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 > > ------------------------------------------------------------------------------ > Try before you buy = See our experts in action! > The most comprehensive online learning library for Microsoft developers > is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, > Metro Style Apps, more. Free future releases when you subscribe now! > http://p.sf.net/sfu/learndevnow-dev2 > _______________________________________________ Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2
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