[Asterisk-Users] Why echo occurs

[Steve Underwood]

Robert Hajime Lanning wrote:

><quote who="Steve Underwood">
>  
>
>>Can you show me an ad for an IP phone which doesn't say it includes an
>>echo canceller? A real phone, I mean. Not some thrown together half
>>baked softphone, many of which do a very poor job.
>>    
>>
>
>I haven't once talked about soft phones.  I don't use them.  I am
>talking about hardphones that talk SIP.
>
>Take the grandstream phones.  Put them back to back, and I gaurentee
>you will never hear echo, unless you are in the same room.  Then you
>can put the handsets together and get all the screech you want.
>
>I have not found anywhere that is says it has an echo canceler.
>  
>
Maybe http://www.grandstream.com.cn/BT100_Spec_cn.pdf is a bit confusing 
if you can't read Chinese, but I think G.168 should be easy to identify :-)

>>Who introduced a loop into the discussion?
>>    
>>
>
>I did.  Because acoustic spill would most likely cause a loop.
>
>Why do I get the feeling you are trolling?  You are the only
>one that brought up acoustic spill.  Which, by the way, is
>usualy controled by directional mics and adjusted gains.
>  
>
Why do I get the feeling you haven't a clue what you are talking about? :-)

Acoustic spill gives basically the same effects as hybrid echo, except 
acoustic spill tends to be more variable over time. Hybrid echo also 
bounces back and forth when both ends are causing echo, but the first 
echo is so much stronger than the subsequent ones that you tend not to 
notice them.

I have worked on echo cancellation, and I know the acoustic spill issue 
is serious. In early GSM phones it was often easy to fool the canceler, 
and GSM to GSM calls would suffer really awful echo. They seem to have 
improved the cancelers a lot in the last few years, and its rare to get 
this problem today. This is a broad issue. Echo cancelers have generally 
improved a lot. The latest version of G.168 is a very different document 
from the early versions, and incorporates tests for a lot of the problem 
issues found in earlier canceler designs.

How do you control acoustic spill within a phone through the use of 
directional microphones? Adjusting gains mitigates the issue a bit, but 
is hardly a solution. These are just bodges, not solutions.

Regards,
Steve