Thanks Steve, that was good reading.
----- Original Message ----- From: "Steve Underwood" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Saturday, May 08, 2004 9:21 PM Subject: Re: [Asterisk-Users] Low Bit Rate Codecs > Craig wrote: > > >Greetings all, > > > >I have searched all over and have found bits and pieces on low bit rate > >codecs, however I have found it very difficult to compare apples with > >apples. > > > >The conclusions I have come to are as follows, I would appreciate if > >anyone has some feedback, or point me to where I might find this sort of > >comparison in black and white.... > > > >G723.1 > >very low bit rate > >used commercially, not avail for * > >(I am currently using this codec in another commercial application and > >therefore it is my reference point) > > > > > G.723.1 is pretty much obsolete. You don't see it being used on anything > new. Most people do VoIP using RTP. The overhead of RTP is so huge, a > small saving on the codec makes little difference. People generally go > for G,729 now, which sounds considerably better. If you compare the > total bit rate for G.723.1 vs G.729 in RTP G.723.1 often comes out > considerably lower. This is because it works in 30ms blocks - you only > have 33 RTP packet overheads per second. You can choose to pack more > G.729 data into each RTP packet and even this up. > > The patent licencing for G.723.1 is a PITA, which hasn't helped it > achieve widespread use. There are two variants of G.723.1, with > different bit rates. The lower bite rate (5.something kbps) sounds > nasty. The higher rate (6.something kbps) sounds more reasonable. Using > 30ms blocks, it is not so compatible with *, which is geared to 20ms > block processing. A lost packet causes a 30ms hole, so it tends to be > less tolerant of packet loss than something working in smaller blocks. > It sounds awful for anything but a single pure voice. > > >G729a > >low bit rate > >slightly higher bandwidth usage than 723.1 ??? > >avail as a low cost add-on for * > >better quality that g723.1 ??? > > > > > Definitely better quality than G.723.1. This is definitely the > mainstream right now for VoIP. It is heavily patented, so free codecs > are not possible. There are several bit rate options, but almost > everyone uses the 8kbps variant. This sounds pretty good for its bit > rate, though I think there have been better codecs. In telephony you > need use something compatible with the far end, and G.729 seems to be > the current common ground. It is rather intolerant of packet loss. Some > people pack several G.729 blocks into a single RTP packet, to decrease > the RTP overhead. That makes it even less tolerant of packet loss. It > sounds awful for anything but a single pure voice. > > >iLBC > >Low bit rate > >slightly higher bandwidth usage than 723.1 and 729a ??? > >open source, no additional cost for * > >quality comparable to G729a > >stands up better ip paths suffering from latency and jitter ??? > > > > > iLBC has a much higher bit rate than G.729, but the voice quality is > about the same. Why does that make it interesting? Well, it is designed > to be much more tolerant of packet loss, and that makes it take more > bandwidth. The design of RTP makes that take so much overhead that the > total bit rate using iLBC isn't a huge jump from using G.729. However, > if you use a more efficient streaming mechanism - say IAX, or an RTP > like format with many calls packed in a packet - the total bit rate > difference starts to look wider. There, the increase in bits is so great > its quite likely to be the *cause* of packet loss, by clogging up the > channel. :-) > > Good old GSM 06.10 is worthy of consideration. Free of patents (at least > ones being actively pursued). Low compute requirements. Reasonable voice > quality. Somewhat more tolerant of background noise than the codecs > above. Although GSM networks don't use it much these days (they mostly > use the newer EFR and half rate codecs) it's still a very servicable > codec. Its bit rate lies between G.729 and iLBC. On a pure voice it > gives poorer quality than G.729. Add some background noise and it can > beat G.729. Its tolerance of packet loss is probably similar to G.729. > > Regards, > Steve > > _______________________________________________ > Asterisk-Users mailing list > [EMAIL PROTECTED] > http://lists.digium.com/mailman/listinfo/asterisk-users > To UNSUBSCRIBE or update options visit: > http://lists.digium.com/mailman/listinfo/asterisk-users > _______________________________________________ Asterisk-Users mailing list [EMAIL PROTECTED] http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
