Ken Pergola wrote:
> Well, I dug up the details. Sorry, using PIC PWM module and 19.6608
> MHz crystal produced only approximations to the musical scale --
> would have drove people with perfect pitch crazy.
>
>
> The two closest frequencies to 440 Hz were:
>         441.38
>         438.86

I don't know where you got that from.  It seems that a fast enough crystal
gives enough resolution so that you can hit any frequency in the 440Hz range
with sufficient accuracy.

With a 20MHz crystal you get a 5MHz instruction rate.  This is essentially
what you get to divide down if using a PIC.  5MHz / 440Hz = 11,363.64
approximately.  This means you could hit

  5MHz / 11,363 = 440.02Hz
  5MHz / 11,364 = 439.99Hz

The last one is off by 32ppm, or less than .0006 of a semi-tone.  Even the
first one is only off by 56ppm or less than .001 semi-tones.  At this rate
the crystal accuracy becomes a significant portion of the overall error.

Isn't that well more than good enough?


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