Microphones in parallel to reduce noise
in [Design]
|
From: Eeyore on 16 Dec 2009 21:14 Phil Allison wrote: > <osr(a)uakron.edu> >> My boss laid a unit on my desk today. Its got ten electret mics fed >> into one low noise opamp. Any ideas how they did this without a ton >> of noise. All I can see from the board layout is it is NOT the classic >> op amp summer. >> >> Any suggestions for papers on low noise signal combining? > > ** Likely all the mics are simply wired in series and supplied with power > from a DC current source and the signal taken from the top most mic. The s/n > advantage comes from *correlation*. > > The 10 noise signals ( from the internal JFET pre-amps) are un-correlated > while the sound arriving at each co-sited mic will be heavily correlated. > So the output signal will be 10 times that of one mic, while the self noise > will be only 3.16 times - so a 10 dB improvement is possible. > > I suspect the mics are arranged to create a directional pick up pattern too. Very likely. > Dynamic mics have much less self noise than typical electrets - cos there is > no internal pre-amp. Means it is easy to get a 10dB or more improvement, > just by using one instead. But dynamic capsules cost more ! Graham
From: Eeyore on 16 Dec 2009 21:17 Jim Thompson wrote: > Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: >> osr(a)uakron.edu wrote: >> >>> My boss laid a unit on my desk today. Its got ten electret mics fed >>> into one low noise opamp. Any ideas how they did this without a ton >>> of noise. All I can see from the board layout is it is NOT the classic >>> op amp summer. >> Electret mikes usually have built-in FET stage. Are you sure they are >> combined in parallel, not in series? Quite so. > > They are available WITHOUT the FET. I have an RFQ on my desk > requesting a complex chip that will use Electrets sans FET's. > > ...Jim Thompson How many Gigaohms input impedance will the complex chip have ? Graham
From: Eeyore on 16 Dec 2009 21:23 osr(a)uakron.edu wrote: > AMBIENT ** noise pickup, not self noise !! >> In any case, YOU are 100% WRONG in your assumption that there ought to >> be a noise problem when combining the mics signals. Wired either in series >> or parallel, correlation still works to improve the s/n ratio. > > Mixer front ends still add thermal noise Uh ? What's a 'mixer front end' ? Obviously you're not an audio professional and haven't a clue about current practice. You can make a mix stage today using inexpensive off the shelf parts that has 100dB or more dynamic range quite easily. So what's the mix stage chip ? Graham
From: Eeyore on 16 Dec 2009 21:25 krw wrote: > "Phil Allison" <phil_a(a)tpg.com.au> wrote: >> "krw" > >>> Some "dynamics" are really electrets in drag. >> >> ** Totally stupid bullshit. >> > In fact, you are. You and DimBulb; a perfect pair. Provide a link to one of these 'electret dynamics' or SHUT UP ! Graham
From: Phil Allison on 16 Dec 2009 21:39
"Eeysore is Baaaaakk" > > But dynamic capsules cost more ! ** But not 10 times more - you wally. ..... Phil |