On 04/22/2013 09:13 AM, Steve Strobel wrote:
It seems to me that the job of the codec itself should be to compress the audio and the job of the FEC should be to make the communication channel robust.Yes, I used to think this as well, and if we had packets as large as IP it would probably be true. Things change because we have such small frames. David has shown that the FEC schemes that we can fit will themselves fail if the bit errors are higher than 3% or so, and will indeed cause good bits to be identified as bad. Fading will frequently bring the errors above this level.
There's a very significant benefit to having the FEC understand the codec data frame, and having the codec itself tuned for bit error resistance.
The most densely compressed version of the codec has vector quantization of the filter parameters. This compresses very efficiently, but even a one-bit error leads to a completely wrong result. So, David removed the VQ. He then applied 16 bits of FEC to the most significant bits only. So, bit errors result in a signal that might be somewhat off but is still readable by ear.
We end up with a fade margin comparable to SSB, where previously it took an S7 signal to work reliably (and everyone complained).
We can only do this because we have access to the codec internals. As far as I'm aware, AMBE uses a data FEC scheme and several times more bandwidth, and would not approach the robustness that we could provide using a GMSK system, not to mention our FDM modem.
Thanks
Bruce
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