the ones on Zach Poff’s page and the one on Wild Mountain Echoes.

In this DIY I’m going to assume you already have a plan for making a mic body. I made mine out of Delrin bar stock on a lathe. Others have used Sharpie pen caps, which also provide a nice clip for clipping the mic to things (see the tutorial on Wild Mountain Echoes), PVC pipe, brass tubing, etc. When mounting the mic in the mic body, make sure the front of the capsule is flush with or slightly proud of the mic body. Don’t recess it. I made that mistake with my first set of mics and wound up with mics that sounded like they were inside a sewer pipe. If in doubt experiment by wiring up the mic completely, plugging it in, and listening to it as you slide it in and out of the mic body you plan to use. After all, this is DIY. Experimentation is part of the deal.

Primo BT-EM172 to P48 XLR Wiring

Credit for the circuit goes entirely to David McGriffy, and credit for the component choice goes entirely to David McGriffy and Ricardo Lee. Ricardo Lee’s writeup, SimpleP48wm61, goes into the theory of the circuit and the reasons for the component choices in depth. It’s the real reference for this. (In order to use that link to download Ricardo’s file, you may need to be a member of the micbuilders group on Yahoo!. If you’re doing this DIY you’re a mic builder, so it’s not a stretch.)

EDIT: A couple of weeks ago Akira So brought to my attention that I had the capacitor poloarity reversed from how David McGriffy and Ricardo Lee have it in SimpleP48. I’ve since corrected the schematic here. Credit where credit’s due.

EDIT: Akira also pointed out that my value for R (120k) resulted in something like 1.3-1.5V at the capsule. I experimented with a number of resistors to see what value of R would produce 7.5V at the capsule on my recorder, and for a Tascam DR-70D, R=40k produces just over 7.5V. When you do this build, you will have to find what works best for your equipment.

EDIT: I also swapped the supplier for the EM172 from Frogloggers to Micbooster (FEL Communications). I haven’t heard from Gene at Frogloggers in a while. Hoping he’s doing ok.

For my build I used the following:

I also used some metal tape (copper in my case, from the local gardening center), heat shrink of various sizes, and the solder I found on the bench in the lab. (My Alphametals solder I’ve been using for the past 20 years isn’t ROHS certified, so I can’t say “use this stuff, it’s great!”)

Not including the tools necessary to fabricate the mic bodies, you’ll also need:

  • Soldering iron (temperature regulated if possible)
  • Source of heat for heat shrink (heat gun, lighter, etc.)
  • Assortment of wire cutters, strippers, fine tip pliers, etc.

Since most of the bodies people use for these require the mic to slide in  from the front end of the housing, we’ll start with the mic capsule.

EM172 Back End

The first step is to strip one end of the cable, trim back the red and white wires to a workable length, and still leave plenty of shield exposed. The red and white wires are then soldered onto the appropriate pads on the capsule.

Warning: The EM172 capsule is sensitive to heat. These two photos were made with a capsule I’d killed using an unregulated soldering iron, which is why the capsule looks a little ugly. If you have access to a regulated iron set your iron no higher than 735C and don’t hold the iron on a pad for more than a few seconds. If you don’t have access to a regulated soldering iron, be sure to get EM172 capsules with stub leads already soldered in place. The tutorial on Wild Mountain Echoes uses capsules with stub leads, so you can see how she did it. Do all your work on the stub leads. Don’t fry your microphones!

EM172 With Wires

Now we build the shielding around the capsule itself. Insulate the sides and back of the capsule with some heat shrink.

Capsule Isolated

Be sure to account for every strand in the shield as you bring it up and over the heat shrink. Wrap with foil tape and trim back the shield so no wires protrude. Be sure no wires cross over the heat shrink and touch the front of the capsule.

Making a Shield

Apply a second layer of heat shrink over the foil tape. I like to apply a short length of colored heat shrink to help me identify which mic is which when I’m running wires and plugging things in out in the field.

Heat Shrunk Ready To Go

At this point go ahead and run the mic cable through your mic body, but don’t mount the capsule just yet. Once you’ve soldered the connector end of the cable, it’s a good idea to test everything to make sure you didn’t make any soldering mistakes, and to make sure the capsule didn’t get damaged during soldering. Strip the other end of the cable, leaving a little more wire to work with than on the capsule end. Thread the wire through the end cap for the XLR connector and set it aside. Since the XLR connector provides its own shield you don’t have to do any metal tape trickery on this end. Gather the wires from the cable’s shield, twist into a bundle, and cover with heat shrink tubing. This is also a good time to apply a length of colored heat shrink to match the capsule end of the cable.

Cable Prepped With Shell

Grab the XLR connector body in a vise or some other holding fixture. If you don’t have a vise, a set of vise-grip pliers with tape over the serrated part of the jaw works well. Just don’t grab it so hard that the connector body is damaged or distorted. Another way to hold these connectors that works great is to have the mating connector screwed into a board. Plug the connector you’re working on into its counterpart and solder to your heart’s content. (I used a vise.)

Trim back the leads on the capacitor and resistor to something reasonable that’ll fit inside the XLR connector. Save the snipped off bits of the leads. One of these works well to bridge from pin 1 to the ground tab.

