[chbot] DTMF tone frequencies - why?

Stuart Brown stuartbrown2 at gmail.com
Thu Sep 16 00:39:57 BST 2021


I found an interesting write up here
(while waiting for a compile to finish)


On Thu, 16 Sept 2021 at 10:52, Robin Gilks <gb7ipd at gmail.com> wrote:

> 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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