Hi Glen, OK so you want to work on reducing PAPR, with the goal of improving performance on VHF channels. Comments/questions:
1/ What do you mean by a "flat VHF channel"? Flat fading or just AWGN? 2/ Can you pls post a wavefile of a demodulated SSB signal with a PAPR of less than 4.5dB? Given a sine wave is just 3dB so I'd like to hear what that sounds like - and measure the PAPR myself. 3/ Writing, testing, and integrating a new modem into FreeDV (and freedv-gui) is a several hundred hour project. I do that exercise perhaps once a year. A MSK wavefile running the same bit stream as FreeDV 700D will have a main lobe bandwidth of 1.5*1400 = 2100 Hz. Have you determined if that will pass through SSB radios without any performance hit? If you lose even a dB in performance you might as well go to non-coherent 4FSK..... 4/ .... we already have 4FSK FreeDV modes - 2400A & 800XA, see README_fsk.md. 5/ By "patch" you mean Pull Request - right? If you get that far I'll help you design the ctests we'll need to see as well. Cheers, David On 6/7/20 7:10 pm, glen english wrote: > Hi David > The goal is to improve FreeDV so that it is at least equal with SSB on a > flat VHF channel. > > On HF, the performance of FreeDV is better than SSB mainly because it > can both a) work at low SNR and b) can mitigate the effects of a > frequency selective channel (which reduces speech intelligibility) . > > At the moment, FreeDV is a bit behind on a flat channel. This is because > the PAPR is costing some ERP. PAPR on SSB on VHF with a good RF speech > processor is about 4.5dB (or less!). If the ofdm modem is pushed, the > performance doesnt improve of course because the BER starts to suffer > (on a non zero BER scenario IE marginal conditions. > > I thought about working on getting the PAPR down on the FreeDV OFDM > modem. The last day I have done some revision and 8dB might be doable, > but 6 to 7dB will only be doable with computational intensity, which the > SM1000 does not have. That still leaves us a few dB in PAPR behind flat > channel SSB. Flat channel SSB is OK down to about -3dB SNR for a good > operator . Actually it is really hard to quantity exactly what the SNR > is because of the time varying power. (IE -3dB wrt to PEP is the usual > statement). > > So now I think probably back to making the MSK modem work. I will also > put in a patch for a band limited clipper for the OFDM modem as soon as > I get the build environment... building. > > cheers > glen > > > > > > On 6/07/2020 3:52 pm, David Rowe wrote: >> Hi Glen >> >> Happy to comment, but there's quite a lot of material in your post below >> and I'm unclear on what you are trying to achieve. >> >> Can you pls tell me what the goal of your proposed work is? >> >> Cheers, >> David >> >> On 5/7/20 6:43 pm, glen english wrote: >>> I've been looking at an coherent MSK modem for Codec2. I have not done >>> any coherent MSK modems since the late 90s. >>> >>> Using an SSB radio , at something like 1600 bps , there will still be >>> appreciable PAPR on MSK due to bandwidth constraint. probably about 3dB >>> PAPR on a 2400Hz wide radio. When bandwidth is constrained, the >>> constant envelope signal is no longer constant envelope... >>> >>> The is quite obvious using MSK441, the meteor scatter mode in WSJTX. It >>> operates at 2000 bps, and struggles a bit on bandwidth with most non-SDR >>> types . PAPR ends up around 4.5dB . Also the phase response of non flat >>> radios is usually aweful. A channel equaliser turned on to model and fix >>> your radio >>> >>> Now... >>> >>> OFDM doesnt need a specific radio equaliser, that's built in of course, >>> the uglyness of most radios can be obscured. . >>> >>> And if the only complication from the existing OFDM modem of David's is >>> the need to constrain PAPR, I think that perhaps constraining the PAPR >>> of David's existing modem , rather than a MSK modem from almost scratch >>> , might be more overall beneficial to everyone (HF as well) . I am >>> thinking an incompatible change to the existing modem to reduce PAPR to >>> something like 4.5dB might be the best of both worlds. yeah it might >>> cost a 0.5dB but oh well, you probably lose that now with PAPR >>> constraints. The existing 2FSK or 4FSK modem should also be brought up >>> to working. >>> >>> David, what are your candid thoughts ? >>> >>> glen. >>> >>> >>> On 6/23/2020 6:16 AM, David Rowe wrote: >>>> Hi Glen, >>> >>> >>> _______________________________________________ >>> 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 _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
