I've uploaded the fruits of today's labours to

http://home.clear.net.nz/pages/joecolquitt/mixer.html

Not finished or polished by any means, still testing, but it's
something to work from. A couple of questions -

As noted, the lowest PWM frequency with a 4MHz clock is
(I believe) 244Hz. The only way I can think of to slow this
down to, say, 0.5Hz is to reduce the clock to (0.5/244) x 4MHz
or 8196Hz. Now, this can be done manually with a pot or
preferably with digital selection, ie the PIC is told to alter
its own clock frequency, perhaps by switching resistors with
a 4016 or by using a pot IC. Assuming make-before-break
switching so that a resistor is always present, any problems ?

Any other ideas to reduce the frequency off-PIC and maintain
the duty cycle ? Perhaps even another PIC as a stretcher ?

The granularity or resolution of the frequency and duty cycle
becomes calculably coarser with increasing frequency. As
the program stands now it's possible (bearing in mind that
granularity) to set by pot the duty cycle at any frequency. At
the lower frequencies the full angular range of the DC pot
is usable, but of course the higher the frequency, the less
the pot needs to travel from the 0% to the 100% position. Is
a table going to be the best solution to scale the pot reading
so that at any frequency the full travel of the pot is usable
(although the % change per degreee will increase as the
frequency increases) ?

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