[aprssig] DTMF on HF?
Stephen H. Smith
wa8lmf2 at aol.com
Fri Jan 10 20:12:15 CST 2014
On 1/10/2014 8:14 PM, Bill Vodall wrote:
>>>> Has anyone had any experience with any success at DTMF working on HF for
>>>> some rudimentary commands?
I have pondered this for some time, having wanted to use some DTMF applications
One approach I did a thought exercise on(but never implemented) would be to
look for the difference between the two tones, rather than the absolute
frequencies themselves. Each tone pair has a different difference value. The
difference tone for a given digit would be the same, regardless of SSB tuning
errors, as long as both tones remain within the SSB receiver passband; i.e. a
tuning error of HUNDREDS of Hz.
Run the received audio through an intentionally highly non-linear audio amp
(perhaps an op-amp running open-loop) to MAXIMIZE the audio inter-modulation
distortion. Then look for the unique difference tones with the same kind of
multiple parallel ultra-narrow software-defined filters used to demodulate MFSK16.
From what I have seen of MFSK16's capability on 30-meter APRS to decode
reliably on completely inaudible signals totally buried in noise, it should
should be possible to decode DTMF signals at S/N far below that usable on the
voice transmissions that presumably would follow.
Stephen H. Smith wa8lmf (at) aol.com
EchoLink: 14400 [Think bottom of the 2-meter band]
Home Page: http://wa8lmf.net
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