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From: brianw@microsoft.UUCP (Brian Willoughby)
Newsgroups: comp.dsp
Subject: Re: Digital Mixer Using Burr-Brown 200 Khz ADC
Keywords: S/H amp stages
Message-ID: <7902@microsoft.UUCP>
Date: 29 Sep 89 23:11:14 GMT
References: <9231@pyr.gatech.EDU> <4513@ncar.ucar.edu>
Reply-To: brianw@microsoft.UUCP (Brian Willoughby)
Organization: Microsoft Corp., Redmond WA
Lines: 69

In article <4513@ncar.ucar.edu> schabtac@porter.UCAR.EDU (Adam Schabtach) writes:
>In article <9231@pyr.gatech.EDU> byron@pyr.gatech.edu.gatech.edu (Byron A Jeff) writes:
>
>As far as using the ADC for 8 channels, from skimming the aforementioned
>article, it seemed to me the chip was best suited for just doing two
>channels.  It could be set up with a multiplexer, but since it's pretty
>cheap (under $30 in lots of $1000, so probably under $50 to you and me), 
>you would probably save yourself time and frustration (and hence money) by 
>using several chips instead of a multiplexer.

Actually, since you would want a S/H stage for analog sampling of each
input (which provides a steady voltage to the ADC), and the multiplexer
could double as a S/H stage, the single ADC looks pretty attractive.  A
Sample/Hold amp is typically a capacitor, some method of charging the
capacitor, and some method of reading the stored voltage without draining
the cap too quickly.  CMOS switches are usually used to diconnect the
charging source (the audio input) from the cap during the Hold setting.
An op-amp voltage follower works well to read the voltage since it draws
very little current from the cap (assuming that the voltage is constant).
I think that a CMOS 8-to-1 multiplexer between the 8 S/H stages would
make an inexpensive circuit.  However, I'm not sure that at the moment
the multiplexer connects the selected S/H stage to the voltage follower,
there might be too much current drain as the voltage follower struggles
to match the input voltage.  If that is the case, then 8 individual
voltage followers would be needed, still far cheaper than 8 ADC chips.
The addressing logic sent to the multiplexer to select the current input
channel could also be set to trigger the Sample/Hold control signals for
the S/H stages.

 CMOS sw.         |\  Op Amp Voltage Follower
--     ----+---X--| \
  |   |    |      |  >-+--
  -----  -----  +-| /  |
    |    -----  | |/   |
    | Cap  |    +------+
    |    -----
   S/H    ---
           -
>>With the output being either high quality audio or DAT compatable digital.
>>The DSP would handle all mixing, EQ, effects, etc.
>>
>You'd probably end up using several DSPs.  It demands most of the processing
>power of one current DSP chip (say the Motorola 56000) to implement a
>fairly complex reverb, OR a graphic EQ, so if you wanted EQ, effects, etc.
>for each channel, I think you'd end up using at least one DSP per channel.

That sounds like a harsh rating of the 56000.  My A.R.T. MultiVerb unit
uses two Z-80 processors to achieve simultaneous low-pass filtering,
pitch-shift and delay OR low-pass, reverb, chorus, multi-tap delay, etc.
This is with the help of a lot of custom TTL logic, but I don't think
that the 56000 would be so helpless doing reverb.  In some instances you
might be able to do some of the processing (i.e. mixing or digital
filtering) before sending the data to the 56000, or on it's output data.
I doubt you'll get independant effects on each channel, though.

>>Any comments?
>>
>There they are.  I don't mean to be discouraging, but it sounds like a pretty
>big project (albeit very interesting -- I wish I had the time to tackle 
>something like it).

I agree, if you could build such a thing for those prices, Yamaha or
someone would probably do it.

Brian Willoughby
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