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[aprssig] An amusing aside (Text Pagers)

david vanhorn kc6ete at gmail.com
Sat Jan 30 15:55:36 UTC 2010


Seems like I unintentionally kicked off quite a discussion!  :)

DTMF:  Some years back I was sending data by DTMF, and I developed an
algorithm to substantially speed that process up.

Normally, one sends DTMF as 50ms on, 50ms off, 10 digits per second.
The thing is, if the next digit is different, you really don't need a 50ms
silence.
So, you can use short silences of 10ms or less between different digits.

Normal send:  1-2-3-4-5-6-6-5-4-3-2-1  (dash = 50ms silence)
Improved send:  1.2.3.4.5.6-6.5.4.3.2.1 (dot = 5ms silence)

In my testing with barcode data, I was getting about 17 digits per second
without shortening the DTMF on times.  Standard decoders need 40ms of valid
tone, so if you try to shorten the tones, you'll hit a wall really fast.

So FWIW, there it is.
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