Hi David, please don’t get unsettled by that post. You are certainly NOT on the wrong track! Of course, each substantial and comprehensible contribution and/or real ‘makers’ are highly welcome, but we need no lessons about the targets and tasks of commercial digital communications in contrast to ham radio. No problem to start here an academic forum, but that wouldn't be very helpful for our hobby.
The last test results with the 700B mode compared with SSB at SNRs <6 dB and additional selective fading on the path are more than promising. In a lot of cases 700B was predominant. We will collect more data for severe conclusions. In every case a very exciting project. 73, Helmut, DC6NY -----Ursprüngliche Nachricht----- Von: Thomas Schiller [mailto:[email protected]] Gesendet: Freitag, 4. September 2015 11:42 An: [email protected] Betreff: [Freetel-codec2] Suggestion to dynamic bandwith adaption Hi David, hello List, thank you for your work on FreeDV and Codec2. I'm strongly interested in a better HAM-DV which must be solve different demands rather than commercial DV. One major DV-drawback is the hard decision cutoff at low s/n. While in SSB and even FM the demodulated s/n is rather proportional of the HF s/n all DV-systems have a stong threshold. This is a good approach for commercial but a poor behavior to HAM. We like to use our brain to listen inside of the channel noise to get the last sample biological-approximated to what we often already expect to know or hear (for example in a contest). Your attempt to use a 2nd carrier points in the right direction. I've written a suggestion to a HAM-defined DV-system in the german amateur journal FUNKAMATEUR ("Analoge und digitale Sprachsysteme für den Amateurfunk"; FA12/12, Page 1260-1263). http://archiv2.funkamateur.de/search The idea is to use at least four D-QPSK carriers shifted to minimize interference. The inner should be modulated with the basic parameters and a strong FEC. The carriers with more distance to the central should carry information to enhance the quality and could get a weaker or at maximum center-distance may be no FEC. The distribution of parameter bits to the carriers must be carefully choosen out of a list of significance of your CODEC2 or any other codec. As far as I know nobody has followed this idea before. Doing so, the RX then has the chance to dynamically reduce the received bandwith if the s/n is getting weak. Using only half of the carriers could enhance the s/n up to 6dB which is very much! Well of cause it will sound then more "spacy" below the decision point. But this is much more similar to what we hear on analog SSB or FM if the HF-signal becoms weak. At least one can listen to the behavior of the cannel and it's a nice gift to enhance the covering range at the same tramsmitting power! This is amateur's most wanted feature at all ;-) This is very different to commercial DV-systems which we currently use to "recycle" or adapt to an HAM application. Commercial DV needs a fixed s/n decision due to their cell-design and the quality necessities of the ordinary telephone user. In fact cell-design is not really compatible to what amateurs need on short wave. Hence we will never get below this interesting cutoff level if we don't put dynamic bandwith adaption to the HAM-DV functional specification. Do you agree? It would be very nice to do some experiments to find out what one can win in practice on a real HF-channel. 73, Thomas - DC7GB ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2 ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
