The View Up Here

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

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.

Tom

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4 Responses to “Clippy EM184 Cardioid Mics and ORTF”

  1. Adrian Hicks said

    Hi Tom,

    Great write ups on here. I’m about to build a pair of EM184’s from FEL myself. Did you only machine one bar up, or did you make a few? I’d be interested in one for ORTF recordings, if you’d sell one?
    As a side, unlike the 172’s the rear of the 184 needs to be open to air, how did you achieve this given the copper shielding? I’m planning on using a strip of Kapton on the mic body then just enough copper tape to cover the capsule, leaving the rear open. Interested to see how you worked around blocking off the rear ports?
    Great blog btw!

    • Tom Benedict said

      I’m not entirely happy with how I solved it, but given another project I just worked on I think it’s the right way to go:

      The Clippy mic body is aluminum. It’s vented at the back so it can work with cardioid capsules, and it has a grill in front so it should be able to provide a complete Faraday cage around the mic. The anodization should keep the mic body from shorting out against the capsule, so the insulating layers shouldn’t be necessary.

      All of which would be great, except the anodizing on the Clippy bodies doesn’t have any gaps, so there’s no path for the cable shield to the mic body, and no path from the mic body to the grill. So right now mine are essentially unshielded. I need to go back and strip away some of the anodizing to make it work correctly.

      As for the ORTF bar, I only made one. Then I went back and modified it to add a pattern of slots for doing XY, so it’s a little weirder looking now. I did mine with hand cranks, which was a real PITA, and not something I want to do again. But let me take a look at it and see what it would take to make some on a CNC. If I don’t get back with you in the next couple of days, drop me a line to remind me.

      • Adrian Hicks said

        Hi Tom,
        I think this discussion is concurrent on micbuilders.
        For CNC, I’m a Catia CAD designer for McLaren, So if you need a 3D model in STEP etc, No worries, I can sort that in a moment. (I just don’t have access to machining :/ )

      • Tom Benedict said

        I’m newly converted to Solidworks, but moving files between Catia and Solidworks is no problem.

        McLaren! Ok, you’re worlds beyond me in mechanical design. I have the luxury of working under design rules like, “If in doubt build it stout.”

        I’ve got a 3D file of the bar, but I designed it around conventional machining. It’s pretty straightforward to convert to CNC, but I need to sit down and design the fixture. I’d probably add two holes outboard of the center for fixturing, then cut the whole thing in one setup. I’d also probably go a little thinner than what I have.

        That’s a shame you don’t have access to a shop. I think that would kill me. I know some places like San Francisco have community shops that you can rent time on a machine. Do you know if anything like that exists where you are? (It doesn’t here, but I’ve got access to other tools.)

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