Perusing, I've found one DTMF generation program for the PIC,
by Stephen Hardy of Canberra.  While this uses an interesting and
heretofore unknown to me method of sine wave generation, it
expects a DAC (typically 8 bits) to generate the summed sinewaves.

I notice that the Basic Stamp 2 has a BASIC command (DTMFOUT) to
generate DTMF via a single digital output (aka 1 bit DAC), which
it says can be turned into proper waveforms with filtering.  (It
also has a command to output one or two arbitrary tones).

My question is: How is this done?

I can see that a single tone can be done via a square wave (and
attenuating the 3, 5'th etc harmonics would approximate a sine wave).
But directly summing two square waves of equal amplitude and
different frequency would take a 3 level output.  I don't see how to
reduce it to 2 levels and keep the desired dual tones even
approximately.

My goal would be to transmit DTMF audio (via low powered radio) from a
PIC (not Stamp) monitoring a pumphouse, to be decoded with a standard
DTMF receiver chip (and a radio receiver) a few hundred feet away.  It
would be nice if I could avoid using a DTMF encoder chip (or 8 bit
DAC) at the pumphouse end.
   Zhahai

@ Zhahai Stewart       zhahai@hisys.com
@ A Meme Gardener      http://rainbow.rmii.com/~hisys/zhahai.html
@ Standard Disclaimer  YMMV - Your Maya May Vary