[Asterisk-Users] Low Bit Rate Codecs

[Steve Totaro]

Thanks Steve, that was good reading.


----- Original Message ----- 
From: "Steve Underwood" <steveu at coppice.org>
To: <asterisk-users at lists.digium.com>
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
> Asterisk-Users at lists.digium.com
> http://lists.digium.com/mailman/listinfo/asterisk-users
> To UNSUBSCRIBE or update options visit:
>    http://lists.digium.com/mailman/listinfo/asterisk-users
>