Subject: Re: [linux-audio-dev] Linux support for IEEE1394 mLAN?
From: Richard A. Smith (rsmith_AT_bitworks.com)
Date: Fri Sep 22 2000 - 18:19:40 EEST
On Fri, 22 Sep 2000 01:36:17 +0200, Benno Senoner wrote:
>> Regarding the float a/d, I originally thought of it only in terms of
>> potentially superior sound quality by making the input passive,
>> providing impedance matching only and eliminating the need for mic
>> preamps, and moving the amplification to the digital side of the s/h
>> where the signal is dc and not reactive. I later realized that if I
Well strain guage systems have been doing this kind of stuff for a while now. However, your
noise floor starts to really become a problem at signals of this level. Usually an on chip
PGA is used. Even if you are talking about line level signals which are large compared to
mic signals 24 bits give you +/- 60 nano Volts (10^-9) of precision. At that level you
begin to see thermal noise of the very wires you use to transfer the signal. God forbid you
get anything with a switching power supply anywhere near it. Even if you were to make a the
input impeadance of your a/d amp .1 ohms you are still talking only .5 micro Amps to
generate +/- 1 bit.
Whats the output level of professional mics now days?
The strain guage people talk about "digital" load cells where they do the A/D as close to
the strain guage as possible. I believe I even read about some company producing gagues
with the A/D on board.
I think that with out doing the A/D directly at the mic you would have a hard time getting a
system that didn't wash the last 4-5 bits of resolution with noise. Of course that noise
would be pretty white so with a little bit of math you might could digitally remove it.
-- Richard A. Smith Bitworks, Inc. rsmith_AT_bitworks.com 501.846.5777 Sr. Design Engineer http://www.bitworks.com
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