I've been talking to some Turbo Coding and LPDC guys and I now think they would be worth testing, for example with block sizes of 256 bits. While these codes are optimal at blocks with several thousand bits (a few seconds delay of Codec 2) they might still be useful at block sizes/delays of a few 100ms.
We could also develop a "weak signal" mode which has all the delay we need to interleave over time, use the best codes available, and just put up with 5-10 seconds delay and no SSB-type break in experience. If it out-performs SSB (say error free, noise free voice at SNR=-3dB) then it might be useful in some applications. - David On Wed, 2013-03-06 at 12:52 -0800, Bruce Perens wrote: > Some of the super optimal coding schemes don't work on really small > blocks. > > On 03/06/2013 11:50 AM, ZPO wrote: > > Are there any LDPC implementations out there that aren't patent > > encumbered? > > > > 73-KY9K/Brian > > > > On Mar 6, 2013 10:05 AM, "Rick Muething" <[email protected]> > > wrote: > > Daniel, > > > > RS and similar block codes are fairly efficient and have > > some nice properties especially when mixed (layered) with > > other types of FEC. E.g. Viterbi (convolutional) type > > decoders work well with random errors. But these type codes > > perform poorly in burst error situations. Sometimes if the > > timing permits bursts can be reduced by interleaving but > > there are limits especially when you don’t want to delay the > > signal much in time as in the case of digital voice. In > > such cases adding a block code “outside” the convolutional > > codes can be effectively in many systems. > > > > Reed Solomon adds two “parity” characters for each carrier > > correction desired. ( a character can be any number of bits > > from 2 on up although often is done with 8 bit characters) > > E.g. if I were sending 8 bit characters and I wanted to > > correct up to 8 character errors I would have to add 16 8 > > bit “parity” characters to my data and these could correct > > up to 8 character errors in the entire block of data (data + > > Parity). The amount of Parity you need is a function of the > > channel and the error rates. So the “efficiency” of a RS > > type code is a function of how many errors you wish to be > > able to correct. For example if I were sending a block of > > 100 data characters and wished to correct up to 8 erroneous > > characters I would need to add 16 total parity characters so > > my overall efficiency would be 100/116 or about 86% which is > > reasonable. As the number of corrected characters desired > > increases the efficiency of course goes down but the > > robustness increases. Correcting up to 32 characters would > > require 64 characters of Parity so the efficiency would be > > 100/164 or about 61%. The beauty of RS and other similar > > block codes is they are tolerant of block errors. It > > corrects by character whether there is one bit in error or 8 > > bits in error of the character. So it compliments the > > convolutional code nicely by correcting bursts that the > > convolutional code can’t correct. > > > > This is why layering two types of FEC is effective and why > > for example NASA often uses layered block coding on top of > > convolutional coding in very weak signal transmission. > > > > In the end the amount and selection of FEC is a function of > > the channel and the type and number and type of errors > > expected. But in general adding the correct type and level > > of FEC can usually provide a net improvement over uncoded > > systems in terms of net BER. This is usually expressed as > > equivalent dB improvement. (the amount of improvement you > > would get by increasing the S/N by the same number of dB. > > > > In digital Voice we have another issue in that unlike in > > binary message transmission some level of uncorrected errors > > may be acceptable (resulting in some speech artifact which > > may not severely impact intelligibility). > > > > I have experimented with this a lot in the development of > > WINMOR and often optimization is the result of iterating > > different promising FEC coding and evaluating them over > > specific (some times standardized CCIR) channels using a HF > > channel simulator. Prediction with theory alone is > > possible on White Gaussian Noise channels but we seldom see > > these channels in HF propagation. > > > > 73, > > > > Rick Muething, KN6KB > > > > > > > > ------------------------------------------------------------------------------ > > Symantec Endpoint Protection 12 positioned as A LEADER in > > The Forrester > > Wave(TM): Endpoint Security, Q1 2013 and "remains a good > > choice" in the > > endpoint security space. For insight on selecting the right > > partner to > > tackle endpoint security challenges, access the full report. > > http://p.sf.net/sfu/symantec-dev2dev > > _______________________________________________ > > Freetel-codec2 mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > > > > > > > ------------------------------------------------------------------------------ > > Symantec Endpoint Protection 12 positioned as A LEADER in The Forrester > > Wave(TM): Endpoint Security, Q1 2013 and "remains a good choice" in the > > endpoint security space. For insight on selecting the right partner to > > tackle endpoint security challenges, access the full report. > > http://p.sf.net/sfu/symantec-dev2dev > > > > > > _______________________________________________ > > Freetel-codec2 mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > ------------------------------------------------------------------------------ > Symantec Endpoint Protection 12 positioned as A LEADER in The Forrester > Wave(TM): Endpoint Security, Q1 2013 and "remains a good choice" in the > endpoint security space. For insight on selecting the right partner to > tackle endpoint security challenges, access the full report. > http://p.sf.net/sfu/symantec-dev2dev > _______________________________________________ Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 ------------------------------------------------------------------------------ Symantec Endpoint Protection 12 positioned as A LEADER in The Forrester Wave(TM): Endpoint Security, Q1 2013 and "remains a good choice" in the endpoint security space. For insight on selecting the right partner to tackle endpoint security challenges, access the full report. http://p.sf.net/sfu/symantec-dev2dev _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
