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Re: PSK demodulation


On Wednesday 11 April 2001 14:39, you wrote:
> Tom Sailer's latest soundmodem package has two modems of interest.
> One is a PSK modem for terrestrial use, which runs at about 7600 BPS
> and is supposed to be compatible with standard FM transceivers
> (ie using the standard audio connections, not the direct connections
> used for 9600).
> The other is a demodulator only for P3D. If the
> 400 BPS downlink is similar to the 1200 BPS PACSAT downlink,
> it might be easy to modify the demodulator to do the PACSAT
> downlink.

I don't think so.
Those two demodulators you picked are both rather unconventional special 
purpose demodulators.
The 7600bps HT PSK modem was designed to enable high speed packet radio with 
unmodified handy talkies, using the standard mic/headphone jacks and 
compensating for the bad frequency response by a data aided (DA) channel 
estimator (training sequence; thus the strange effective bit rate) and a MLSE 
(maximum likelihood sequence estimator; viterbi decoder; the same principle 
as in e.g. GSM cellphones). Pretty clever concept, you can read about it in 
the transcript of last year's PR convention. The bottom line is you would 
very likely end up spending more time removing the "overhead" stuff than 
writing a normal modem from the bottom up.
The 400bps AO-40 telemetry demodulator does the frequency and phase 
synchronization by looking for the 32 bit preamble. I don't know the frame 
format of those 1200bps sats, but as long as they don't employ similar 
preambles you won't have much luck converting this piece of software either.

  -- Jens
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