The View Up Here

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

Archive for August, 2016

Microphone Self-Noise vs. Recorder Equivalent Input Noise

Posted by Tom Benedict on 25/08/2016

Yet another attempt at combining math and sound recording… Ye have been warned!

A number of threads on a number of field recording forums revolve around a simple question: I have X amount of money. Where do I throw it to improve my recording?

An obvious and common answer is, “upgrade your pre-amps!” This can be done a couple of different ways: The first is to trade out your recorder for one with better pre-amps. The second is to send your recorder to a shop to have the pre-amps changed out for better ones. The third is to buy an external pre-amp like the Sound Devices MicPre or MicPre-D, and plug it into the Line-In jack on your existing recorder.

But is that always the right approach?

A bunch of head-scratching, web-searching, and number-crunching led me to the conclusion that it’s not as obvious as it might appear. A number of factors come into play: noise level, sound quality, build quality, ergonomics and convenience, useful features of the gear, battery life, etc. Of these, the easiest to tackle from a quantitative standpoint is noise, so that’s where I’m starting.

Most of the calculations I’m doing are spelled out in an article on the RANE web site titled, Selecting Mic Preamps. The first set of calculations help you determine the maximum pressure levels a particular mic/pre-amp combination can handle. Since the field recording I’m doing involves quiet sources I skipped that bit and went to the second set of calculations. These help you determine the level of self-noise a given combination of mic and pre-amp will have.  To run the calculations you need information about the mics as well as the pre-amps.

(If you’re recording loud sources that first set of calculations may be of use to you! You don’t have to skip them just because I did.)

Right now all of the mics I own are based off of Primo capsules: BT-EM172, BT-EM158, and BT-EM184. The data I used for the mics all comes entirely from the Primo datasheets.

I currently own two recorders: a Tascam DR-05 and a Tascam DR-70D. In the spirit of this question I’m looking at two competing solutions: one is to buy a new recorder, a Tascam DR-680 MkII, and the other is to buy a used Sound Devices MicPre to use as an external pre-amp. The data I used for the recorders comes from a mix of sources, the most important being the Avisoft Bioacoustics Microphone Input Noise Comparison website. The rest came from the manufacturer’s datasheets.

The RANE calculations require the self-noise and sensitivity of the mics in question. From these you can use Table 3 in their article to calculate the mic output noise. For all of these I’m using A-weighted noise values for the mics and recorders. A-weighted noise levels are scaled for the auditory response of a normal human. They tend to be about 5dB more optimistic than their non-weighted counterparts. So long as I stick to A-weighted for both, I’m comparing apples to apples. The numbers for my mics and for the DPA 4060 omni by way of comparison are:

  • DPA 4060
    • Self Noise 23dBA
    • Sensitivity -34dB
    • Mic Output Noise -103dBu A-weighted
  • EM172
    • Self Noise 14dBA
    • Sensitivity -28dB
    • Mic Output Noise -106dBu A-weighted
  • EM158
    • Self Noise 20dBA
    • Sensitivity -32dB
    • Mic Output Noise -106dBu A-weighted
  • EM184 Cardioid
    • Self Noise 22dBA
    • Sensitivity -39dB
    • Mic Output Noise -110dBu A-weighted

The RANE article says that when you compare the output noise of the mic to the equivalent input noise of the pre-amp, you really want to see a factor of -10dB lower noise in the pre-amp or better. A -10dB lower noise in the pre-amp means it’s only contributing 0.4dB of noise to the final signal. Looking at the recorders I’m using, along with the two I’m considering, their EIN levels are:

  • Tascam DR-05 EIN -109dB A-weighted
  • Tascam DR-70D EIN -120dB A-weighted
  • Tascam DR-680 MkII EIN -127dB A-weighted
  • Sound Devices MixPre -126dB A-weighted

Here’s how I’m reading this:

If I plug any of these mics into my DR-05, the noise from the recorder’s pre-amps will be the limiting factor. Getting a better mic won’t improve my sound with that recorder.

My DR-70D is -12dB lower noise than the EM172 that my go-to mics are built around. In this case the mic’s own self-noise is the limiting factor. Switching to a DPA 4060 won’t help from the standpoint of noise, either. (I’m not mentioning any improvements in the character of the sound, mind you.) This does imply that I’m coming up on the limits of my pre-amps with the EM184 cardioid mics.

Switching to either a DR-680 MkII or a MixPre certainly wouldn’t hurt, and the higher quality amplifiers on either device may improve the sound in other ways, but it probably wouldn’t help the noise much overall because the mics would still be the limiting factor. At most I could improve my noise levels by a tenth of a dB.

Conclusion:

Unfortunately what this means is that to make any substantial improvement in the noise level of my recordings, I need to upgrade both my recorder and my microphones. Upgrading either one without the other really won’t buy me that much.

The Real Conclusion:

This leads to the next obvious question: Have I reached a point from which the only way to improve my gear is to throw orders of magnitude more money at it than I already have? (Or to word that only slightly differently, more money than I have at all?)

In short, is this it?

(Or is this the excuse I need to stop improving the gear I’m using and start building parabolic mics?)

Tom

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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.

Tom

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.

Tom

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