Hi. Ignoring the fact that iLBC is not open source, and concentrating purely on voice quality...

Does anyone have quantitative info / comparisons or opinions (I am going to regret I said that word) on the quality of Speex versus that of iLBC. Also, is one more suited to low bandwidth application than the other?

Cheers.

(Disclaimer: I'm the author of Speex)

I've set up a
Speex codec comparison page on the Speex site that compares codec features (not quality). You can also listen to Speex samples here.

As for Speex vs iLBC specifically, I think I won't be the only one think that Speex sounds better at equal bit-rate. This is not because I'm saying I'm better than the guys who wrote iLBC, but because of a basic design decision taken by the iLBC author(s). The main difference between iLBC and other CELP-based codecs (like Speex and most other codecs) is that iLBC encodes frames independently from each other. This tends to increase robustness to packet losses (can't say how much) at the price of higher bit-rate for equal quality. However, it's been suggested that a better way of handling packet loss is to use the lower bit-rate of CELP codecs to add redundency (send some packets twice).

agree.
In addition, packet loss compensation technique for voice aslo usefull.