Thomas, See "Suggestion to dynamic bandwidth adaption" thread above. It would be interesting to add QAM constellations to the cohpsk modem. The pilots symbols could be used as an amplitude as well as phase reference.
-/- Glen, Reports of a quite a few NVIS contacts already, in fact we have a websdr here in Adelaide without a ground wave path from my QTH that I regularly use for just that purpose. Power as low as 500mW with 0 BER. Curiously, I haven't had a _single_ report of a high SNR path that breaks FreeDV 700(B). For example a path that works well for SSB, but makes the FreeDV 700 modem fall over, would indicate the modem can't handle the channel. Cheers, David On 07/09/15 07:46, glen english wrote: > Hi Thomas > You are talking about hierarchical coding and transmission, and it has > been discussed here widely. > > We have considered it and I think it is in the informal roadmap, IE > when conditions are good, we have a low priority stream that enhances > the user experience. > > The frequency selective narrowband HF channel represents rather > different character compared to the VHF narrowband channel which is > primarily a flat fading condition. > > Frequency diversity is the most efficient system wide method of solving > the problem on HF > > I suspect when the frequency selective performance is deemed OK by the > users, and that is the frequency selective channel properties generated > by say, a >600km ionospheric path, then the users will begin to push the > vertical incidence ability of FreeDV, this is a high random doppler > profile and will likely spur another round of development to cope with it. > > regards > > glen english VK1XX > > > On 7/09/2015 2:01 AM, Tomas Härdin wrote: >> Hi >> >> (I tried sending this a few days ago but forgot to confirm my >> subscription to this list - oops! So resending) >> >> My name is Tomas and I've been following the development of codec2 on >> and off for a few years now, and since getting my ham license earlier >> this year I find myself thinking about it more. A few days ago I had >> what I thought was a clever idea for fixing the "digital cliff" problem >> Mike mentioned in a talk that's up on YouTube (I forget which). Today I >> see on the roadmap post[1] that this is currently being worked on using >> two GMSK streams, but I thought "hey, maybe someone will find it >> interesting". So here goes: >> >> The idea assumes that codec2 can make use of bitrate peeling. That is, >> that we can split the stream up into two or more streams where the first >> one provides a rough but usable quality, and subsequent streams improve >> upon this. H.264's SVC would be an example from the video world >> >> So the idea is take these streams and modulate them onto a hierarchical >> QAM system. The simplest would be to take the current FDMDV modem and >> instead of using 14x QPSK (aka 4-QAM) carriers you use 7x 16-QAM >> carriers. You then code the more important bitstream into the most >> significant bits in each 16-QAM symbol, and the improvement stream into >> the lower bits (assuming the two streams have identical bitrate). Since >> you now have half the number of carriers you can put twice the amount of >> power into each carrier >> >> That's about it. Lots of variants are of course possible, but this >> should get the point across. I may experiment with the idea once I get >> some suitable SSB equipment, but for now I'm interested in feedback even >> if it's just shooting it down :) >> >> /Tomas, SA2TMS >> >> [1]http://www.rowetel.com/blog/?p=3931 >> >> >> >> >> ------------------------------------------------------------------------------ >> >> >> _______________________________________________ >> Freetel-codec2 mailing list >> Freetel-codec2@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > -- > - > Glen English > RF Communications and Electronics Engineer > > CORTEX RF > & > Pacific Media Technologies Pty Ltd > > ABN 40 075 532 008 > > PO Box 5231 Lyneham ACT 2602, Australia. > au mobile : +61 (0)418 975077 > > > > ------------------------------------------------------------------------------ > > > > _______________________________________________ > Freetel-codec2 mailing list > Freetel-codec2@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
