The View Up Here

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

Posts Tagged ‘ORTF’

(Yet More) Microphone Tests

Posted by Tom Benedict on 19/12/2016

Since writing my last post Homero Leal pointed out that I could mitigate some of the harshness of the Alice microphone (for field recording, mind you) by adding a capacitor across the 2.2k drain resistor. I didn’t have the size I needed (6.8nF), but I had everything to make a similar modification by adding an 8.2nF capacitor in series with a 750 ohm resistor, both across the 2.2k drain resistor. All of this is spelled out in Ricardo Lee’s ChinaMod+U87.doc file on the micbuilder forum.

Alice with ChinaMod U87

I walked out into the park behind my work, recorded for about five minutes, and headed back inside to modify the microphone. After adding the HF EQ mod I walked back out into the park and recorded again.

Prior to the mod my recording had a background hiss that sounded like microphone self-noise. I knew from testing the mic inside my car that it’s not, and is actually a sound from the environment. While testing the mic with a 22″ parabolic dish a couple of weeks ago I panned around to try to identify the source. I’m almost certain the hiss comes from the sounds of tree leaves rattling against each other in the wind. It only takes a breath of wind to make the leaves rattle, so the sound is almost always there. After the mod, that background hiss was reduced quite a bit. Enough so that I wanted to try it more rigorously out in the field.

Last night conditions were almost perfect. We had a storm system rolling in, the air was still, and the sky was overcast. Perfect conditions for people to stay home, get off the road, and let people like me lurk in the shadows with headphones on. I packed both my Alice microphones along with my SASS and Olson Wing, and headed out to an old cane haul road to record coqui frogs and insects. I was rained out in the end, but even that worked to my favor.

The tests!

Alice with HF EQ vs. Stock Alice

This is an A-B test between the Alice with Ricardo Lee’s HF EQ mod (thanks for the pointer, Homero!) and an unmodified Alice. The mics alternate every ten seconds, with a two-second cross-fade. That’s probably excessive on the cross-fade, but c’est la vie. Keep in mind there was very little wind during the test, so the difference is subtle. But it’s there.

SASS vs Olson Wing

While I was there I also tested the SASS against the Olson Wing. In this case both were populated with Primo EM-172 capsules. After I got home I realized I had wind protection on the SASS, but none on the Olson Wing. So this isn’t a fair test of frequency response, but it should be a fair test of the depth of stereo imaging, and to some degree, sound localization. (The frogs really don’t move around that much.)

I was content to let this setup run for a while, but it started to rain. Without any rain protection on either array, I knew the rain would eventually soak the mics. So I packed it all in and pulled out my rain gear.

Rain Gear

I’m still trying to get a good, clean recording of rain. A while back I took a tip from Gordon Hempton and built a microphone rain shelter. It’s a hard aluminum plate covered with two inches of non-woven air filter material. The aluminum plate keeps the mics dry, and the filter material diffuses the rain drops to a soft “fuff” sound. I also added a layer of carpet foam underneath to cut down on the residual “fuff” sound. It’s set up to take my DIY shock isolator, a small ball head, and my ORTF bar. (Sorry, no pictures of the whole setup just yet.) With the whole mess set up on a tripod or c-stand, it protects the mics from rain while minimizing the sound of the drops hitting the rig.

Finally finally I had a chance to use it in the field. And it worked! It worked great!

Only problem is that I managed to damage one of my EM-184 cardioids while testing the Alice mics. It barely responded at all, and produced a deep wumping noise in the recording instead. So the stereo recording is rubbish, unfortunately. I thought the wump sound was the mic picking up rain drops hitting the tripod legs, so I switched to a c-stand, re-arranged, tied up cables, did all sorts of things. None of it helped. After about half an hour I finally admitted to myself that the mic wasn’t working, and packed it all in.

But the rain gear worked! It worked great!

And once I dried the EM-184 mics out they worked great again, too. (Lesson learned:Don’t let it rain on your mics. DOH!)

All in all it was a good night of testing. I have one other test I’d like to do with the two Alice mics (ocean waves!), and I’d like to do one more side-by-side of the SASS and the Olson Wing to see if I can shorten the length of the Olson Wing and still get a good boundary effect out of it. But I’m pleased as punch with the rain gear.


P.S. I also learned that I need to finish this project before watching another season of Stranger Things. There’s something about driving way the hell out on some abandoned road to some spot in the woods in the middle of nowhere with fog and rain and nothing but the buzz of the insects and the calls of the frogs to… WHAT THE HELL WAS THAT?!

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SASS and ORTF Side-by-Side

Posted by Tom Benedict on 09/08/2016

Or top and bottom, rather.

I had the opportunity to stick my SASS rig and my newly minted ORTF bar on the same mount, one right over the other, and use them to record coqui frogs in a eucalyptus forest on the Big Island of Hawaii.

Of course whenever you record in a forest here you also get insects.

