Multiple Condenser Mics in the same room (paralleled) - Page 1
Forum section: electronics / projects

author: kolbep
author_rank: Frequent Contributor
timestamp: December 18, 2017, 06:48:38 pm
content: Here's a new one for the Hive mind.In a conference room, I have a Electret Condenser Mic Capsule mounted in the ceiling above the main conference table.This Capsule is then hooked up to an OrangePi Zero, with Linux running Sox, to automatically record whenever there is 'voice'. It uses sox's VAD and Silence to automatically record.When people talk at the Conference desk, it works fine, and the recordings are clear.The problem is that there is a desk about 3m away from the main conference table, as it is a bit far from the microphone, conversations at the desk are very muffled, and hard to make out.I was thinking :1. A Seperate OrangePi Zero, Seperate Mic Above the Desk, basically a seperate system. Messy. , You also have 2 recordings of almost all the conversations (One from the far mic, one from the near mic)2. Put a USB Soundcard on the OrangePi Zero, Seperate Mic Above the desk, and run a second Sox instance to record from that Input (Again you have 2 recordings of the conversations, and also I think it will hammer the Pi's Processor (I do quite a bit of Companding, etc, before saving)3. Parallel the Condenser Microphones. Put a Mic above the desk, and seprate Biasing for each of the Capsules, then sum them together using 10uF Capacitors. That way the closer the person is to a specific microphone, the more of their level will be from that microphone, and the further microphone, although sound not clear, will have a lower level because of the distance from the person.I would prefer no 3, Do you think that will work, it would save money and make it a lot easier. I will be relying on the Proximity to the microphones, instead of an Automixer or Expander.There might also be Phasing issues, but due to the distances from Mic to Person, and from Mic to Mic, this should be minimal..

author: Audioguru
author_rank: Super Contributor
timestamp: December 19, 2017, 01:21:47 am
content: When you parallel mics the unused ones pickuplotsof background noise and probably lots of acoustical feedback howling and squealing. The differing wavelengths and distances cause cancellation of some important frequencies and other frequencies are boosted.I worked for a conferencing company. Good systems use lots of mics, a mic on the table in front of each person but the audio mixer turns one on only when it is spoken to. They also use an echo canceller IC to prevent acoustical feedback howling and squealing. The echo canceller is programmed for each conference with a model of the acoustics of the conference room which changes with the number of people in there.

author: raptor1956
author_rank: Frequent Contributor
timestamp: December 19, 2017, 02:52:13 am
content: It's a fact that audio is MUCH harder than video, particularly outdoor location audio. I mean audio is a good 10X harder than video much of the time.Brian

author: Delta
author_rank: Super Contributor
timestamp: December 19, 2017, 07:00:53 am
content: I would say that option three will probably be fine. K.I.S.S.Try it. If it not deemed good enough, you haven't lost anything, then try another option.

author: DaJMasta
author_rank: Super Contributor
timestamp: December 19, 2017, 08:11:33 am
content: The trouble with the potential phasing issues is also potential destructive interference being mixed in with your capacitor mixer - btw, if you can determine the input impedance of the mic on the orangepi, you may not need that large a cap.I think the best sounding solution would be separate mics on separate lines, whether you digitize both or have a little analog circuit that chooses the one with more signal or whatever. Depending on usage cases, though, you may be able to get something better that can use a single mic. I would carefully consider the pickup pattern of your mic and the housing you're putting it in - it is not hard for the grill or baffle for the mic to cause real pickup issues from certain directions or at a distance. It's also worth mentioning that while a full omni mic will cover sound from anywhere, it will also get noise from anywhere - if you can be clever with your positioning as to only need half of that range, or less, you can pick a mic and orientation with a response pattern that effectively blocks out much of the external noise.As mentioned, the big name, big dollar conference room audio systems are going to use a lot of mics and some DSP that monitors the channels and changes to the ones with the "best" signal for what they're looking for. Just a good mic will go a long way, but fairly close, directional micing is almost always the best choice for low noise, clear sound. Up close means the best signal to noise ratio and going directional means you can ignore a huge amount of background noise without DSP or other algorithms, of course, if you're going in close with directional mics, you're going to need a lot of them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2017, 08:18:34 am
content: How are you taxing that poor little thing so hard that it can't handle a little audio DSP?I mean, a 386 PC could do that back in 1990, assuming of course at least a modest level of optimization (straightforward code written in C, or preferably, using the sound card's buffering, mixing and DMA features).Like... have you measured actual CPU consumption? Are you doing this with massive bloated Python libraries or something?If things are, in fact, all nice and happy and lightweight, then it should be no problem to stream in multiple mic inputs and do whatever you like in DSP (diversity, delay compensation, mixing..).Tim

author: jonovid
author_rank: Super Contributor
timestamp: December 19, 2017, 09:46:31 am
content: directional mic is almost always the best choice for low noise, clear sound ,truean aluminum tube cut slots in to it with a hacksaw. see image.

author: StillTrying
author_rank: Super Contributor
timestamp: December 19, 2017, 02:41:26 pm
content: "The problem is that there is a desk about 3m away from the main conference table, as it is a bit far from the microphone, conversations at the desk are very muffled, and hard to make out."Move the desk much closer to the conference table.

author: Kalvin
author_rank: Super Contributor
timestamp: December 19, 2017, 05:24:00 pm
content: If you can connect a USB audio dongle to the Raspi Zero, just record in stereo. After that, you can make some fancy stuff to remove any phasing issues etc. if needed. If the Raspi Zero cannot do things in real-time, just stream the raw audio data to a faster computer.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 19, 2017, 05:46:39 pm
content: Taking two distant-positioned mics (vs. mics in front of the subjects' face) which are widely separated (several m) and doing a simple sum is quite likely to have phase, echo, and reflection problems. But only you can try your own situation and make your own judgement call whether the resulting quality is adequate for your purposes. For example "surveillance-grade" or "transcription-grade" audio may be adequate vs. "documentary-grade" or full "production-grade" audio quality depending on what you are doing with the recordings.Hard-wiring the two microphones together with stiff (10uF) capacitors may not be as good as using a "summing" circuit with at least a couple of series resistors (maybe 5K?).Recording in "stereo" would seem much more suitable. Where each mic has its own channel, but the channels are intrinsically and automatically synced together and bundled together.As @DaJMasta mentioned, the commercial conferencing systems (and even Amazon's $30 Alexa hockey-puck) use extensive DSP to extract intelligible speech out of the acoustic mess that most spaces are.