[amsat-bb] Re: D STAR - try freeDV instead

Ross Whenmouth 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.

73's ZL2WRW
Ross Whenmouth

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