Resistor and Capacitor

Solder a leftover component lead from pin 1 to the ground tab. Next, solder one end of the resistor to the ground tab as well. Next, solder the (+) end of the capacitor to pin 2. Finally, tie the two free ends of the capacitor and resistor together.

XLR Plug with McGriffy Components

All that’s left is to solder the cable onto the plug. Red goes to pin 3, white goes to the (+) lead of the capacitor as well as the free end of the resistor, and the cable’s shield is soldered to the ground tab. (In this photo the connector is rotated 180 degrees from how it’s drawn in the schematic, but that’s how the solder cups are oriented. Flip it around in your mind and it’ll make sense.)

XLR Plug with Cable

At this point your microphone’s electronics are finished. Put the connector together and screw things tight.

This is a good time to test the mic to make sure nothing went wrong. Plug it into your recorder, turn on phantom 48v power, and dial up the gain. If all went well you should have a low noise microphone ready to be installed in its mic body. If not, go back and check each step to find out what went wrong.

Finished Mic

Have fun recording!


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Powering the EM172 Capsule – Part 3: Capitulation

Posted by Tom Benedict on 23/02/2016

I made up my mind about powering my EM172 microphones. Ultimately this decision had less to do with how I was powering the microphones than how I was plugging the mics into the recorder. One of the things I discovered when I wrote my last post was that the Tascam DR-70D uses completely different amplifiers for the XLR inputs and the 1/8″ inputs. Different form factor, obviously; different impedance; different gain. It’s that last part that really drove this decision.

The gain ranges on the 1/8″ plug are +3dB, +11dB, +26dB, and +38dB. The XLR gain ranges are +21dB, +36dB, +51dB, and +63dB. While I was performing side-by-side tests I kept having to crank back the gain on the XLR input to match the levels on the 1/8″ input. As I tested with quieter and quieter subjects it finally hit me: +38dB of gain just wasn’t enough to bring up the levels of some of the subjects I want to record. The XLR input gave me more gain to play with. The last test I ran was what finally convinced me. Even with the gain cranked all the way up on the 1/8″ input mics, I couldn’t get the sound levels over -25dBFS. The recording was just too quiet to use. I cranked up the gain on the XLR input, and was able to get -12dBFS with the same subject.

Good news is the mics really do perform better with the 9.6v bias voltage David McGriffy’s circuit provides. So this is a win-win.

The lavalier mics were no problem to convert. I bought a stash of Neutrik XLR connectors when I started this whole investigation, so it was just a matter of lopping off the 1/8″ connectors and soldering up the XLRs with the resistor and capacitor from McGriffy’s circuit.

XLR-Converted Lavalier

My SASS was another story. I really hate having things with cords that can’t be unplugged, so I wanted to connectorize everything and use extension cables. Only problem: I’m a beginner! So I had no idea how all the connectors worked.

After some Googling and image searching I learned that:

  • XLR extension cables are gender-inspecific. One end is male, the other is female.
  • Female XLR connectors are the ones with the latch. This is true of both panel and cable connectors. So female panel connectors have a latch, but male panel connectors don’t. (This confused me.)
  • Neutrik makes a crapload of XLR connectors you can choose from. It’s worth looking them up in multiple catalogs to find out which series were developed to fix the bugs in previous series. Though it’s really hard to go wrong, so long as you get all the genders right. These things are built like tanks.

I picked up a pair of pre-built 10′ extension cables for a little over the price of the connectors themselves along with some male panel jacks to install in the SASS. Installation meant cutting into the back of my SASS, but it went quite smoothly and the results look (and sound!) nice. (Yeah, this is an infrared photo. Ironwood trees look like Dr. Seuss trees in the IR, so I just had to play.)

SASS Back in the Field

Meanwhile I figured it was finally time to solve the issue of wind protection. A few months back I learned I’m really REALLY bad at sewing fake fur. I did some reading since then, so I think I know what I did wrong. But rather than getting stalled on my own lack of sewing skill I ordered a pair of lavalier windscreens from Cat Ears. They fit over my oversized mic bodies, but they’re too small to go over a foam windscreen. I probably needed the larger ones. They do a decent job by themselves, but in wind over 15-20kts the mics still suffer from wind noise. Good enough to use the lavs as tree ears, but not enough to use them at the beach in solid wind.

Cat Ears Windscreens

Now I just need to solve the issue of wind protection for my SASS. Back to learning to sew fur…

In any case my gear and I are off the soldering bench and back out in the field. Finally. YAAAAAAAY!


Posted in Audio, Electronics, Engineering | Tagged: , , , , , , , , , | 3 Comments »

Friggin’ Cable Releases

Posted by Tom Benedict on 24/03/2015

A year or so ago I bought a remote cable release for my camera. I wanted something that would do time lapse, long exposures, delayed exposures, you name it. Turns out there are scads of these things out there. They all seem to use the same basic electronics. The difference mostly lay in the packaging and form factor. So I picked one, used it, and got a lot of good use out of it.

In my post about batteries I mentioned that I managed to kill my cable release by letting the alkaline batteries I’d put in it go stale. And leaky. And corrosive. And… >deep breath< Whew! Let the past be the past.

