[chbot] DTMF tone frequencies - why?

[Robin Gilks]

That nails it - Thanks Stuart, I'll sleep better tonight knowing what the
original rules were ;)


On Thu, Sep 16, 2021 at 11:41 AM Stuart Brown <stuartbrown2 at gmail.com>
wrote:

> Robin
>
> I found an interesting write up here
> https://www.specialtyansweringservice.net/wp-content/uploads/resources_papers/dtmf-tone/Dual-Tone-Multi-Frequency-Signalling.pdf
> (while waiting for a compile to finish)
>
> Stuart
>
> 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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