[chbot] DTMF tone frequencies - why?

[Robin Gilks]

Greetings all

I've used DTMF tones for several projects coding transmitters (usually with
sine wave lookup) and receivers (usually with a simple Goertzel decoder)
but the actual frequencies I've taken for granted as a part of the spec
(ETSI ES 201 235-2).

My question is, why these frequencies ? apart from being in the speech band
for signalling over telephone wires.

Why 697, 770 etc and why not 700, 777 and so on. Looks like the frequencies
are 10.5% apart in each of the low and high ranges and they avoid 50, 60 &
400Hz harmonics (from PSUs) as far as I can see.

It all looks pretty arbitrary and just happened to be what someone at Bell
chose for the original TouchTone system. I'm sure someone in the group
could give some historic context to these common beeps we hear in our ears
so often !!

-- 
Robin Gilks
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