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Sat Jan 1 15:44:35 EST 2011

Synth: the real problem

After doing some experiments with analog circuits, I am now convinced
that the only real problem is the D/A conversion for the control
signals.

For A/D (controller feedback measurements) there is the PIC's A/D
convertor, and digital frequency measurement (timer, counter).  I
suppose these are going to be good enough, as they are mostly needed
for low time-scale measurements, i.e. oscillator stability.

The question is: will D/A errors be audible?

My huch is that: yes, they will be audible, but we want them to sound
random.  I.e. random pitch fluctuations are not so problematic, and
might even sound good.

The next thing to do might then be to perform some analytical tests.
Given 1-pole filtering, what does the noise sound like? 

Roadmap: 

    - Build a real modulator (i.e. dithered S/D) and _listen_ to the
      noise ripple, i.e. when generating a ramp signal:
        * does the noise sound white?
        * does the noise change when the D/A steps up?

    - Use this to modulate an oscillator, i.e. an exp-converter -> 555
      sawtooth generator.

    - Build an analytical model and perform some randomness tests.




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