[sdiy] super cheap synthisizers

[Andrew Martens]

This is so bizarre - I was just thinking about a non-hardware-intensive
synth today at work.  Specifically, I think I figured out how to implement a
microbass synth-in-a-PIC.  Specifically, the 16F877 @ 20 MHz (my favourite,
and I happen to have a couple on hand).  My design calls for one OSC (but
with two octaves above mixed into the waveforms) that can do
sine/saw/tri/square, two LFOs, a 24-dB LPF, and a basic DCA.

I did some calculations, and I think I can fit it into the cycles available.
I need to do some more interrupt calculations to make sure I can accurately
reproduce the right note frequencies (re: tuning - my initial design had
horrible horrible errors that left two sequential semitones outputting the
same freq), but I believe I've addressed most of the timing issues.  The
output will be _very_ lo-fi, between about 4-8 kHz (depends on the note,
hard to explain), but it should be functional.  Oh yeah, and I think I have
enough spare cycles for MIDI input (and it's got 8 ADCs, the USART for MIDI,
etc. etc.).  Hell, there's I2C/SPI available, so you could even hook up a
serial EEPROM and save/load patches in there :-)

The 24-dB IIR filter code is grabbed from piclist.com, and it claims that it
only takes a maximum of 282 (iirc) cycles per sample.

If anyone is interested in this, I could get off my butt and actually spend
some time working on it over the next few days / weekend / whenever I have
time.

I'm still wondering how to connect from my 8-bit parallel TTL out to a
proper line level one (DAC080x? anyone have a better idea?), but I know
there are a bunch of schematics floating around there.

Cheers,
Andrew Martens