🚀 go-pugleaf

If you want chips to do this, several companies make them.  The transmitters
usually take in four or five bits and produce the analog tones, while the
receivers take in the analog tones and produce four or five bits of data.
Silicon Systems is one company that comes to mind, but there are others.

If you want to implement a transceiver on a general purpose DSP chip, it's
not too hard.  For the transmitter you just need two tone generators.  The
receiver is trickier.  One possibility is to use ganged bandpass filters
followed by a power estimator.  Another is to use the Goertzel algorithm,
which is essentially a "single frequency" FFT/DFT, on the frequencies
of interest.  Motorola's "Dr. Bub" bulletin board has the source to a
DTMF receiver using the Goertzel algorithm.

(I implemented an MF receiver using this technique with Gabriel on
a DSP56001 last summer, so I could decode the MF signals at 3:14 into
the Pink Floyd song "Young Lust", from their album _The Wall_.  If you're
curious, the numbers were something like KP + 044 + 831 + ST).

References:

1. Geoffrey Smith, "DSP Workshop: DTMF Receiver Using the WE DSP16 Product
   Family", AT&T DSP Review, Summer 1989, Vol. 2, Issue 3.

   This is a pretty good overview of the implementation of the Goertzel
   algorithm for DTMF reception on a DSP16.  You can probably get a copy
   from AT&T at (201) 949-3409.

2. Oppenheim and Schaeffer, _Digital Signal Processing_, Prentice Hall,
   NJ, 1975, pp. 287-289.

   Explanation of the Goertzel algorithm.

3. Silicon Systems Communications Products Databook, 1991, Ch. 4 and 5.

   Data sheets on SSI's DTMF and MF transmitters/receivers/transceivers.
   You can probably get a copy from SSI at (714) 731-7110.

Phil Lapsley		phil@ucbarpa.Berkeley.EDU	...!ucbvax!phil