Re: Software DTMF /tone detection

> I would simply generate the I and Q waveforms for the eight (4+4)
> tones, multiply with the input waveform, square the result for each
> frequency, add them together (to get the magnitude) and perform some
> comparisons.

You mean that version ( sorry text is in german, but
pictures should be obvious ):
http://www.embeddedFORTH.de/dtmf.pdf
It was said to run on a 4MHz Z80 decades
ago. But getting it up on a 8MHz 68HC908 again
has proven difficult.

As picture 5 shows during the integration
phase there are only additions. Only
on dumping ( every 20msec ) the contents
of the partial accumulators are mulitplied
and added to the main accumulator.
That approach works only for coarse
approximations of sin/cos but is suited
to CPUs without MUL-opcode. Needs sufficient
RAM.

The wellknown approximation for the
magnitudes are also used:
http://www.embeddedFORTH.de/quad.pdf

MfG JRD
.