Yes, Of course there are block codes and convolutional codes. Convolutional codes have a memory that is equal in length to the code length. e.g a K=7 rate 1/2 code (NASA Voyager) has a length of 7 so each data bit affects the current codec output and the next 6 bits. In practice this means that you need to supply 6 "flush" bits and send an additional 6 bits (or use some other tail biting scheme) to get full effectiveness of the Viterbi (most likely hood) decoder. That can be additional overhead but sometimes can be "burried" in the leader or inter frame gap so it has minimal effect.
Very often when the most powerful FEC is desired it is done in layers with different (complementary) codes. E.g. Viterbi inside code surrounded by a block code (e.g. Reed Solomon). These can be very effective in some channels. Modern HF data modes (e.g Pactor 2, 3, 4, WINMOR) often use layered convolutional and block codes for maximum efficiency and performance. But this whole coding and FEC thing has to be evaluated using the following: 1) What is the nature of the channel we are trying to code for. E.g. WGN on VHF/UHF or poor multipath on HF. Deep fading, burst errors etc. 2) What is the information content of the source. This is complicated using CODEC2 since some bits may be more "important" than others (not usually the case with text) Dave has done a good job of squeezing down the bits so it may be a reasonable approximation that they are all now of "equal" importance. 3) Since we are dealing with a audio signal the type and frequency of "acceptable" errors may not be simply related to standard Bit Error Rates. What is desired of course is FEC coding that maximizes the intelligibility of the voice using a particular propagation channel and S/N. I would suggest that there be more testing done on a HF channel simulator to evaluate the most effective coding scheme. E.g. A block scheme may be effective but it may be that a convolutional code (using flush bits where needed) might actually give better speech intelligibility even though it propagates a net higher bit error rate due to the way is is decoded. Although ideally one would select the best code for the current channel we really need to standardize on one (or perhaps 2) FEC schemes. (e.g. it is very likely different FEC schemes and different modulation schemes will be optimum for HF or VHF/UHF) Because of the complexity of the CODEC2 in forming the bits and the human factor in determining the intelligibility of speech at certain error rates we probably can't use some of the conventional WGN channel and BER analysis tools. The good news is that all the tools are available and coding up test implementations and using the HF channel simulator are pretty straightforward exercises. 73, Rick Muething, KN6KB ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