Wireless Timer Release

Anyway, while shopping for a new one I saw that Yongnuo had a wireless version for not too much money. I picked one up off of Ebay, tested it, verified that it worked, and… promptly had it fail when I took it out in the field. It would focus on a half-press, but wouldn’t trip the shutter on a full-press. The weird thing is the display said “Release”, so I knew the switch was good. But it wouldn’t actually do anything.

The wireless release comes as two components: a handheld transmitter with a display, button pad, shutter button, etc., and a receiver that you stick on the camera. The receiver doubles as a cable release, complete with a shutter button of its own. When I tested it it worked perfectly! So I wasn’t entirely dead in the water. Just mostly.

The Yongnuo MC-36R also allows for a cable to be used instead of the wireless connection. Today I made a cable using some spare 1/8″ stereo headphone plugs and some spare wire I’d salvaged from a dead sensor at work. I built the cable, plugged it in, and… had the same exact behavior! Half-press would focus the camera, but the full-press did nothing!

Digital devices are usually pretty self-contained. Except for witnessing the battery-driven demise of electronics, there’s typically very little you can do to salvage something that has stopped working. But this was sounding a lot less like a logic fault in some chip and a lot more like a failed connection. So I opened the unit up.

The MC-36R has two circuit boards inside. One houses the LCD, buttons, and processor. The other houses the 2.4GHz transmitter, the channel-selecting DIP switch bank, and the 1/8″ stereo jack for the optional cable. I expected the connection between the two to be some sort of three-wire UART. Instead I found Vcc, Gnd, 1, and 2, and the #2 wire had popped out of the connector. ??! Each state of the switch had its own discrete wire! I shoved the wire back in and everything worked perfectly!

When I put the thing back together I saw what the underlying problem was. There’s almost no room inside the thing. The connector for the 2.4GHz board bumps up against the big honking half/full press switch for the shutter. So if one of the wires is even slightly out of place when the unit is assembled it’ll get pulled out of the connector. In my case the #2 wire was the one who lost. A little care during re-assembly and I avoided the problem.

I have to wonder how many of these things fail during QC testing. I wonder how many more are eventually returned when they quit working. In the event mine dies again I can replace the connector with a new one. It’s a 4-conductor micro-JST. I have a bag of them. Meanwhile I’m back up and running. And now I have a cable I can use, too.

– Tom

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Video Downlink for KAP

Posted by Tom Benedict on 04/12/2013

This is the third and final installment of KAP articles. In Then and Now I described the evolution of my KAP rig from my first flight to the rig I use regularly today. In A Progression of Kites I described the additions I made to my kite bag over the years, and what those kites provided for in terms of KAP. This article describes my video downlink system.

The previous two articles were written almost as a history: “First this happened, then this happened.” But over the years as I solved the various problems I ran into with my video downlink system, I wrote articles describing what I did. So the history version of this has already been written. Rather than repeat what I wrote, this article is more of a how-to for adding a video downlink system to a KAP rig.

In its very simplest design a video link is a wire that connects the video output of a camera with the video input of a display device. It’s possible to set up a KAP rig this way, but the wire would have to be long, heavy, and would be a nasty thing to have in your hand when lightning strikes. For obvious reasons, it’s preferable to send the video signal to the ground some other way: radio.

It bears mentioning at this stage that unlike a wire, leaping into the world of radio places the operator in a new situation. At this point you’re transmitting your signal in such a way that it can potentially interfere with other wireless devices. In most countries radio transmitters are regulated, and typically fall into two categories: Either the device has been declared fit for use by an untrained person or it has not. In the US this declaration comes in the form of FCC Part 15 approval. If the device has an FCC Part 15 sticker on it, you can use it without a license. If it doesn’t, you can’t. The laws in other countries will vary, but that’s how it is here.

FCC Sticker

As was pointed out to me when I started down this road a couple of years ago, if you wind up using equipment that doesn’t carry your country’s seal of approval, it’s up to you to get whatever licenses are necessary for you to stay on the right side of the law. Unfortunately most video transmitter gear requires a license. (In case you’re wondering, that’s a picture of a 2.4GHz transmitter module from an RC radio. Most RC gear is approved for unlicensed use. The manufacturers are well motivated to make sure their stuff is certified. Video manufacturers? Not so much.)

There’s another benefit to studying for and getting your radio license. The whole point of the exercise is to give you the information you need to successfully and safely experiment in the world of radio. And that’s precisely what you’re doing! You’re exploring the world of radio by setting up an amateur TV station and a receiver. Questions that you’ll need to answer will include how much power to use, what kind of antenna to use, how much radiation exposure you’ll get, etc. By virtue of studying for and receiving your amateur radio operator’s license, you’ll find the answers to these and other questions. It’s well worth it.

Back to the video link!

The next question you’re probably asking is, “But what do I have to get to make it work?” Unfortunately there’s no right answer because there’s more than one way to do it. Here are some things to consider:

Most video hardware is built to handle either NTSC, PAL, or both of these formats. When looking at your gear – camera, video transmitter, video receiver, and monitor – keep in mind which system you’re using and make sure all your hardware will handle it. I got hardware that will handle both, so I can switch the entire setup from NTSC to PAL and back without penalty.