And if you happen to be within a hundred yards of a bunch of… dinosaurs? You also get them.

And the rain.

Ok, just a bunch of stuff. Anyway, here’s the recording. It’s an A-B test, switching between SASS and ORTF at thirty second intervals with a two second cross-fade.

My take: The two are different. (Well duh!) They provide different sounds. Neither one is “right” to my ear, just… different. But I’ll let you decide for yourself.


P.S. No I didn’t say which is which in the recording. What would be the fun of that?

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Clippy EM184 Cardioid Mics and ORTF

Posted by Tom Benedict on 05/08/2016

I’d planned to write an article describing my trip to Edinburgh for SPIE 2016, but I got side-tracked. That article is yet to come.

I did some audio recording while I was there, but not nearly as much as I’d have liked. I wound up packing all of my sound gear, including my SASS, but the few times I pulled it out it rained. The one time I thought I’d get to use it for sure – poking it out of my hotel room window to record traffic sounds – I found it was too big to fit through the window. I wound up using spaced omnis to record traffic sounds, but the SASS didn’t get used even once. I found myself wishing I had other options.

A number of common stereo techniques require the use of cardioid microphones. Up until my trip to Scotland I only had omni microphones in my bag. There are still some stereo techniques that use omnis that I haven’t tried, but I’ve been wanting to play with cardioid mics for some time. Step one was to buy or make some cardioids.

The same circuit I used to make my EM172 omni mics can be used with other FET-enabled Primo capsules, including the EM184 cardioid capsule. FEL Communications (micboosters) sells these on their site either as individual caps or as matched pairs. I picked up a matched pair along with a pair of Clippy mic bodies, clips, and windscreens. I still had some Mogami cable and Neutrik connectors on hand, so I just drew from that stock to build out the new mics.

The Clippy mic bodies work nicely with the cardioid capsules, and the resulting mics have very little pickup at the back. It’s not zero, though, so you do have to be aware of everything that’s not directly in front of the mic. I’d been warned that cardioids are more sensitive to wind than omnis, and these mics bear that out. They’re stupid sensitive to wind. Even with the foam windscreens and some furries I got from Cat Ears, the slightest bit of wind kills them. I need to figure out some other solution for wind protection.

Step two was to come up with a way to hold the mics so they record a clean, well separated stereo image. There are plenty of choices for this, but the one I chose was ORTF, a technique designed around 1960 by Office de Radiodiffusion Télévision Française (ORTF) at Radio France. (See? Astronomers aren’t the only ones to recycle their acronyms!)

ORTF requires the microphones to be separated by 170mm and angled away from each other at a 110 degree angle. It’s a bit of a pain to set up in the field without some way to gauge the angle, so many people favor other setups such as NOS (Nederlandse Omroep Stichting) in which the mics are separated by 300mm and are angled out by 90 degrees. I wanted to play with ORTF, though, so I decided to solve the setup problems with a fixture.

Clippy ORTF Bar

Since the Clippy mic bodies register nicely with their lapel clips, I used the clips to orient the mics both in location and rotation. The clips have a tab on top that’s just over 6.2mm wide. I made 6.5mm wide slots at either end of a bar to receive the clips.

Clippy ORTF Bar With Mics

I wanted to keep things simple so I didn’t have to fuss with stuff in the field, and this lets me do that. With the clips fully seated in the slots the mics are angled out at a 110 degree angle and are 170mm apart. It takes more time to unroll the cables than it does to install the mics on the fixture. And the flat bar packs down a lot smaller than my SASS.

Clippy ORTF Bar Slot Detail

The bar I used was just over 4mm thick. I cut the slots to leave 2mm of material for the mic to clip to. This wound up being a little thin, but it made for a nice, deep slot to register the clip in.

Clippy ORTF Bar Velcro

The bare metal of the bar was too slick for the clip to get any real grip, so I put a tab of Industrial Velcro on the bottom of the bar under each of the slots so the clips would have something to grab onto.

I’m pleased with how easy it is to use this setup, and it’s tough to beat how compact it is. But I’m not 100% satisfied with how it works in the field just yet. I already mentioned the wind issue. Even with double protection the mics saturate when almost any amount of wind touches them. They’ll probably fare better inside  a Rycote or a Rode blimp, but for now I’ll have to save them for wind-free environments.

The sound is also significantly different from that of my SASS. (Sorry, no side-by-side comparisons yet.) The SASS picks up more reverberation than the ORTF setup, so there’s more of a sense of the space with the SASS than with the ORTF. But you don’t always want that sense of space. During an earlier test I had one of my omnis and one of the cardioids in a car. The omni picked up so much of the car noise, it was difficult to hear the people in the car speaking. The recording from the cardioids was much cleaner.

Needless to say there’s still plenty of testing to be done. Once I learn the strengths and weaknesses of this setup and have a better handle on wind protection, I’m sure it’ll see plenty of use.


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