[amsat-bb] Re: D STAR - try freeDV instead
ross at topwire.co.nz
Mon Nov 11 23:01:39 PST 2013
Rather than trying to get a D-Star radio to QSY in less than 5 kHz
steps, you could try freeDV with your existing SSB transceivers?
D-Star occupies 6.25 kHz bandwidth on the transponder, but freeDV only
needs 1.25 kHz (half of a normal SSB voice emission).
You can get freeDV (experimental) software here:
Unlike the early days of SSB (which has been the subject of various
patents) AMBE is unlikely to ever be free, even after the patents expire
as DVSI Inc. treats AMBE as a TRADE SECRET, and the only way to get the
codec is to buy the chip (a locked TI DSP pre-loaded with AMBE). DVSI's
patents cover some of the technology used in AMBE, but these patents do
not explain enough about the AMBE process or protocol for someone to
write their own AMBE-compatible codec.
Once the patents expire, someone with the right resources might be able
to hardware reverse engineer the firmware in the AMBE chip (very
difficult!) and write a specification of the AMBE process/protocol,
which someone else who has never seen inside the AMBE firmware can then
use to write a "clean room" AMBE compatible codec. Depending on the laws
in force at the time, it might also be illegal to de-bond the AMBE chip
and reverse engineer the codec therein. However, all this is probably
requires far more effort than it is worth - better to spend time, energy
and money on developing an alternate codec that is truly "open".
FreeDV uses an OFDM modem (16 QPSK carriers + 1 BPSK pilot), designed to
have strong resistance to the multi-path propagation typical on the HF
bands that it was originally intended to be used on. However, this means
that is has a ~ 12dB peak to average power ratio, which imposes
linearity requirements on the transmit signal path. For satellite
operation, where path loss is a much greater concern than multi-path,
the use of a serial tone modem (single carrier QPSK, BPSK, GMSK, etc)
for freeDV would reduce the linearity requirements of the transmit
signal path making it easier for an otherwise marginal ground station to
produce enough EIRP to work the bird.
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