Success!!!!
Well at least sort of. I received an informative message from PE4WJ, Wouter Jan Ubbels, who is part of the team working on Delfi-C3, which is scheduled to launch in April. As part of that project he has made available a sound card based program, called Warbler, to demodulate the 1200 baud telemetry downlink. It uses a Costas Loop to demodulate and for Doppler correction. He suggests that the program could be expanded, or at least adapted for voice DSB signals. While he is busy with Delfi-C3, he seems interested in our discussions. Perhaps we can come up with some ideas that are worth trying to add to future versions of Warbler.
I had originally thought that the idea should have been considered before, and I am pleased to know that it is actually in use. The fact that it is in use by the digital "community" but I could not seem to find any reference to it, is a yet another example of a communications problem that has spanned my three decade engineering career: different technical fields have such specialized languages that the members of each cannot recognize when they are talking about similar things. I should have recognized that I needed better understanding of just why AO-16 has a DSB transmitter. It makes a lot more sense after I read an article by G3RUH called "the shape of bits to come" which Wouter suggested. I still have a lot to learn, but I am starting to comprehend the following quote from the article:
Binary data DSB modulating an RF carrier" and "binary phase shift keying (BPSK)" are exactly the same thing.
What I might add to try to clarify the statement for those, like me, for whom it was not immediately obvious is that "Binary Data" in this context comes in the form of +1 and -1 instead of 1 and 0 . For some of us it makes the difference between BPSK and OOK.
If I am understanding Warbler, it brings in the 1200 baud BPSK, which "looks" like DSB with 2400 Hz bandwidth, into a sound card. The signal is tuned so that the "carrier" would be between 1400 and 1800 Hz. Once the Costas Loop is locked, it will then compensate for 400 Hz of Doppler shift. If the "carrier" drops below 1400 or above 1800, the extremes of DSB signal would get attenuated by the passband of an SSB receiver, which is assumed to be used to pick up the signals from satellite.
Modifications for voice DSB, would start by the use of a wider receiver, perhaps an AM mode instead of SSB. Assume for the moment that we have a receiver audio bandwidth of 10 KHz. If the audio used to modulate the DSB transmitter is limited to 3 KHz, then the DSB signal will be 6 KHz. Set the Costas Loop to track "carrier" from 3.3 KHz to 7.0 KHz then you get a 3700 Hz Doppler correction range. The sound card would then process the upper and lower sidebands into the original speech which would then be produced by the sound card. You could whittle that down some by communication quality audio trimmed to 2.7 or even 2.5 KHz. The Doppler range increases to 4300 and 4700 Hz. A wider receiver band would also be a benefit, limited by the high frequency limit of the sound card. And of course a wider receiver lets in more noise. The additional noise suggests digital processing.
Wouter also suggested that software defined radio could be used instead of the scheme outlined above. SDR is the concept that I had in mind. Perhaps a simple system that has the I and Q channels fed into the L and R sound card inputs, Costas Loop and additonal processing in the computer, sound card or serial output to control the VCO. The bandwidth would be limited by the sound card audio inputs, but the Doppler tracking correction would then be limited only by the range of the VCO and its update rate.
I think this is worth the time to experiment. The situation with AO-16 certainly makes it easy to get voice DSB signals on the downlink. One only needs FM transmissions. It could be used on a translating repeater, but I would suggest that it would be courtesy to not try it on a crowded area. If DSB gets a bad reputation for "hogging" limited bandwidth, it will never get fair consideration for current or future satellites. DSB does not use as much spectrum as FM, and I hope that it can demonstrate a benefit for Doppler compensation. It has yet to prove its value the amateur satellite community. I think it will be fun to try.
I certainly hope that I have presented some ideas that encourage thinking, learning, and experimentation.
Best Wishes.
James n5gui