Hi Dan, as Codec2 is a digital codec, a drop-down of quality like on analog transmission seems hard to archieve. Normally, a good squelch-filter with amplifier would work better instead of trying to modulate it again as an analog narrowband signal. Instead of 16QAM, I would prefer 4QAM, as it is much more robust than 16QAM in case of an angular phase shift. That's why 64QAM and 256QAM only work fine in a stable connection and 16QAM is standard in DVB-T (64QAM in DVB-C/S).
Am 15.01.2013 15:03, schrieb Daniel Ankers: > Hi David, > > > On 15 January 2013 05:58, David Rowe <[email protected] > <mailto:[email protected]>> wrote: > > > I'm in the middle of hacking the code in fdmdv.c to give an > indication > > of the quality of the received symbols as well as the symbols > > themselves. This means that if the CRC check fails, we can flip the > > bits most likely to be incorrect and see if it passes. > > This is also something I would like to look at so perhaps we can > collaborate. There is some other structure in the parameters (e.g the > LSPs) that could be treated the same way as you have described above. > > > That would be good. My approach is to change > float qpsk_to_bits(int rx_bits[], int *sync_bit, COMP > phase_difference[], COMP prev_rx_symbols[], COMP rx_symbols[]) > to > float qpsk_to_bits(int rx_bits[], int *sync_bit, COMP > phase_difference[], COMP prev_rx_symbols[], COMP rx_symbols[], float > errors[]) > > I calculate the mean magnitude avg_r for each frame, and then define > the error in each symbol as the distance between the symbol and a > "perfect" symbol at avg_r, theta (where theta is one of pi/4, 3*pi/4, > 5*pi/4 or 7*pi/4, depending on the quadrant the symbol is in.) > > At the moment I'm returning the error on a per-bit basis rather than > per-symbol. This has the disadvantage that it ignores the fact that > errors are going to affect whole symbols not individual bits, but it > means that the whole approach can be used with more complicated > modems. (One idea I had was that the codec2 bits which have a more > minor effect on the voice quality could be carried in 16QAM carriers > to reduce the overall transmitted bandwidth and giving a more > analogue-like drop in quality as signal levels dropped instead of a > digital all-or-nothing effect.) I'm hoping to just start simple and > then possibly improve in the future to a more intelligent approach > which carries data about which bits come from which symbol and the > quality of that symbol. > > I'm also very deliberately sticking to a frame-by-frame approach > rather than gathering data across several frames. I'm trying to avoid > doing anything which will reduce the sync speed of the system. > > > To the list in general: over the next few months I will be looking at > making FreeDV more robust on HF radio channels. This will involve > work > to FreeDV, Codec2, and the fdmdv modem > > > One thing which I think would be valuable is some kind of indication > of the format of the data being sent over the fdmdv modem. > At the moment, if we change the data format without changing the modem > then existing FreeDV users will hear odd noises (since the data will > demodulate from the modem correctly but will not make sense when > decoded by the codec.) It would be nice if in future we are able to > change the data format and FreeDV mutes the sound and possibly brings > up a notification that a new modulation scheme is being used and it > may be time to go online and check for updates. > Obviously if the modem is changed then this will be shown in the > waterfall looking different and existing copies of FreeDV would > probably not be able to decode anything. > > Cheers, > Dan MD1CLV > > > ------------------------------------------------------------------------------ > Master SQL Server Development, Administration, T-SQL, SSAS, SSIS, SSRS > and more. Get SQL Server skills now (including 2012) with LearnDevNow - > 200+ hours of step-by-step video tutorials by Microsoft MVPs and experts. > SALE $99.99 this month only - learn more at: > http://p.sf.net/sfu/learnmore_122512 > > > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2
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