[Speex-dev] Prebuffering best practices

David Barrett dbarrett at quinthar.com
Tue Jun 14 23:34:17 PDT 2005

Ah, I'm sorry, I have read the manual and believe I have a reasonably 
good grasp on how to use the Speex encoder and decoder altogether.  In 
fact I've been using it with great success in my P2P SIP/RTP VoIP 
application for almost a year now; it's been working wonderfully and I 
can't thank you enough.

However, the manual makes no mention of the jitter buffer, nor does it 
(so far as I can tell) address the questions I've raised.  The list 
archive has been more helpful in this regard, but I still have holes in 
my understanding.

Specifically, I'm trying to refine my working system to work even better 
over high-jitter connections.  I'm eager and open to using the jitter 
buffer as you suggest, and I see how it can impove playback quality in 
high packet-loss situations considerably, but I haven't yet wrapped my 
head around what benefit it offers in reliable, high-jitter environments.

So far as I can tell, the only solution to jittery transport is an 
adequate prebuffer, and thus I'm looking for advice on how to determine 
what "adequate" means.

Likewise, I can easily broadcast anywhere from 33ms to 500ms audio 
packets (I currently use 50ms), but I'd like to hear your real-world 
advice on what the ideal packet size is I should be using.

Thanks for all your help!


Jean-Marc Valin wrote:
> I strongly suggest you start by reading the Speex manual (you can skip
> the technical parts about CELP). If you still ask questions, then post
> them.
> 	Jean-Marc
> Le mardi 14 juin 2005 à 22:30 -0700, David Barrett a écrit :
>>Ok, this is a silly question, but what does the jitter buffer do?  I'm 
>>really new to audio, so please bear with me.
>> From what I gather (primarily from the list archive), the jitter buffer 
>>is a wrapper around the Speex decoder.  I give it the packets I receive, 
>>in whatever order I receive them, and then it gives me back a clean 
>>stream of audio samples.  But what I don't entirely understand is how 
>>this is different from just working with the decoder directly.
>>Right now, I dump my RTP packets direct into the Speex decoder, and then 
>>queue the output for playback.  This works reasonably well.
>>However, it doesn't accomodate dropped packets well.  If I drop samples 
>>10-20, I'll just queue 0-10 and then 20-30 immediately after, which 
>>isn't great.  I think I read the jitter buffer will fabricate a fake 
>>replacement for the missing samples 10-20, and thus improving quality of 
>>playback.  Is this correct?
>>But what else does it do?  I see mention of "clock skew", but I don't 
>>know what that means in this context.  What am I missing?  Most 
>>importantly, what does it have to do with jitter, and how can I use it 
>>to solve my problems?  Specifically:
>>1) Assuming lossless, in-order, but highly irregular delivery of packets 
>>(as I'm witnessing), what advantage does the jitter buffer offer over 
>>going straight to the Speex decoder?
>>2) Assuming samples arrive at an average rate of 22KHz, but arrive in a 
>>highly irregular fashion, is there any way to ensure regular playback 
>>other than to just wait some "prebuffer" duration before beginning 
>>playback?  How do I pick the smallest prebuffer duration to accomodate a 
>>given connection's jitter?
>>3) Assuming I want to deliver samples at a rate of 22KHz, what's the 
>>best graularity at which to encode and broadcast?  Granted, I need to 
>>stay beneath the MTU.  But should I be going for the largest granularity 
>>that fits under the MTU, or should I be going for the smallest 
>>granularity that my CPU can churn out?
>>Jean-Marc Valin wrote:
>>>Have you looked at the Speex (adaptive) jitter buffer? See
>>>	Jean-Marc
>>>Le mardi 14 juin 2005 à 17:50 -0700, David Barrett a écrit :
>>>>What is the best way to pick a prebuffering length for a streaming audio 
>>>>application using UDP transport?
>>>>I'm using Speex in a VoIP application with RTP transport, currently with 
>>>>a fixed 500ms prebuffer on the playback side.  However, I'd like 
>>>>something a bit more adaptive to accomodate high-jitter connections.
>>>>For example, in one test configuration there is a very low average 
>>>>round-trip latency (50ms), but it spikes all over the place (sometimes 
>>>>10ms, sometimes 500ms).  Thus I can't make my prebuffer duration 
>>>>proportional to latency, but somehow proportional to "jitter".  But I'm 
>>>>not sure the best way to quantify this, nor how to tranform that into a 
>>>>reasonable prebuffer length.
>>>>Thus I'm curious what experience you've had in this area, and what you 
>>>>can recommend as a good way to adaptively compute a prebuffer duration. 
>>>> Thanks!
>>>>Speex-dev mailing list
>>>>Speex-dev at xiph.org
>>Speex-dev mailing list
>>Speex-dev at xiph.org

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