[Speex-dev] Acoustic echo cancellation

LiMaoquan2000 LiMaoquan2000 at 126.com
Sun Apr 17 18:07:41 PDT 2011

> Hi, 
> I have a scenario in a mobile VoIP app that requires echo cancellation but 
> is somewhat different from what's described in the docs. 
> Audio is received from and sent to the network at 8000Hz. Each packet 
> contains 160 samples worth a playback of 20ms. 
> But the hardware requires aggregation for both playback and capture. So for 
> playback, I coalesce 4 packets in a buffer and queue them as a larger buffer 
> for playback. 
> On the send side, I read a large buffer (worth 4 packets) and send them out 
> over time 20ms apart. 
> I tried using speex_echo_playback just when a 160-sample packet arrives from 
> the network, before coalescing and speex_echo_capture just before a packet 
> is sent out to the network but that doesn't seem to work properly (doesn't 
> cancel any echo). 

The most likely reason is that you didn't align the far-end and near-end samples.
So the filter can not converge.

> So, in this scenario above, please recommend a good place to insert 
> speex_echo_playback and speex_echo_capture. Should I be just before the read 
> and write to hardware? In that case, should I use a larger "frame size" of 
> 160 samples x 4? 

Of course you can set frame size to 160*4. Otherwise you can feed samples 4 times
to the AEC if you don't want to modify the frame size.

> Thanks in advance, 
> Daniel. 
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