Fri Dec 24 12:07:00 EST 2010

PWM expo

Next problem: how does a RC filtered pwm interact with an exponential
converter?  Should the output be filtered too?

The real question is: is there some form of modulation that will be
optimal in some form?

It seems to me that the most important property is not that the error
signal is small, but that it is free of audible patterns.

I already have 2 that are not so optimal:

   - PWM: pretty clear period ;) However this should work if the
     modulation frequency is well above 20kHz.

   - integrating SD: advantage: frequency is very high in the
     mid-range, which might be ok for audio as that's where we're most

What are the numbers?  I think the PIC can do 100ns PWM resolution.
That gives 500 slots in a 20kHz wave.  That's not enough resolution.
Probably need between 10 to 100 times more, so it looks like some
decorrelation is necessary.

About SD: how much of the dynamic range do you need to give up to not
run into degenerate cases with very small duty cycle, and long period?

Is it possible to temporally "rearrange" a binary wave form in a cheap
way, i.e. random transpositions.  See dithered PWM.