[chbot] DTMF tone frequencies - why?

Charles Manning cdhmanning at gmail.com
Thu Sep 16 00:35:53 BST 2021

I expect there's a good reason for this in terms of picking frequencies
that fall in the middle of FFT bins or such.

But it could be something along the lines of the guy who started it just
happened to have certain value capacitors on his desk.

On Thu, Sep 16, 2021 at 10:52 AM 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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