All of this hardware will have to be powered. In the world of FPV RC aircraft, there’s an advantage to running your video system on a completely different power supply than your radio system. That way when your video system drains your batteries dry, your aircraft doesn’t suddenly fall out of the sky in an uncontrolled heap. This isn’t as big a consideration in the world of KAP since the kite will fly regardless of the state of the batteries. (A single line kite with a dead battery flies exactly the same as a single line kite with a fully charged battery. The kite really doesn’t care.) So there’s an advantage to choosing gear that can all be powered off of a single source.

Between the transmitter and the receiver you will need a pair of antennas. At their simplest an antenna is a piece of wire trimmed to a particular length (1/4 the wavelength of the transmitted signal). At their most complex they can be fairly complicated pieces of equipment that preferentially transmit and receive in a particular direction, polarize the signal in a particular way, etc. Keep in mind that the size of the antenna is always dependent on the wavelength of the transmitted signal.

And while keeping that in mind, consider that radio gear for sending and receiving video signals can be built to operate on a wide range of wavelengths. At the longer end of the spectrum (literally) you can get 900MHz hardware. At the shorter end you can get 5.8GHz hardware. In between you can find hardware built to 1.2MHz, 1.3MHz, 2.4MHz, etc. The longer your wavelength, the better the signal will penetrate solid objects and bend around corners, and the less power it will take to get a particular range. This favors longer wavelengths. Conversely, the shorter your wavelength, the smaller your antenna. This favors shorter wavelengths. And no matter what, you don’t want your video system to interfere with your RC transmitter, either at the primary frequency or at a harmonic (n * freq). So if you’re using 2.4GHz RC gear, don’t use 2.4GHz or 1.2GHz video gear.

Finally, you’ll need some form of display on which to see the image from your camera. Options for this range from fully enclosed headsets like the ones from FatShark to standalone monitors. The display you choose will depend greatly on what you’re doing.

Regardless of whether you’re planning to build a video downlink for KAP or an FPV setup for RC aircraft, it’s really not safe to use a self-enclosed headset if you plan on operating your gear by yourself. The AMA’s safety guide for FPV specifically says you should use a spotter when flying FPV, and that they should maintain line-of-sight on the aircraft at all times. It’s really no different for KAP. If you’ve spent any amount of time flying single line kites, chances are you’ve seen them do something unexpected. If you’re watching the kite you can typically do something to recover. If you’re staring into a self-enclosed headset, you won’t notice until it’s way way too late. If you plan to use a headset for KAP, also plan to bring someone along to operate the kite. I’m a solo KAPer, so I built my system to use a monitor.

Something to look for when getting a monitor is what it does when it loses signal or encounters a glitch. Many of them switch to a blue screen. Because radio signals are almost always glitchy, this means that the monitor will spend most of its time being blue. Look for a monitor that will show static when it loses signal or encounters a glitch. Most of the ones that do will proudly tell you of this on the packaging or in the advertisement, and may even say, “Made for FPV use!” My first two monitors did the blue screen thing. Skip the pain and get a good monitor from the get-go.

Now for the real question: How do you wire all this stuff together? Fortunately the world of FPV has made these systems a lot more turnkey than they used to be. Unfortunately they’re not as turnkey as, say, plugging servos into a receiver and moving them with joysticks. Some soldering is typically required. Here’s the diagram for the system I built using Boscam 5.8GHz hardware:

FPV Wiring Diagram

I used a 3-cell LiPoly battery to power the video system, so the video transmitter is getting between 12.6V and 11.6V. The UBEC steps that down to 5V for the RC receiver and servos. I don’t use the Vcamera output on the transmitter to power the camera, and rely on the camera’s own internal battery for its power.

Keep in mind this is for a KAP system. I built out a similar system for FPV. In that case I omitted the UBEC and didn’t provide any connection between the video system and the RC system. This was in order to avoid RF interference from the airplane’s ESC, motor, and servos, and to keep the video system from draining the flight battery, as I said earlier.

When setting up your radio gear, carefully check the channels the gear can operate on to make sure the frequencies are legal in your country. Not all countries allow amateur use on all bands, and not all countries define the bands the same way. You should be able to find the frequencies used for each channel in the manual for your hardware.

When setting up the channels on mine I ran into another problem: The Boscam 5.8GHz Rx and Tx use banks of DIP switches to set the channel. Unfortunately on one of them 1 is up and 0 is down, and on the other 1 is down and 0 is up. Even worse, the setting for channel 1 on the transmitter and the setting for channel 1 on the receiver are completely different! So for those using the Boscam 5.8GHz system, this diagram appears to be correct if you are looking at the numbers on the DIP switch banks (original source):

Boscam VTx VRx Channels

Because I knew I wanted to be able to swap cameras on my KAP rig, I wired the three video/audio/ground wires to a male servo connector and built individual camera cables that ended in female servo connectors. To swap cameras, all I have to do is swap cables.

Quick aside: If you plan to do anything with any sort of servo gear – KAP, RC airplanes, robotics, whatever – get a bag of male connectors, a bag of female connectors, a spool of servo wire, and a crimp tool. Places like Servo City carry these as stock items. I have never regretted getting mine. The only regret I’ve had is that I didn’t get bigger bags of connectors! You’ll find these come in handy for more than just servos.

Ground stations vary from person to person. I built mine for KAP, so it’s small, self-contained, and can be attached to my RC transmitter.

