[asterisk-dev] Jitterbuffer and FAXes with E1

[Klaus Darilion]

Zoa wrote:
> Because the jitter buffer will never be size zero if its a fixed jitter 
> buffer and will fluctuate if its a variable length jitter buffer.
> So whatever you choose, the timing will be altered.

I would think that a fixed jitter buffer is a fixed jitter buffer - 
thus, only buffering voice frames adding a fixed delay (thus reducing 
jitter but increasing delay) and do not change the audio data at all.

A dynamic jitter buffer of course is bad. Because of the dynamic 
behavior the audio samples have to be played back sometimes 
faster/sometimes slower.

Thus, why does static jitter buffer manipulate the audio?

@Martin: What happens if you call the Milliwatt() application via a 
jitter-buffer link? Do you hear frequency changes?

regards
klaus

> 
> Zoa
> 
> Martin Vít wrote:
>>>> So i've tried simulate fixed latency on my router (hold data for 200ms 
>>>> from ATA to asterisk) and disable jb at all. FAX are OK. If jbenable = 
>>>> yes - problem.
>>>>
>>>> There is something wrong with jbenable or am I missing something?
>>>>
>>>> Maybe there is somewhere activated PLC or some another filter? 
>>>> whenever jbenable is turned on?
>>>>
>>>> Codec is alaw
>>>>     
>>> It is correct that a perceptual jitter buffer will play with the timing, 
>>> and that will break things like faxing. 
>> I need to investigate this, because without jitter there is no chance 
>> to send fax on a little jittery network. (and there is no way to 
>> enable or disable jitter or change jitter on the fly yet)
>>
>> On production system I have asterisk 1.2 patched with slavs 
>> jitterbuffer implementation and digium 4port PRI. I've enabled SIP 
>> adaptive jitterbuffer and with the same ATA reregistred to this 
>> production system, i've send fax to my 1.4 with success.  (Jun 28 
>> 16:16:04 VERBOSE[8475] logger.c:     -- ***[JB LOG]*** adaptive 
>> jitterbuffer created on channel Zap/32-1)
>>
>> Complete scenario:
>>
>> FAX -> ATA SIP -> Asterisk 1.2 adaptive jitter -> Zap PRI ---> TELCO 
>> SWITCH ---> Zap PRI asterisk 1.4 ---> iaxmodem.
>>
>> this work even though jitterbuffer is enabled on patched asterisk 1.2 
>> which is almost the same as in 1.4. But there have to be some 
>> differences which broken fax transmitions.
>>
>>
>>> You need to disable clever 
>>> jitter buffering algorithms when you need an unbroken stream. Typically 
>>> VoIP equipment will do this automatically when they hear FAX tones.
>>>   
>> This is true for my testing ATA which is switching from adaptive to 
>> fixed jitterbuffer.
>>
>> So the question is, why are frames modified when jitterbuffer is 
>> active in 1.4 even though there is NO jitter (100mbit eth in lab 
>> environment). I've disabled PLC in codecs.conf.
>>
>> Festr
>> ------------------------------------------------------------------------
>>
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