Hello David, I would be interested to be a beta tester if your interested.
If there is working code I can do some testing in the real world.
73
Karel Fassotte
HR-HC1AKP/PE2KFA
2012/3/28 David Rowe <[email protected]>
> Hello List,
>
> Over recent weeks I have made some more progress on a HF modem designed
> specifically for Digital Voice (DV) over HF radio, based on Mel
> Whitten's FDMDV spec.
>
> Most other modems out there are designed for data. Specific differences
> are:
>
> + We don't care if there are bit errors in DV data, data always needs to
> be perfect. FEC is optional for DV. We would prefer to hear
> _something_ from the system than throw the whole packet away if it has
> bit errors. Let the Ham's head do the decoding in the presence of bit
> errors - just like it does for SSB.
>
> + We want fast sync (around 100ms) and low latency for DV. Data can
> tolerate several seconds of sync time and several 10s of seconds of
> latency.
>
> Progress over the last few weeks:
>
> + The nominal operating point of the modem (1% bit errors) is a SNR of
> 4dB.
>
> + Frequency offset estimation using a DBPSK pilot signal. The pilot
> adds about 1dB of transmitted power on top of the 14 FDM carriers which
> is reasonable I think. It can pick up +/- 200Hz frequency offset and
> track frequency drift of several Hz/s
>
> + I added the BPSK odd/even frame sync decoding. Two 20ms frames make
> one 40ms codec frame at 1400 bit/s. Combined with the freq offset
> estimation, the frame sync hangs on beneath 0db SNR. At 0dB SNR the BER
> is 8%, which still might be usable for DV, but that's a long shot.
>
> + The demod syncs up in around 4 frames (80ms), which should be fast
> enough for PTT operation and get us back quickly after a fade.
>
> + I spent some time looking into peak to average power ratio (PAPR) of
> the modem TX signal. I could see some big spikes in the tx signal from
> time to time (a high PAPR) which would mean a big back off in power
> amplifier drive to maintain linearity and a good bit error rate. After
> some experimentation I worked out a simple technique to improve PAPR by
> 5-6dB with no change in the bit error rate. I think this means we can
> drive the PA to 4 times the power output. If this translates to real
> world power amplifiers it could be significant. Would certainly make set
> up easier if the system was less sensitive to PA drive.
>
> The modem simulation code is all checked into codec2-dev SVN, try
> running octave/fdmdv_ut.m using Octave. At the top are channel
> simulation parameters like EB/No (SNR), frequency offset, and power
> amplifier clipping. I have attached some screen shots of the simulation
> outputs.
>
> Next step is to code this modem in C to get it running in real time. We
> can then make a first pass at a real time PC based DV system. Anyone
> like to help converting Octave to C code?
>
> Thanks,
>
> David
>
> On Mon, 2012-03-05 at 18:13 +1030, David Rowe wrote:
> > Hello List,
> >
> > To gain some understanding of the problems facing Codec 2 over the HF
> > channel I have been coding up an Octave simulation of the FDMDV modem
> > that has been used with some success in the past for digital voice over
> > HF. It's a 1400 bit/s modem with 14 carriers, each carrier being DQPSK
> > at 50 symbols/s.
> >
> > Just starting to test it with an open source Windows/GUI based HF path
> > simulator called PathSim (thanks Peter Lawrence for finding this):
> >
> > http://www.moetronix.com/ae4jy/pathsim.htm
> >
> > Pathsim and the modem simulation work with 8 kHz wave/raw files so I can
> > feed various signals through the simulator and see how they go.
> >
> > The idea is to get my head around the modem/codec requirements for the
> > HF channel the tune it for best performance.
> >
> > I have attached a spectrogram of my simulated modem data through a "CCIR
> > good" channel at 4dB SNR (equivalent to DQPSK Eb/No=7dB over AWGN
> > channel).
> > I can see segments of several seconds where the signal is wiped out.
> >
> > Also visible are some "clicks" due to clipping of the input signal when
> > multipath actually amplified the modem signal.
> >
> > Cheers,
> >
> > David
> >
> >
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