Hi Helmut, Thank you for your report. Oh I'm happy to discuss HF DV any time, respectful debate helps push the art forward :-)
Cheers, David On 05/09/15 00:19, Helmut wrote: > 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 > ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
