Does G.729 provide better voice quality than GSM?It depends. The bit rate of G.729 is a lot lower, so it starts with a disadvantage. To overcome that, they made it a lot more complex and tuned to the human voice. The result is for a single person talking G.729 sounds pretty good. When there are other sounds mixed in, GSM degrades more gently. The bit rate advantage of very low bit rate codecs isn't much of an advantage in most VoIP work. A single audio RTP stream carries a huge overhead, if the latency is kept low enough for a two way conversation. 8kps vs 13.2kbps sounds like a big advantage. Add the RTP overheads and the difference looks much smaller.
(a question for people who have tried both)
Actually, the GSM we use here hasn't been used in GSM networks for years. The use either EFR (enhanced full rate) or half rate. EFR is some sort of CELP based codec (I can't remember the details) running at the same bit rate as the original GSM codec - same bit rate; higher quality on a single voice; less tolerant of more complex sounds. Half rate is a 5.95kbps VSELP codec. I think the half rate codec can beat the 8kbps G.729, but it depends a lot on the implementation. Nokia phones suck on half rate. Some others sound pretty good.
Codec performance is difficult to compare, as circumstances affect the results a lot. Just a little background noise can often make a big difference. Codec developers spend a fortune on trials of new designs before drawing any real conclusions about them. As a crude illustraion of the problem, look at comments people post about iLBC. They range from awful to excellent. Personally, I know I would have to use such a codec a lot before making a meaningful comment.
Regards, Steve
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