[Speex-dev] Speex-dev Digest, Vol 83, Issue 17

Li Maoquan limaoquan2000 at 126.com
Fri Apr 22 02:40:06 PDT 2011

>> Simply to say, in a quiet room, you can play a impulse signal and then find 
>> it's impulse response signal from the 
>> microphone. For example, if the delay between the impulse signal and its 
>> response signal range from 500 to 
>> 3000 cycles, you can buffer the far-end signal to 0-300 cycles and set the 
>> filter length to 4000. It is also called 
>> to align far-end signal and near-end signal. 
>> BTW: Speex AEC is sensiive to mismatch between sample rates of capturing 
>> and rendering. But most low-cost 
>> computer soundcards have this problem. 
>Based on your explanation above, I have attempted to measure the delay 
>between the near-end and far-end signal. I send in a sharp impulse sound, 
>have it play on the speaker and see when it comes out of the mic. I am 
>looking at peak amplitudes in each frame (160 samples) to identify the 
>impulse sound. 
>The time from when the sound enters the stack (before it is queued with the 
>speaker) to the time when it comes out of the mic ready to go out, varies 
>between 0.44 to 0.57 seconds. So I queue the far-end samples for 0.4 
>seconds, and then send them to speex_echo_playback. Also, I am using a 
>filter length of 0.2 seconds (1600 samples). Each outgoing (near-end) sample 
>coming from the mic is passed through speex_echo_capture to cancel the echo 
>and the output is sent out of the device. 
>Despite this, the cancellation does not seem to work. I have also tried 
>using speex_echo_cancellation. That doesn't help either. 
>What could I be doing wrong? Any suggestions? 
>Also, I am not using the speex encoder/decoder, just the echo cancellation. 
>I hope that's not an issue. Please correct me if it is. 
>Thanks for helping me with this! 

I guess you are not very clear on the concept of delay or align.

1. Each time, when you call speex AEC, you feed same samples of far-end signal
and near-end signal to it, right?

2. Far-end signal is a continuous flow sent to AEC and the speaker. Near-end
signal is also a continuous flow from microphone. You cannot take away any
samples from the flows or insert zeros into them when AEC is started.

3. The delay (let's call it D) between samples at the same position of near-end
signal and far-end signal is constant if acoustic echo path is fixed, and ADC
and DAC of the soundcard have exactly the same sample rate.

4. If the echo reverberation time is A cycles, We know the length (L) of AEC
filter must be longer then A, and delay between near-end signal and far-end
signal cound be set to (L-A)/2 in order to deal with drift of echo path.

For example, echo reverberation time of a meeting room is 2500 cycles, you can
set the length of the AEC filter to 4000, and buffer far-end signal to make
it lag (4000-2500)/2 cycles after the near-end signal.

If the filter length and delay are set properly, the filter coefficients curve
will be at the center of its range and it also have the maximum tolerance to the
echo path drift.

>> At 2011-04-21 03:00:01?speex-dev-request at xiph.org wrote: 
>> >> >>> 
>> >> >>> 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. 
>> >> > 
>> >> >Thanks for your response. Can you please explain what you mean by 
>> >> >align samples from near-end and far-end? And how is that usually 
>> >> >accomplished? 
>> >> 
>> >> You need to know the total delay caused by DAC buffer before speaker, ADC 
>> >> buffer 
>> >> after microphone and acoustic path between speaker and microphone. Simply 
>> >> to say, 
>> >> if you play an impluse signal and its first echo appears after N sample 
>> >> cycles, 
>> >> you can call N as the delay between y (echo in near-end signal) and x 
>> >> (far-end 
>> >> signal). Then you can buffer far-end signal for N-M cycles before sending 
>> >> to AEC. 
>> >> M is a little number (such as 100) in order to avoid filter failure when 
>> >> echo 
>> >> path drifts. 
>> >> 
>> >> 
>> >Thanks again. I am trying to model the delay between the near and far end 
> > >signals using a circular queue of length n. Every time a frame is received 
>> >and queued for playback, it is also entered into the queue. Each frame being 
>> >read from the mic is echo-cancelled ( speex_echo_cancellation ) using the 
>> >oldest frame in the queue if the queue is filled up, thus I am cancelling 
>> >the recorded frame using a playback frame that is N-frames old. 
>> > 
>> >I have played with different values of N from 2 to 50 (320 samples to 8000 
>> >samples), attempting to align the input and output but the cancellation 
>> >doesn't seem to work. The echo is steady as ever. 
>> > 
>> >Is this model correct and expected to converge with a right value of "N"? Or 
>> >do I need some other adaptation to account for drifts here. Right now, it's 
>> >a black box for me. I am not sure how to get some feedback from this system 
>> >to tune the AEC (and the delay parameters) correctly. 
>> > 
>> >Also, I did not follow the use of "M" in your description above and how it 
>> >helps with drifts. My queue stores frames (160 samples each). So a number of 
>> >100 samples seems too small. 
>> > 
>> >Btw, I am assuming that speex AEC API can be used even though I am not using 
>> >the speex encoder/decoder. 
>> > 
>> > 
>> > 
>> >> >> 
>> >> >>>> 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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