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Muxing 4 mic channels vs mixing 4 mic channels (6 replies)

Nicholas Evans
6 months ago
Nicholas Evans 6 months ago

My questions raises the issue of a replacing a very expensive Murideo Mux Pro $1495 (which has about $50 worth of parts) which allows for connection of 4 microphones and the ability to rotate through the mics automatically and  pass samples ultimately onto AudioTools software. Why are we not able to replace this with an ordinary $50 Mic mixer that allows us to adjust the 4 channel mic mixing within that piece of hardware, versus the mux which samples each microphone, pass each piece onto the Audiotools software where the 4 samples are mixed at that point. What am I missing here ... $50 box versus $1495 box????  It should be noted that there are hundreds of mixers on the market vs only one Mux device on the market.

Thanks

jamin
6 months ago
jamin 6 months ago

The reason you can't swap a mux for mixer is phase. Typically spatial averaging utilizes the magnitude of the measurements and ignores the phase aspect. 

In one sense it is no different than direct and reflected sound combining to create a phase based anomaly like SBIR. Likewise sound arriving at multiple microphones that are simply mixed can result in cancellations and peaks.

Another view is that the low frequency room modes have um, polarity, associated with them. This is the reason that placement based modal cancellation works. That same polarity mechanism can create measurement nulls if the mic signals are merely summed making you think there is a null/peak  when there is not. 

PS - if a weird avatar or name shows up with this post my apology--I'm working to fix it !

Acoustic Mafia -- hear no evil

jamin
6 months ago
jamin 6 months ago

Hah, I left out a paragraph. 

When an input signal is digitized and processed in the Studio Six framework it is dealt with in a magnitude only fashion. The acquisition averaging function then takes care of those different signals fed sequentially thru the mux from the different microphones. 

 

Ok, better now. 

Acoustic Mafia -- hear no evil

Nicholas Evans
6 months ago
Nicholas Evans 6 months ago

Thanks for the response, much appreciated. However I believe the situation is a little bit more complicated than meets the eye. On further research, the mux box does nothing more than generate multiple analog samples from various mics to the IAudiointerface2 box where the 'Transfer Function' is applied. This Transfer Function in part does the averaging of the incoming samples. The Doco states that the Iaudiointerface2 converts the analog signal into a digital stream, looks at the amplitude of the incoming stream, and applies a number of functions to that stream (averaging, phasing, etc). So in essence the Iaudointerface2 simply does the averaging in the digital domain before passing it onto Audiotools for recording. What the doco does not state is how the averaging is carried out. The Iaudiointerface2 can handle upto 16 samples. By replacing the mux box with a cheaper analog mixer (say up to 4 mics), the averaging is done in the mixer unit in the analog domain, then passes it onto the Iaudiointerface2 where the  'Transfer Function' is applied. All normal functions will be applied, however averaging part will in effect be simply be null and voided as it already in essence be carried out. So again I ask the question, why do we need the mux box. What am I missing here!!!

Gerry Lemay
6 months ago
Gerry Lemay 6 months ago

Hi Nick,

The added wrinkle is the concept of mixing versus averaging.  Mixing dB levels is mathematically different than averaging sound pressure levels which is our true aim.  The signals fed to AudioTools are digitally averaged based upon their calculated pressure levels.   A mixer is basically summing not averaging dB levels. 

The signal level of the measurement, in effect, represents the logarithmic dB level from which the sound pressure can be calculated within AudioTools.  The pressure values can be properly averaged because they are linear and not logarithmic quantities.   That's not to say that some bias can't be applied to normalize the results, it's just something that needs to be accounted for in the process.   

 

 

Gerry

Nicholas Evans
6 months ago
Nicholas Evans 6 months ago

Gerry,

  Thanks for the response, appreciated. I would love to see the maths behind how the averaging is done within the Iaudiointerface2 box. No doubt that would never be released from the studiosixdigital people. I remain a little skeptical !!!! As a follow on from what I previous stated, by definition a one microphone input into a mux box vs into a mixer box would identically be the same as all mixing (there is none) is carried out within the audiointerface2 unit. It is only when  we add the additional mics where the issue raises it head.

Gerry Lemay
6 months ago
Gerry Lemay 6 months ago

Yes, the issue is how the mic's are combined; summing versus averaging; a single mic would not be an issue.  There remains an error in terms of averaging dB levels.  An analogy is averaging Power versus Voltage.  For example the spatial averaging function in AVPro ATLink for FFT converts sound levels to pressure levels to properly average the signals.  My guess is that AudioTools completes a similar conversion in its averaging.

Gerry

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