Am 2012-03-05 08:43, schrieb David Rowe: > 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.
With Codec 2 hopefully doing 1000 bit/s in the future that would give some headspace for channel coding. > Just starting to test it with an open source Windows/GUI based HF path > simulator called PathSim. > The idea is to get my head around the modem/codec requirements for the > HF channel the tune it for best performance. I see different requirements for proposed transport methods like SIP/VoIP, HF, V/UHF. I think Codec 2 should do what it can do best: source coding of speech. The modem/channel coding should make do the best to transport the bits from source to destination. > 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. Deep fades are nasty, they tend to get more the narrower the HF channel is. You can guard against fades with interleaving. Phil Karn Ka9Q used 16 seconds interleaving for his BPSK1000 scheme which was used at Arissat-1, but that seems to be a bit exagerated for voice communication ;-) I guess with the new VQ approach not only lower bitrtes are possible but also even better quality at higher bitrates. Any plans to have different quality grades with differnt bitrates? With some metadata about the content of a transmission the receiver could accomodate to different modulation and coding rates. At good conditions (not much fading, good SNR, low noise) one would use all the badwidth for the raw unprotected data and get high quality sound. If conditions get worse, you would switch to higher compression with lower speech quality and put in some FEC to compensate for the bad channel. You would write the parameters in the header anyway. > Also visible are some "clicks" due to clipping of the input signal when > multipath actually amplified the modem signal. This is where multiple carriers have a strenth. You can use this multipath constructive interference if you spread your bits over frequency and time. I expect this to bee same for VHF/UHF channels, where you can even use more bandwidth. I think it would be nice to have a modulation scheme that could be parametrized: Bandwidth, datarate, code rate, modulation (BPSK, QPSK etc.), SNR requirements (which is a result of the other parameters). The same encoder/decoder could be used for HF, VHF, UHF or even higher frequencies. For every band you would have say 3 recomended parameter sets. I know this is ambitious, but we see that digital transmission in in that profile can be handled for several years now, and Codec 2 brings new opportunities and requirements. Regards Patrick ------------------------------------------------------------------------------ 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