KAP Video Ground Station Rear

I use the same ground station for FPV. It consists of the video receiver and antenna, a monitor, and a battery. The wiring is typically a little more straightforward than on the transmitter gear. Both video receiver and monitor usually come with RCA connectors, so it’s just a matter of plugging everything in. In my case, though, the RCA cables were all male! Rather than use a bunch of female/female gender changers to connect the cables together, I made all new cables. But it’s not necessary to go this far with your gear.

Once you have all your bits and have wired them together, test it thoroughly on the ground. Having a KAP video downlink fail in the field is a bummer, but it’s not the end of the world. Most of us start off either using autoKAP or aiming by looking up at the camera, so not having a video downlink isn’t a show-stopper. It’s just not fun. If you’re building out an FPV link for an RC aircraft, losing your video feed mid-flight can be a lot more disorienting and may lead to a crash. So test first. Then test again. Then, just for grins, test it again. I spent way too many KAP sessions chasing video problems to chance it these days.

Whatever route you go, remember to fly safely whether you’re flying a kite, a plane, a helicopter, or a multi-rotor. Using a video downlink of any sort means you’re taking your eyes off of your aerial platform. In the case of a kite, finding it again is just a matter of looking up the line. In the case of an airplane or helicopter, it may take you a while to pick it out of the sky. Use a spotter. And expect things to go wrong. They always do.

– Tom

Posted in Electronics, Engineering, Kite Aerial Photography, Radio, RC Airplanes | Leave a Comment »

Offroad Camera Project Revisited

Posted by Tom Benedict on 22/03/2013

I did some more work on the offroad camera platform I wrote about back in 2011. As delivered back then, the platform had a couple of serious weaknesses. First, having a highly compliant suspension meant that it never truly sat flat. So no matter where you drove, no matter what you took a picture of, your horizon was never, ever, going to be level. Second, the pan/tilt head from Servo City was inherently unbalanced, so the servos were constantly fighting the weight of the camera. In addition to burning through the batteries and making a pretty loud racket as the servos fought for position, this also meant that when power was removed, the camera slammed down either forward or backward onto its hard stops. Toss in a number of other minor annoyances like that bundle of cables on the side of the camera, and you can see it was far from ideal.

Crawler 1

So the 6WD camera platform came back for a mod job. Here’s the list:

Remove the Servo City tilt axis, replace with a tilt/roll unit:

The new unit is a variant of the HoVer KAP rig I made for my A650 years ago. In the case of my KAP rig, it was set up so the two extremes of motion on the servo gave me horizontal or vertical orientation for the camera. By flipping a switch on the transmitter, I could toggle between those two modes.

HoVer Axis

In the case of the offroad camera platform, the roll axis was centered on the horizontal position, with +/-45 degrees of motion to either side to level the horizon. The only real mechanical differences are that the tilt frame had to be a little wider to clear the video and shutter cable on the side of the camera, and I used a beefier arrangement for the roll axis using Servo Blocks from Servo City.

Servo Block Built

Unfortunately, when I put everything together I found that the Brooxes KAP gear was too flexible to work in this application. But let’s face it: In the world of KAP, having a slightly compliant rig is a bonus, not a detractor. And lightweight is the name of the game. So it actually works really well for what it’s designed for. But this thing is intended to roll over rough terrain and potentially get thrown around a lot. Rather than start over from scratch, I beefed up the main frame with corner bracing and a brace across the back to stiffen it. The tilt frame didn’t get any bracing, but if it looks like it’s necessary down the road, it’s easy enough to add.

Roll Axis

Replace UBEC voltage regulator:

In testing the changes to the camera mount, I ran across another problem: Occasionally the servos would jerk and shake, and sometimes while driving at full speed the motors would slam to a stop, then resume at full throttle a moment later. Scary! I wound up checking the servos, the speed controllers, and finally checked the voltage regulator I’d installed when the unit was first built. GAAAH! It was noisy! Time for new hardware.

A while back I converted the video system on my KAP rig to a 3S LiPo battery, but kept the 4xAAA battery pack for driving the RC receiver. On the advice of a fellow KAPer (hi, Bill!) I’ve been wanting to install a UBEC to power the radio instead. What a perfect opportunity! I ordered two 3A continuous / 5A burst UBECs from Hobby King, and when they arrived one went in my KAP rig and the other in the robot. The KAP rig worked perfectly first try (thanks again, Bill!) and the installation on the 6WD robot was a snap. Most of the noise went away. (Yeah… most… more on that in a sec.)

Replace the antenna support:

Back when I started on this, there was limited selection for radio and video gear. The only video link that fit the bill was a 2.4GHz system. This precluded using a 2.4GHz RC radio, so we went with a 75MHz ground radio from Hitec. I’d post a link, but it has since been discontinued. The one gotcha with dropping to a lower frequency is that the antenna gets longer. And for best reception it needs to be mounted well clear of the vehicle. The first rev used a fiberglass rod I mounted in a Delrin block to support the antenna. But it was hard to transport, was fragile, and finally snapped off at the base.

