[Ground-station] Opulent Voice - Demo at DEFCON and demodulator plans

[Michelle Thompson]

Greetings all,

I hope you've had a chance to see the video of the over-the-air
demonstration of the uplink protocol Opulent Voice. We took the c++
implementation of M17, parameterized it, eliminated the LSF frame, combined
voice and data (no separate packet mode), and increased the bitrate by
switching from CODEC2 to OPUS. Bitrates are no longer hardcoded in, but are
not a runtime option.

If you haven't seen the demo, then here it is: https://youtu.be/i7k-jKtU_n8

Paul KB5MU advises that we need to start over on the lower layers of this
demodulator. He explains that the M17 code is too limited by the assumption
that the analog part of the receiver already does FM. This is a reasonable
assumption given the use case for M17 (VHF/UHF).

"Accurate tones are hard for HTs with analog modulators, so M17 was right
to make the demod as forgiving as possible. I think we can afford to be
pickier." (ptw)

An area of concern is the way carrier detection is done in M17. It's an
energy measurement made in two places, in and out of band. I'd like to not
depend on comparing to a fixed "out of band" location in order to signal
the presence of a carrier.

This work will start after DEFCON. We are not going to make major changes
to the (working) demodulator right before a show.

What are your opinions?

The Opulent Voice code is here:
https://github.com/phase4ground/opv-cxx-demod

-Michelle Thompson
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