[asterisk-dev] Zaptel Echo Cancellation

[Steve Underwood]

Andrew Kohlsmith wrote:

>On Thursday 03 August 2006 11:09, Eric "ManxPower" Wieling wrote:
>  
>
>>>Is there not an inherient delay of several ms due to CPU and encoding
>>>overhead etc or is this not a factor in the calculations? I'm just
>>>wondering what kinds of delays play into this or is the only thing that
>>>matters the delay on the PSTN before the signal hits the card?
>>>      
>>>
>
>  
>
>>The PSTN delay is what matters.  Echo must be removed at the PSTN
>>interface.
>>    
>>
>
>When the echo cancellation is being done in software on the PC (i.e. in 
>zaptel), you need to account for the PCI delay and processing delays as well, 
>do you not?  That is the advantage of the hardware echo cans; they work on 
>the echo "closer" to the source.
>
>  
>
The closer to the echo issue has little to do with that. Its about the 
adaption process of canceller's filters.

Zaptel cards work in 8 sample chunks. Thus only about 2ms of your 16 ms 
canceller is lost in latency.

Steve