Am 2011-11-02 00:29, schrieb David Rowe: > On Tue, 2011-11-01 at 21:52 +0000, Trevor . wrote: >> --- On Tue, 1/11/11, ZPO <[email protected]> wrote: >>> Should the FEC be considered part of the baseline CODEC or >>> a part of the external transport protocol? >> >> FEC is nothing to do with the CODEC it's in the transport protocol. > > For low bit rate digital speech at low channel SNRs you can get much > better performance by considering source coding, FEC, and even > modulation together. > > It's very wasteful just to apply blanket FEC to a > 2000 bps digital voice stream.
I do not think that just trowing "some FEC" on a codec yields a sensible combination. One needs to know the system very well to choose the right components that fit exact together and take advantage of each other. There are many questions to be answered before. Let's say the source coder fits the signal and does it's job well. * Are all bits of the source coded signal equally susceptible to bit errors? What about the difference between burst errors and static errors? How does the coder/decoder behave with different bit error rates; which bit error rate is required for understandable decoding? All in all you do not simply look at the bit rate alone, but take the channel bandwidth, SNR, modulation and calculate a channel capacity at a given BER, and try to match that with your target bit rate and BER (or the other way around...) * Which modulation should be used? What are the channel characteristics? How much delay is acceptable? How much bandwidth, SNR, what type of expected fading? Is continuous envelope necessary? Which amplifiers are to be uses (linear, non-linear)? Can special coding/modulation improve the robustness of the signal (trellis coding) * If not all bits of the source coded signal need the same protection, which would need how much protection? How would you pack the protected and unprotected signal parts? * Do we need some other parts, like encryption (not for ham operation), whitening, interleaving etc.? If DSTAR should be tried with codec2, which poperties has a DSTAR channel? Doesn't do DSTAR already error correction for packets? What's the expected BER for a standard DSTAR link? Is the error correction enforced or can it be switched off/circumvented? If other RF systems should be used, what's the plan? HF, VHF, UHF? What about delay and multipath/spread? Regards Patrick 73 de Patrick -- Engineers motto: cheap, good, fast: choose any two QTH: JN77rb http://sat.mur.at/ Patrick Strasser OE6PSE <oe6pse at wirklich priv at> ------------------------------------------------------------------------------ RSA(R) Conference 2012 Save $700 by Nov 18 Register now http://p.sf.net/sfu/rsa-sfdev2dev1 _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