For this rev I looked to the RC car and truck world for inspiration. Not only did I find it, I found nice off the shelf components that fit the bill perfectly! Now the radio antenna is supported by a Dubro antenna tube that mounts in a nice machined aluminum collet system from Integy. It holds the antenna out of harm’s way and up in the air for good reception, and is easy to break down for transport. In the end I bought two: one for installation and one as a spare. But I wound up mounting both because it’s handy to have a second mount on the drive system for those times when the upper deck is removed for servicing.

Antenna Collet

Build a new video cable

As you can see in the photo from the original 2011 build, the cable from the camera to the video transmitter was the stock Canon A/V cable that came with the camera. “Ungainly” is a kind word to describe that bundle of wires. But with the addition of the roll axis to the camera head, “disaster” came a little closer to the mark. The stock Canon plug wouldn’t clear the tilt frame, and the cables constantly tangled on everything. So I took the last of my Ebay cables from building the T2i video downlink for my KAP rig and made a custom cable for this camera. Since there’s already a shutter cable running from the video transmitter to the camera, I made them both the same length and zip-tied them together. Now they’re one neat little bundle that fits fine and doesn’t cause tangling.

Custom Video Cable

Track down that @$%! NOISE!!!

I mentioned the servo noise in the section on replacing the voltage regulator. I got most of it, but not all. The pan axis continued to exhibit noisy behavior that I couldn’t track down. After double-checking the UBEC (fine), I tore the transmitter and receiver apart.

Here’s another one of those areas where hobby bleed-over is a good thing. A while back I bought a cable on Ebay that lets me use my RC transmitter as a controller for RC flight simulators on the computer. It came with a bunch of different plugs for different transmitters. When I got it I plugged in the one for my radio, and I was up and running.

The cables work by picking up the pulse coded signal off of the radio and turning it into positions for each of the axes via the USB port. Aha! What a great way to test the RC transmitter! It turns out I had a Hitec plug for this cable, so I plugged in the transmitter and took a look. The potentiometer driving the pan axis tested out fine on my meter, and when I tried it with this cable, it came out completely clean. Most radios tap the pulse coded signal right before it goes into the transmitter electronics, so at that point I figured the transmitter was fine.

The receiver also turned out to be fine. For what it’s worth, the noise filtering on the Hitec 75MHz receivers is remarkably clean – almost pretty. This one was no exception, and tested out fine. No blown caps, no cold solder joints, no scorch marks. Nothing!

Eventually I broke down and tested everything using an independent radio, battery, and servos out of my parts drawers. The only pattern I found was that the pan servo on the camera head started to have jitter if I plugged in the tilt servo from the camera head. No other combination caused the noise. And it didn’t matter which channel either was plugged into. The tilt servo was the culprit!

Unfortunately the only matching servo I had was in my KAP rig. For the sake of the project I gutted my KAP rig and installed the new servo in the camera head. It tested out fine. But I had to order another servo for my KAP rig!

After re-installing all the servos, tying down all the wires, and bolting on the upper deck for what I hoped was the last time, I gave it a test run. Jitter free at 250′! YAAAAAY! The noise was finally GONE!

Lock it down and send it home!

The last step was to remove every single screw in the thing, apply blue Loctite thread locking compound, and replace it. No sense having this thing fall apart in the field! I also neatened up the wire runs, tied down the servo wires, and applied rubber to the spots where the camera would contact the camera mount. At long last, the offroad camera was ready to go home.

6WD DSLR Revised

This was, and continues to be a fascinating project to work on. I’ve learned a lot in the process, and will doubtless keep learning as I service this in the years to come. But for now I’m ready to fix my KAP rig and get back in the air.

– Tom

Posted in Electronics, Photography | 2 Comments »

Changes to the Video Down-link Hardware

Posted by Tom Benedict on 05/03/2012

From 2007 until a couple of months ago, I’ve done kite aerial photography using an RC radio to control pan, tilt, and sometimes yaw on a camera rig suspended from a kite line. Aiming has been done by looking at the camera rather than by looking through it. And for the most part this has worked well. I did some good photography, made some good panoramas, and had a truckload of fun flying kites.

Kiholo Bay

A number of events over the past year or so have made me want to try using a video down-link. I ordered the hardware, found out I needed a ham radio license in order to use it, studied for my license exam, passed it, and finally built a video down-link for my rig. It wasn’t completely smooth sailing, but it worked. More to the point, it worked well enough to convince me this is a direction I’d like to go. So I set out to fix the problems I’d run into and re-design the system so it fit in with how I like to do KAP.

My style of kite aerial photography is a little rough and ready. Everything goes in a backpack so I can hike in to wherever I need to be. When I’m flying I can tolerate holding a remote and a winder, but that’s it. Most of the time I’ll clip the winder off to a waist strap so I can have both hands free to operate the remote. I like to keep an eye on the kite. If things get rough, I want to be in a position to drop my transmitter on the ground so I can devote 100% of my attention to the kite. This has saved me from losing both kite and camera in the past, and will continue to save me in the future. I have stayed with this philosophy since I started doing KAP in 2007, and I see no reason to change.

