On 09/13/2011 08:56 AM, Gustavo Santos wrote:
I'm trying to use Asterisk as a PSTN simulator to run performance tests
for echo cancellation algorithms. I'm using the following configuration:
SIP <-----> Asterisk 1 <----> Asterisk 2 <----> Echo()
Asterisk 1 and Asterisk 2 are connected using E1. Echo() is the dialplan
application.
The problem is the high delay using this configuration: 20 ms only in
Asterisk 2. I've read the source code of chan_dahdi, and I think the
channel has a 20 ms "buffer" (160 samples). Algorithms like mg2 and kb1
are configured to accept 128 taps (16 ms), so 20 ms is too high.
Someone knows how I can reduce the delay to at least 10 ms? Should I
change something in the source code?
20 milliseconds is far from a 'high' (long) delay. Asterisk handles
audio in packets, it does not directly switch TDM streams. As a result,
there is always going to be (at least) the delay of one packet time for
audio passing into Asterisk and back out via the Echo() application.
This is unavoidable.
An alternative solution would be to send a call into Asterisk2 and have
it dial back to Asterisk1 (and then back to the originating endpoint)
and bridge those two calls in Asterisk2; if both calls are on the same
E1, then Asterisk will let the DAHDI hardware directly connect the two
channels, resulting in a 1 or 2 millisecond delay.
But realistically... configuring an echo canceller with only a 16ms
window of operation is not very practical. Sending a call through *any*
network element that packetizes the audio will result in a delay longer
than 16ms.
--
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kflem...@digium.com | SIP: kpflem...@digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at www.digium.com & www.asterisk.org
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