The echo canceler needs to 'remember' what you just said so that it
can apply the inverse of that to the returning echo. They train up
based on the latency but when the latency is jumping all over the
place (jitter) they cannot train up properly. Lets say you said
something and 1 second later it came back. The echo canceler will
remember what you said 1 second ago and apply the inverse waveform to
cancel out the returning echo. So, now the echo canceler is trained
up at 1 second. Lets say the latency changes to 2 seconds, your echo
canceler is still applying the inverse wave form from what you said 1
second before. The echo and inverse wave form don't line up so the
returning echo isn't fully canceled and you hear it. Your echo
canceler is going to be constantly retraining to adjust for changes
in latency (jitter) and for small amounts of jitter it does a pretty
good job.
On Mar 10, 2006, at 8:45 AM, Herman Webley wrote:
On Fri, 2006-03-10 at 08:30 -0500, Matthew Crocker wrote:
On Mar 10, 2006, at 8:24 AM, Hugh L. Johnson wrote:
It's all about latency. The roundtrip ping times across my DSL
circuit
(Verizon ATM cloud) to my ISP's router (One hop away) are approx
25ms.
My ISP has a PRI which I use for all incoming and local outgoing
calls.
For these calls the quality is better than when I had an analog
phone at
home.
No, it is all about jitter. Latency is irrelevant unless it is above
150-200ms. Rapidly changing latency (aka Jitter) is what kills VoIP
quality because the echo cancelers can't train up properly.
How does jitter hinder the echo canceler training?
As an aside more jitter means more jitter buffer to avoid voice drop
outs. Of course a jitter buffer introduces additional latency equal to
the buffer. And of course additional latency equals worse echo :)
Loads of fun this VoPI is ...
-Herman
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Matthew S. Crocker
Vice President
Crocker Communications, Inc.
Internet Division
PO BOX 710
Greenfield, MA 01302-0710
http://www.crocker.com
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