AVR-Chat

I'm working on a project with similar constraints (even
uses 2200Hz as one of the tones). There are not very many
ways to improve things. 

I am using an 6-bt R-2R DAC driven by six port pins. Since
the DAC output only has small steps in it, the harmonic
content is much lower and the rate and filtering are much
less demanding.

You can add a smoothing filter with more sections (poles).
I am actually using a 6-pole switched-capacitor filter that
is "tuned" according to an external clock (also provided by
the micro). A good filter like this can do wonders. With
such a small number of samples (points) per cycle, a simple
2-pole active filter just won't hack it.

You can decrease the TOP value (that is, reduce the number
of bits) so that the PWM period is shorter and you can get
more of them in per tone cycle. With such few samples, it
makes little sense to try for a full 8-bits. Every halving
of TOP (from the standard value of 255) drops one
resolution bit and doubles the repetion frequency of the
PWM, doubling the number of samples per tone cycle.

Use the fastest possible main clock (that is, highest
frequency crystal). This will give you the fastest possible
PWM clock and that will give the highest possible PWM
repetition rate. 

Not many other choices than these! That is why I chose not
to use PWM for this kind of application.

Jim Wagner

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