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Hi all, I have few questions on Low Delay speech coders. As we all know that G.728 is a Low delay speech coder and is very complex interms of computations. This speech coder is extensively used in Cable Modem applications. Here are my questions - Why is the "low delay" factor so important in Cable modem systems despite of the fact that it is very complex interms of computations - Why can't we use either G.729AB or G.723.1 coders (Less complex than G.728 Vocoder) in Cable modem applications. Somebody, please answer these questions?? Thanks Rao

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Rao: The big advantage of G.728 is the voice quality. It rates as "toll quality" with a MOS score of 4.0. G.723.1 won't produce nearly as good a quality; MOS will be around 3.7. G.729A will achieve 3.8 or 3.9. These are clean channel scores. With tandeming, G.729 will degrade much more than G.728. G.729 and 723.1 also perform poorly with music whereas G.728 does quite well. The delay issue can be important. The frame size of G.723.1 is 30 ms. When analyzing echo, the round trip time is therefore near 60 ms for G.723.1 but can be less than 5 ms for G.728. The difference may allow you to eliminate echo cancellers in some networks, or at the least have a shorter tail length with resultant improvement in performance. Over a satellite, the coding time makes little overall difference since the satellite contributes most of the delay but in an enterprise network the coding time may dominate total delay. By the way, I do not know of any cable modems that use G.728. All the modems I know of are using plain old G.711. Rao wrote: > Hi all, > > I have few questions on Low Delay speech coders. > > As we all know that G.728 is a Low delay speech coder and is > very complex interms of computations. This speech coder is > extensively used in Cable Modem applications. > > Here are my questions > > - Why is the "low delay" factor so important in Cable modem > systems despite of the fact that it is very complex > interms of computations > > - Why can't we use either G.729AB or G.723.1 coders (Less > complex than G.728 Vocoder) in Cable modem applications. > > Somebody, please answer these questions?? > > Thanks > Rao > > > To Join: > > To Post: > > To Leave: > > Archives: http://www.egroups.com/group/speechcoding > > Other DSP-Related Groups: http://www.dsprelated.com

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Hi Low delay speech coders have the following advantages : 1) Delay increases the necessity of Echo cancellers. So if a low delay coder is used, there may not be much need of echo canceller. Even if an echo canceller is needed, its complexity will be low for the low delay case.(This appears to me to be the reason for using g728 in cable modems) 2)Higher delay causes discomfort to the conversing parties 3)When multi-party conferencing is done, at the bridging site, the different channels are first decoded, then summed up & then encoded again. Clearly low delay coders will ensure lower time spent in this process. 4)Lastly, if delay of speech coder is less, more delay can be provided to the channel coder, permitting more interleaving to be done. Regards, satheesh.s -----Original Message----- From: saiprasad [mailto:] Sent: Sunday, October 22, 2000 12:58 PM To: Subject: Re: [speechcoding] Question on G.728 Codec Dear Rao, - In G.728 is also frame based codec , frame is defined 20 -samples caters 2.5 milliseconds delay --------------> to invoke this codec U have to wait for 2.5 ms only. - In G729 starting 15 ms later for every 10 ms delay to invoke this codec. - In G723.1 caters 35 ms delay from above figures , u can say thay G.728 is low delay to start processing - To perform algorithm , one may need more time to process, - In processing time wise G729 algorithm has less time. - next G723.1 - next G728 so processing time is more for a G.728. Saie. -----Original Message----- From: Rao <> To: <> Date: Thursday, September 21, 2000 9:37 PM Subject: [speechcoding] Question on G.728 Codec > >Hi all, > > I have few questions on Low Delay speech coders. > > As we all know that G.728 is a Low delay speech coder and is >very complex interms of computations. This speech coder is >extensively used in Cable Modem applications. > > Here are my questions > > - Why is the "low delay" factor so important in Cable modem > systems despite of the fact that it is very complex > interms of computations > > - Why can't we use either G.729AB or G.723.1 coders (Less > complex than G.728 Vocoder) in Cable modem applications. > > Somebody, please answer these questions?? > >Thanks >Rao > >To Join: > >To Post: > >To Leave: > >Archives: http://www.egroups.com/group/speechcoding > >Other DSP-Related Groups: http://www.dsprelated.com To Join: To Post: To Leave: Archives: http://www.egroups.com/group/speechcoding Other DSP-Related Groups: http://www.dsprelated.com

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Dear Rao, - In G.728 is also frame based codec , frame is defined 20 -samples caters 2.5 milliseconds delay --------------> to invoke this codec U have to wait for 2.5 ms only. - In G729 starting 15 ms later for every 10 ms delay to invoke this codec. - In G723.1 caters 35 ms delay from above figures , u can say thay G.728 is low delay to start processing - To perform algorithm , one may need more time to process, - In processing time wise G729 algorithm has less time. - next G723.1 - next G728 so processing time is more for a G.728. Saie. -----Original Message----- From: Rao <> To: <> Date: Thursday, September 21, 2000 9:37 PM Subject: [speechcoding] Question on G.728 Codec > >Hi all, > > I have few questions on Low Delay speech coders. > > As we all know that G.728 is a Low delay speech coder and is >very complex interms of computations. This speech coder is >extensively used in Cable Modem applications. > > Here are my questions > > - Why is the "low delay" factor so important in Cable modem > systems despite of the fact that it is very complex > interms of computations > > - Why can't we use either G.729AB or G.723.1 coders (Less > complex than G.728 Vocoder) in Cable modem applications. > > Somebody, please answer these questions?? > >Thanks >Rao > >To Join: > >To Post: > >To Leave: > >Archives: http://www.egroups.com/group/speechcoding > >Other DSP-Related Groups: http://www.dsprelated.com

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