[sdiy] A/D --> D/A conversion and audio

[phillip m gallo]

Bret,

Lot's of reading on the Web, Google "sampled audio microcontroller ADC DAC"

regards,
p

-----Original Message-----
From: Bret Truchan [mailto:clone45 at mailblocks.com]
Sent: Saturday, January 03, 2004 6:41 PM
To: philgallo at attglobal.net
Cc: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] A/D --> D/A conversion and audio



Thanks!  That will keep me busy for a week or so! :-)

  I can't say that I understand how to do half the things that you 
suggest, but maybe I should hit the public library and net to see if I 
can brush up on those topics.


-----Original Message-----
From: phillip m gallo <philgallo at attglobal.net>
To: 'Bret Truchan' <clone45 at mailblocks.com>
Cc: synth diy <synth-diy at dropmix.xs4all.nl>
Sent: Sat, 3 Jan 2004 18:30:03 -0800
Subject: RE: [sdiy] A/D --> D/A conversion and audio

Bret,

Address the quality of input first.

At least capacitively couple into a resistive divider so that zero input
equals 1/2 of the converters range.  You'll find audio get snappy real
quick if you center the zero cross.

Next i would adjust the audio input level to use every bit of the 10 bit
input conversion range you have.

Look like your in a "dead loop" inputting and outputting.   Is 40kHz the
resulting rate of this loop or are you padding it to achieve ~ 40kHz?
If you are padding perhaps you have time to average 4 input samples to
provide 12 bit output data.

Also run the ATMEL at the highest possible speed.

Regarding numeric formats i was referring to the data format i.e. the
BB-DAC your using is straight binary so 800H  is mid-scale so your
audio "zero cross" will be at 1/2 VCC  so your going to want to
capacitvely couple the output.

Once all this is up and running and if this is not just to play around
with there are filters, and potentially, companders left to talk about.

regards,
p



-----Original Message-----
From: Bret Truchan [mailto:clone45 at mailblocks.com]
Sent: Saturday, January 03, 2004 5:40 PM
To: philgallo at attglobal.net
Subject: RE: [sdiy] A/D --> D/A conversion and audio



Ohh... lots of info!!!  Happy! Happy!

I'll inline my responses....

> 1) Are you driving the 12 bit output DAC at the same sample rate of
the
> input ADC?

Yes, I think so.  My program basically says:

a) do a/d conversion
b) clock the "load DAC registers" pin of the D/A converter
c) goto a

Is that what you mean?

> 2) As there is no input circuitry, how the does the input relate with
> the spec's for the ADC? For instance is the A/D "bi-polar" accepting
+/-
> inputs or uni-polar ? If uni-polar you are going to have to bias the
> audio to 1/2 of the ADC acceptance range .

The way I have it set up, it's unipolar.  Harry Bissell also
recommended biasing the audio signal, and hopefully he'll guide me
through the process.  I would love to see a schematic of how that
biasing is done.

> The signal swing of the Audio? Is it scaled to use the maximum bit
> range of the A/D Converter ?

No.  Is this different than just changing the audio's volume?

> 4) The numeric formats of the ADC and DAC.  Are they compatible?

Do you mean how many bits they use?  If so, then no, my A/D converter
is 10 bit and my D/A converter is 12 bit.  I'm tying the two most
significant bits of the D/A converter to 0.  Should I instead by
rounding the LSB's as you've suggested?

Many thanks.  This is my first time playing with this stuff!  I'm
excited.  This can certainly lead to huge fun.
- Bret


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