[sdiy] Another protecting inputs question...

Seb Francis seb at burnit.co.uk
Thu Nov 20 03:24:11 CET 2008

Eric Brombaugh wrote:
> Seb Francis wrote:
>> This is the audio input buffer for the CODEC:
>> http://burnit.co.uk/sdiy/stuff/audio-buffer.png
> Mind if we back up a step and ask a more fundamental question?
> This isn't AH, so why use an analog VCA for feedback? Why not do the 
> feedback internal to the dsPIC and implement gain control digitally?
> OK - you can argue that analog gain control is going to have more 
> resolution than a gain CV quantized to 12 bits via the dsPIC ADC, but 
> is that _really_ a driver here? Is it worth the extra expense & board 
> space? Can you hear the difference?

It's nothing to do with being an audio-purist ;)

Since this is intended to be part of a modular synth, it's gonna be 
pretty nice to be able to patch things into the feedback loop.  No way 
to do this if the feedback is digital.

The biggest problem of digital delays IMO is how identical and clean 
every repeat is.  Sure, you can implement some filtering in DSP, but 
giving access to the feedback loop adds more possibilities (+ things 
like ping-pong delays .. which I know you can also implement in DSP, but 
hey I still like plugging in wires :)

Of course if someone doesn't want the expense of the THAT2162 
(admittedly they are about £5 each in small quantities), then the part 
can just be omitted and feedback done in software, or indeed done with a 
simple pot like it is in the 4xD.


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