Hi Gullik I don't think that the transceiver needs any modification or improvement. In the last months we did a lot of tests using freeDV together with really advanced DDC/DUC SDRs. We evaluated the assistance of adjusted bandwidths, CESSB algorithm and in-band-distortion-free compression: All works absolutely well down the SNRs above 4-5 dB, but selective fading - typical on HF-bands and for the propagation principle - disturbs freeDV dramatically even when one carrier dives below this level.
73, Helmut, DC6NY -----Ursprüngliche Nachricht----- Von: Gullik Webjörn [mailto:[email protected]] Gesendet: Mittwoch, 21. Januar 2015 13:32 An: [email protected] Betreff: [Freetel-codec2] Some thoughts on a transceiver Dear list, Currently the typical freedv / codec2 gear is a SSB transceiver and a codec2/fdmdv modem. Observing the signal on the waterfall, I can see that a fdmdv signal is about half the bandwidth of a SSB signal. Thus, I was thinking about changing the filter in the transceiver, to gain an extra dB or so by eliminating the difference in noise bandwidth. However, maybe this is NOT the way to go, maybe the signal processing in the fdmdv modem is so much superior due to cpu power, that THAT would be the area to focus on. We have discussed I/Q modulation/demodulation, and that even though this is easily implemented, it has not been done (but maybe tested??) yet. Is it your belief that a "dedicated" transceiver would significantly improve codec2/freedv operation? Where is the biggest mismatch? Crystal filters ( passband shape / group delay) , AGC, general amplitude/phase in the signal chain (that is optimised for CW/SSB/RTTY)?? Gain / AGC ( passband mismatch)?? In short, how would a "near optimum" transceiver for this mode look like, what can be eliminated, what should be enhanced? Would a signal chain with independent carrier processing of the 16/17 carriers gain anyting, I am here thinking of individual carrier erasure due to multipath, or are we close to optimal here? Your thoughts... Gullik / SM6FBD PS Saw the video from linux-dev NZ, nice... ---------------------------------------------------------------------------- -- New Year. New Location. New Benefits. New Data Center in Ashburn, VA. GigeNET is offering a free month of service with a new server in Ashburn. Choose from 2 high performing configs, both with 100TB of bandwidth. Higher redundancy.Lower latency.Increased capacity.Completely compliant. http://p.sf.net/sfu/gigenet _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2 ------------------------------------------------------------------------------ New Year. New Location. New Benefits. New Data Center in Ashburn, VA. GigeNET is offering a free month of service with a new server in Ashburn. Choose from 2 high performing configs, both with 100TB of bandwidth. Higher redundancy.Lower latency.Increased capacity.Completely compliant. http://p.sf.net/sfu/gigenet _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