When I added the video down-link to my setup, I wanted it to follow this same model: All of the ground-side video hardware had to fit on my RC transmitter. No extra stuff. No batteries in the backpack. No stray cables to prevent me from dropping some or all of my gear so I can deal with an unruly kite. All of the air-side video hardware had to fit on the KAP rig as discretely as possible. Once installed, I had to be able to wrap both KAP rig and RC transmitter up in cloth and shove them into my backpack for transport. If things went wrong in the field, I had to be able to drop the RC transmitter and ground-side video gear and not have it break. Everything had to be robust enough to handle this kind of treatment or it wouldn’t survive my style of KAP.

I came up with a good arrangement.

KAP Rig as of 5 March, 2012

The monitor is hard-mounted to the RC transmitter. It has a 1/4″-20 socket on the back, which made this easy. The hood is formed from some ABS plastic I had in the scrap bin. The 5.8GHz video receiver is attached to the back of the transmitter with industrial strength Velcro. I’m not 100% happy with the wiring between the monitor and the receiver, but it’s at least neat and tucked out of the way. All of the ground-side electronics are powered off the same battery supply. At the moment this is 8xAA Eneloop rechargeables located in the RC transmitter, but a future plan is to replace this with a LiPoly transmitter flat pack battery. For now it works, and appears to give hours of service. When the time comes to upgrade, I already have a battery picked out. (Thanks again to Bill Blake for pointing me in the right direction on batteries!)

The KAP rig has the video transmitter mounted above the RC receiver, on the opposite side of the rig’s aluminum frame. I am not convinced this offers sufficient RF shielding, but for now it works. A single cable runs between the video transmitter and the camera. A second cable made by James Gentles runs between the RC receiver and the camera’s remote shutter jack to remotely trigger the shutter. When installing the camera, these are the only two cables that need to be installed. Keep it simple.

5.8GHz Video Antennas

I replaced the stock antennas on the video transmitter and receiver with a cloverleaf and skew planar wheel, respectively. There were custom made by These are right-hand circularly polarized antennas. They offer a reasonable amount of gain over a simple half-dipole, and also offer some amount of rejection for linearly polarized signals. As an added bonus, PNPRC tests their antennas for good SWR match at 5.8GHz. Put it all together, and they’re a nice upgrade for a video link system. I haven’t had a chance to test these in the air, but already on the ground I can see a benefit.

These antennas are somewhat fragile. One was slightly bent during shipping. Easy enough to bend back, but I can see that some form of protection will be needed when I’m using them in the field. For the moment I’m removing them in order to pack my gear. As a longer term solution I’m leaning toward some radomes. Considering the size of the antennas, ping pong balls might be the perfect size.

For early tests I powered the air-side unit using a separate 9V battery. This was bulky, it added weight, and rechargeable 9V batteries have nowhere near the current capacity of an alkaline 9V so I was forced to use alkalines. Not my favorite solution. But it worked well enough to convince me the video down-link was a good idea.

More recently I completely re-vamped the video power system on the ground and in the air:

KAP Video Switch

Here’s how the new setup works:

The ground-side unit has a single master switch. When the RC transmitter is powered off, everything on the ground is powered off including the video hardware. When the RC transmitter is powered on, power is also available to the ground-side video hardware (monitor and receiver). Power to the ground-side video hardware is controlled using the quad-pole double-throw switch shown above. One of the poles is used to switch the power to the ground-side video hardware.

The other three poles are wired to a pair of 5k potentiometers. This lets the switch toggle between two fixed set-points for an RC channel at the same time it is controlling power to the ground-side video hardware. When the ground-side video hardware is powered on, that RC channel goes to 2.0us timing. When the ground-side video hardware is powered off, that RC channel goes to 1.0us.

On the KAP rig, the RC channel associated with the switch is wired up to an RC MOSFET switch from Pololu Robotics. Above 1.7us timing, the MOSFET passes the rig’s Vbatt on its outputs. Below 1.5us, the MOSFET passes 0.0V on its output.

The output of the MOSFET is connected to a 2.5-9.5V variable boost regulator, also from Pololu Robotics. Powered by anything above 1.5V, it’ll output a fixed voltage that can be set using an on-board potentiometer. I set mine to 9.3V and plugged it into the video transmitter on the KAP rig.

The net effect of all this is that the RC transmitter has a single switch that can be used to control power to the video hardware, both on the ground and in the air. This opens up a method of doing KAP that saves on batteries, and very closely follows how I do photography from a tripod:

When I’m using a tripod, I like to scout a scene handheld first. Once I find one or more vantage points from which I’d like to do photography, I’ll attach my camera to my tripod, rough-position the camera, then look through the viewfinder to see where I need to move it to get the composition I’m after. Once I’ve composed the photograph, I’ll do a final focus and finally trip the shutter. Then it’s on to the next vantage point.

When I’m doing KAP, I can power off all of the video hardware and set down the radio so I have both hands on the winder while getting the camera to altitude. Once it’s at altitude I can walk the camera into position, flying it by eye the same way I have done since 2007. Once I think the camera is in position, I can power up all the video hardware and take a look at the viewfinder. If I need to adjust my position I can. Then I compose, focus, and trip the shutter. Then the video hardware goes back off while I walk to the next camera location. With the exception of the kite, it’s almost the same procedure.

From past experience I know I can use my KAP rig without video hardware for many hours in the field without running out of battery on the ground or in the air. Video hardware places an additional drain on the batteries of both the KAP rig and the RC transmitter. But by having the power switchable in this way, it’s minimized.

