> -----Original Message-----
> From: Eric Wieling [mailto:eric@fnords.org]
> Sent: Friday, March 11, 2005 1:52 PM
> To: Asterisk Users Mailing List - Non-Commercial Discussion
> Subject: Re: [Asterisk-Users] Re: Incoming echo cancel
>
>
> Nenad Radosavljevic wrote:
>
{clip}> >
> > Anyone have an idea, why this type of echo happens ? As far as I have
> > read on the lists this type of echo should not occur at all, but it
> > simply does !
>
> I consider all this MMX stuff to be voodoo for echo problems.
>
> If you have a PRI and SIP phones then the echo occurs on the far end
> analog phone.
>
{clip}
I concur. The echo cancellation code is syncronised to the stream of samples
going to/from the T1 card so if it takes longer to process then it simply takes
longer to deliver the audio, much like the transcoding times of GSM vs Speex
(2ms vs ~30ms). I can only think of a few possible reasons the echo cancellation
code may not work as expected:
- A echo signal too complex for the style of filter
- Continuous variations in the 'length' of the echo tail on the
_incoming_side_ of the T1 causing the filters to continuously adapt (perhaps
caused by 'slips' in the interface/timing system onboard the card?)
- Coding errors in the MMX vs. non-MMX routines
- Fundemental differences between the audio signals being passed to the
customers depending on the service providers facilities (eg. variations in level
padding: http://yarchive.net/phone/digital_pads.html)
Of all of these, I remain convinced that in most cases the real issue is
something like the last. Namely that subtle differences between facilities are
interfering with the assumptions the mathmatics in mec2 use - it's a very
rigid model and makes use a number of fixed assumptions about average signal
levels, offsets and so on.
Kris Boutilier
Information Systems Coordinator
Sunshine Coast Regional District
