> modulation and coding rates. At good conditions (not much fading, good > SNR, low noise) one would use all the badwidth for the raw unprotected > data and get high quality sound. If conditions get worse, you would > switch to higher compression with lower speech quality and put in some > FEC to compensate for the bad channel. You would write the parameters in > the header anyway.
This is a bit of a brainstorm, but here goes: An approximation of the HF channel is really good SNR or really bad SNR where nothing gets through. A way to get over fades is to interleave bits over several seconds and use FEC to make up for data lost during fades. Down side is delay. However when the channel is good (between fades) we can handle much higher date rates than the nominal rate. Rather than switching codec rates (which requires a feedback channel), we could send both low and high latency data at the same time at double or triple the nominal codec bit rate. When a fade kicks in, we use the high latency data. Not sure how to implement this in practice, like handover between the two modes. Maybe in periods of silent speech we could "take up the slack" and switch back to low latency transparently to the listener. > I think it would be nice to have a modulation scheme that could be > parametrized: Bandwidth, datarate, code rate, modulation (BPSK, QPSK > etc.), SNR requirements (which is a result of the other parameters). The > same encoder/decoder could be used for HF, VHF, UHF or even higher > frequencies. For every band you would have say 3 recomended parameter sets. I am working on a first pass at this that can be used to test a few scenarios. Feel it's crucial to get something "on the air" to get subjective feedback of the entire system. For VHF we should be able to use coherent PSK which means a SNR of < 1dB and a 1kHz bandwidth. Not sure how to compare that to FM or competing DV systems. Getting some kind help on modems from a couple of HF modem gurus like Rick Muething (Winmor) and Peter Martinez (G3PLX, PSK31 etc), and Bill Cowley (my PhD supervisor and a professor of digital communications) . - David ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-dev2 _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
