[aprssig] DTMF on HF?

Stephen H. Smith wa8lmf2 at aol.com
Fri Jan 10 21:12:15 EST 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 
on HF.

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
Skype:        WA8LMF
EchoLink:   14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

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