– Tom

Posted in Electronics, Engineering, Kite Aerial Photography, Photography | 7 Comments »

A Second and A First

Posted by Tom Benedict on 11/02/2012

I got out again at lunch today and flew my video assisted KAP rig for the second time. But before I did that I flew my first IR-converted camera.

The camera is a full-spectrum modified Canon Rebel XT. The conversion was done by a guy at work who did the conversion for the CFHT Cloudcams. (The camera I used is actually a backup cloudcam.) The conversion involved removing the camera’s IR blocking filter from the front of its detector, and replacing it with a piece of BK7 glass. This lets infrared light reach the camera’s detector, but still maintains the same optical depth so that the autofocus hardware in the camera still works.

The flight itself went great. It flew exactly like an unconverted camera, except I couldn’t use my video down-link with it. The camera predates live view on the Canon DSLRs. So it was back to aiming by eye. I came away with some keepers, so I’m pleased.

CFHT Headquarters in the Near IR

The flight took place near the headquarters of the Canada-France-Hawaii Telescope Corporation where I work, which is the subject of the photo. No, it’s not snowing here. That’s grass on the ground.

The camera focuses like a champ, but the exposures aren’t quite on. My guess is the exposure sensors have a different response to infrared light than the focal plane detector. For this flight I metered manually, but I think a better approach in the future would be to figure out what the exposure offset is and just dial it in with exposure compensation.

Once the IR camera was safely landed and put away, I stuck my T2i on the rig and plugged in all the video hardware. The wind was stronger than during my first flight, so no surprise landings. But it was also more turbulent. So getting the compositions I was after was harder. Still, it worked out well. Here’s the visible light version of the above photograph from a slightly different vantage point:

CFHT Headquarters

I wanted to get the whole building in the frame without the foreground building intruding, but the wind wasn’t cooperating. Still, this was a nice tight composition. Except for normal RAW processing, this is straight off the camera. No cropping, no rotating, no nothing. I am super super pleased with how well this video downlink setup is working out.

Several years ago when I first flew over CFHT HQ, I did some directly-down photos of the indoor garden and the lanai area. They were completely blind-aimed. For some of them I couldn’t see the building at all, and was going entirely by feel. I got lucky and got some good images. So I tried to do the same with this setup, where I could compose them at will.

The first, the indoor garden, worked out quite well:

Complex Rooflines

Except for a little horizontal perspective control tilt to get my horizontals horizontal, I didn’t do much of anything to this photo. It really was rotated that straight from the get-go. Ok, ok, to be fair the wind was really cooperating for this one. I had plenty of time to half-press and focus, do the final composition, and trip the shutter. Not so for the lanai:

CFHT Lanai

I really wanted this one to be as well-aligned as the previous photo, but the wind was not as cooperative. By the time I’d walked in to position to make this one, the kite was bucking around and the rig was swinging. I could get the focus and composition, but not rotation. Two out of three. If I rotated, the tilting of the rig meant I wouldn’t always get the full courtyard in the shot. I did the best I could and walked away with some good photos. As with the full building photo, this one is almost straight off the camera. Just normal RAW processing. No cropping, no rotation.

Right after tripping the shutter on this one, the wind dropped. Kite and camera started coming down a little too fast for comfort. When I set up I knew that might happen, so I used too much kite for the wind. After the wind dropped I was glad I’d made that call. It was a bit of a mad dash to bring in line and land the camera safely, but everything made it down without a scratch. Camera went back in the camera bag, KAP rig and radio went back in the KAP bag, and the kite went back in the kite bag. With ten minutes left in my lunch hour, time to go back to work.

All in all I’m liking this video down-link more and more. It’s a good direction for my KAP to go. But as I hinted at in my last post, there are some changes I’d like to make to my gear. I spent the last of my earnings from my last Getty sale picking up the hardware I need to get all this done, so it’s not a question of if. It’s a question of when:

  • I’m adding a new switch to my radio. Flipping the switch will power on all the ground-side video gear (receiver and monitor), and simultaneously trip one of the servo channels on the KAP rig. On the KAP rig I’m installing a relay switch that will power on the video transmitter power when the switch on the ground radio is flipped. So flipping this single switch off will power off all the video gear on the ground and in the air. And flipping it on will power everything up.
  • I also picked up a boost power supply. It pegs at 9.5V, but I might swap it out for a 12V unit at some point. The idea is I can feed it 6V (or anything else between 1.5V and 9.5V) and it’ll output a nicely regulated 9.5Vdc. Add a line filter to get rid of the harmonics from the switching power supply on the boost board, and I can power my video gear off of the rig’s battery.
  • Finally, I ordered some reverse polarity SMA plugs with coax tails on them. I’m planning to make a 5.8GHz cloverleaf antenna for my video transmitter and a 5.8GHz skew planar wheel antenna for the video receiver. These are circularly polarized antennas, so they should reject a lot of the linearly polarized interference from other 5.8GHz devices. They also offer some modest gain, which should make for a stronger signal overall.

Yup, I’ve caught the bug. Now I just need a weekend with good weather and clean wind to fly in. Here’s hoping it’s the one that starts tomorrow!

– Tom

Posted in Electronics, Kite Aerial Photography, Photography, Radio | 1 Comment »