[Asterisk-Users] G.729? Worth it?

[Steve Kann]

On Jan 19, 2005, at 2:49 PM, Stewart Nelson wrote:

> The MOS (Mean Opinion Score) scale is:
> 5=Excellent; 4=Good; 3=Fair; 2=Poor; 1=Bad.
>
> Some values, taken from "Carrier Grade Voice over IP" by
> Daniel Collins:
>
> G.711          4.3
> G.729          4.0
> G.729AB        3.9
> GSM(full rate) 3.7
>
> The above scores assume no packet loss, minimal delay, no echo.
>
> However, IMO such scores are generally only useful for choosing
> among compression codecs.
>
> If you have plenty of bandwidth and minimal packet loss, you
> should use G.711, not only for better quality, but because it
> avoids issues with conferencing, DTMF relay, etc.  Also, if your
> ITSP has upstream routes that use a different compression scheme,
> G.711 avoids cascaded codecs, which sound really awful, MOS < 3
> for sure.
>
> If you don't have enough bandwidth to handle the desired number
> of simultaneous calls with G.711, you obviously need to use
> compression; IMHO G.729 is a good choice.
>
> If you have >1% packet loss (or packets effectively lost due to
> excessive jitter), then G.729 may actually sound better.  Lost
> G.711 samples are replaced with silence, sometimes with pops
> at the transitions.  OTOH, most G.729 implementations have
> "packet loss concealment", which continues the previous sound,
> gradually fading out.  With 5% loss, a good G.729 system sounds
> like a mediocre cellular call, but G.711 sounds terrible.

You can do PLC with any codec;  codecs like G729, speex, and iLBC 
include the actual "guess what sound is supposed to go here" part, but 
you can write a generic "guesser" for any codec, and the G.711 
specification Appendix 1 includes sample code for the "guesser" for 
G.711.   Any phone that is implementing PLC for G.729 should (imho, I 
don't really know) also implement PLC for G.711, unless the vendor just 
doesn't care about quality..


-SteveK