The View Up Here

Random scribblings about kites, photography, machining, and anything else

Archive for September, 2016

Job Satisfaction

Posted by Tom Benedict on 20/09/2016

One of the questions I’ve asked myself over the years is whether I would be proud to tell my kids what I do for a living.

Much of the time the answer has been yes. When I worked for Academic Computing at the University of Texas I spent close to a year working in the Student Health Center. We took care of the servers that maintained medical records, handled customer satisfaction surveys, and made sure the desktop and handheld computers of the doctors, nurses, and administrators all worked. We helped them help the student body of UT stay healthy. Feel good about it? You bet!

Some of the time the answer has been a resounding no. The last year I was with IBM my job was to spy on my co-workers. I was the weenie who read all the logs from all of our servers and flagged “security risks”. During that time we never had an actual outside attack, and I think we had fewer than ten internal “ethical hacks”, all of which we caught. But I lost count of how many times my co-workers had a typo or tried to do something as root out of innocent ignorance. I had to report them all. Not one was a malicious act, and yet my job was to ding them for it anyway. Feel good about it? You gotta be kidding me…

Working at CFHT is solidly in the yes category. I’ve had downer days. Heck, I’ve had downer months, if not years. But at the root of it all we’re in the business of exploring the universe to better understand how the whole thing works. When people ask what I do for a living I tell them that my job description basically amounts to doing whatever is required so we can collect science-grade photons at night. Sometimes this means designing and building new instruments; sometimes it means sweeping the floors. When things get floor-sweepy it’s easy to lose sight of the fact that all of the things we do here contribute to our understanding of the universe.

Yesterday I brought in my camera bag and lighting gear so I could photograph a set of filters for customs paperwork. We’re shipping the filters to France to be scanned on a better spectrophotometer than the one we have here. Customs had a set of requirements for the photographs, so I was taking my time to make sure I got everything right. Toward the tail end one of our resident astronomers came in to see what I was doing. I explained about customs, about their need for documentation and serial numbers, etc. Not exactly sweeping floors, but documentation photography is pretty mundane stuff.

He listened patiently, then said, “You understand the importance of what you’re doing?”

“What do you mean?” I asked as I moved the last of the filters from the lighting scoop, back to its packing crate.

“Right now the tightest constraint on the cosmological constant is the SNLS survey, made with these filters. The scans they’re planning to do will further refine our understanding of the cosmological constant.” He pointed to the filter I was holding, the r’ filter. “That filter is key.”

Gulp!

Yeah. Even the act of photographing these filters so the customs agents can identify them and their serial numbers was helping to contribute to our understanding of the universe. No “Eureka!” moment. No lone genius. Just a lot of people doing a lot of seemingly mundane tasks, all of which is further refining our knowledge of how the universe works.

Today I’m processing the pictures, putting together documentation packets for customs, and packing the filters lovingly in their cases for shipment to France. Am I proud to tell my kids what I’m doing for a living?

You bet your ass.

Tom

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Building New Mics

Posted by Tom Benedict on 17/09/2016

When you’re faced with a dilemma like choosing the next step to improve your recording gear, instead of finding the right answer to the question, sometimes it’s more fun to dodge the issue completely and go off on a tangent.

So I went off on a tangent! Building new microphones!

I’ve currently got two projects in the works. A parabolic mic and a self-contained mid-side mic.

The Parabolic

I’m basing my parabolic mic off of the family of parabolic mics  from  Telinga Microphones. The mics from Telinga offer all kinds of neat options. One contains a cardioid mic facing the parabola and an omni facing away from the parabola. This lets you record a distant sound and the ambient sound field at the same time on two different channels. Another contains two omnis on either side of a baffle plate so you can record a distant sound in stereo.

But at its simplest, a parabolic microphone uses a parabolic reflector to direct pressure waves at a single microphone located at or near its focus. That’s where I’m starting.

I picked up a 22″ parabolic dish from sdill471 on Ebay. He sells them for around 50USD and ships all over the place, including Hawaii (yay!), using USPS shipping.

The microphone for this project is the first EM-172 lavalier mic I built back when I started building external mics for my DR-05. It’s since been converted to XLR and received the full shielded treatment the rest of my EM-172 mics got when I did that conversion.

The rest of the project will be to make all the mechanical bits to place the EM-172 mic at the focus of the dish. I’m drawing a good bit of inspiration from WW Knapp’s Homemade Parabolic Mic page, though I’m making two big departures Knapp’s design: The first is to think more in terms of parts I can make in a machine shop rather than what I can find at the hardware store. (This departure is called “needlessly complicating a good, simple design”.) The other is to take a tip from Klas Strandberg at Telinga: You don’t always want the mic to be at the exact focus of the paraboloid. Having the ability to rack the microphone through focus gives you some much needed flexibility in the field to widen or narrow the pickup pattern of the mic, or even to tune which frequencies are focused on the mic by the dish.

I’ll post the design and build articles once I’ve finished the mic.

Mid-Side Microphone

The entire idea for the self-contained mid-side microphone comes from an Instructables article written by Jules Ryckebusch. Jules took a BM-800 mic – about ~20USD off of Ebay depending on the seller – gutted it, and replaced its innards with two Pimped Alice amplifier boards and three TSB-165 capsules. The really clever part is how he did it, but for any of that to make sense it helps to understand how mid-side microphones work.

The easiest way to understand mid-side recording is to read a really good article about it. What I wrote below won’t be nearly as good, so I urge you to follow that link. That being said, here’s my take on mid-side:

Back when recording was in its infancy no one even thought in terms of stereo recordings, quadrophonic, 5.1, 7.1, or any of the other immersive formats we’ve since come up with. Mid-side was one of the earliest stereo techniques, patented by Alan Blumlein in 1933.

Mid-side uses two microphones: one to pick up the center part of the sound field (the “mid” mic) and another to pick up the sound on either side (the “side” mic). In most cases the mid microphone is a cardioid, which preferentially picks up sound in front of the mic. In all cases the “side” mic is a figure-eight – a microphone that picks up sound in two opposite directions, but nowhere else.

To create what we consider a conventional Left-Right stereo image from a Mid-Side (M/S) recording requires a little math. The equations look like this:

Left = Mid + (+Side)

Right = Mid + (-Side)

In the equations the Mid channel is taken as-is. The Side channel is used twice: first it’s used as-is (+Side) and the second time it’s used inverted (-Side).

As wonky as that sounds, and as convoluted as the post-processing sounds, it offers some distinct advantages when mixing the tracks afterward. Want a wider stereo sound? Mix in a little more of the Side channel and a little less Mid. Want to focus the listener’s attention on the bird in front of the mic and down-play the forest full of frogs chirping in the background? Bump up the Mid and turn down the Side. Want to mix a mono track to go with an accompanying video on Youtube? Use only the Mid channel for clean mono without any phasing issues. The real strength of mid-side is the flexibility and versatility it offers after the fact.

The one catch with mid-side, as with all stereo techniques, is that it requires two distinct microphones. ORTF requires two cardioid mics and a bar to mount them on. A/B requires two widely spaced omnis. Even my SASS consists of two omni mics mounted in an admittedly rather large baffle. M/S is no different, requiring a cardioid and a figure-eight.

What makes M/S special is that you want the microphones to be as close to each other as you can get them. By its very design it’s inherently physically compact. (Side note: This is true of X/Y as well, which uses two cardioids pointing 90 degrees to each other, and of the Blumlein arrangement, which replaces the cardioids with figure-eights.)

Which leads us back to Jules’s M/S microphone, which takes “compact” to a new level by cramming multiple microphones into just one mic body. That makes for a light, portable recording kit that’s quick to set up and tear down; perfect for traveling, or for recording subjects that require substantial hiking to reach.

So why three capsules instead of two? Jules realized that if he took two of the TSB-165 cardioid capsules, faced them in opposite directions, and wired them 180 degrees out of phase with each other in series, they act like a single microphone with a figure-eight pickup pattern. Add a third TSB-165 capsule in the center and you have all the makings of a well matched mid-side microphone.

Where Things Stand Now

My parabolic reflector arrived last week. The mic for the parabolic project is already in-hand, though I may have to (yet again) cut it out of its housing and install it in a new one. I’m in the process of designing the mechanical bits, and should be able to start making them in the next couple of weeks.

I ordered the BM-800 donor mic for my mid-side mic just this morning. Jules posted a link to download the Pimped Alice PCB files that Homero Leal designed based off of Scott Helmke’s original Alice design. Once I have the board mounting hole pattern off of the BM-800 microphone, I’ll add those to Homero’s PCB layout and send the files off to OSH Park for fab.

Work on both of these is contingent on my getting a number of other gotta-do’s off my plate, but I hope to make some progress on both in the next couple of weeks.

Tom

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