[Speex-dev] Echo Cancellation

[Jean-Marc Valin]

And all this time I thought that you couldn't really do any worse than
resampling using linear interpolation. You've certainly been very
creative here! Also I don't quite understand what your "nibbling" thing
is, but it sure sounds like it would be completely desynchronising the
AEC before it even has a chance to converge...

So the moral of the story is: start by doing the basic audio work right
and then see if AEC (or anything else) works.

	Jean-Marc

zmorris at mac.com a écrit :
> Hi All, I have tried using echo cancellation, and I think it doesn't
> work well, because of how I resample.  I take the raw 44100 Hz on my mac
> and just do a loop averaging every 5.5 samples to 1 sample to get
> 8018.18 Hz, and feed that to speex in 160 sample buffers.  I realize
> that this introduces some aliasing, and I may try the new speex
> resampler, but I find it hard to believe that the noise intruduced would
> be higher than normal background anyway, and it sounds clear enough when
> I just do playthrough for testing.
> 
> The problem is, this uses nibbling, which means the input jitters in
> time in relation to the output.  I also don't know what Apple does under
> the hood for playback, they almost certainly nibble as well.  This means
> that the echo canceler will nearly always see a different time offset
> for the current buffer pair, than for the previous ones.
> 
> So it seems to me that in order for echo cancellation to work, it has to
> tolerate nibbling up to roughly 160/8000 or 1/50th of a second. 
> Assuming that 2 nibbles can happen, one on input and one on output, this
> becomes 1/25th of a second.  In other words, feeding the input and
> output buffers shifted in time +/- 1/25th of a second to the echo
> canceler should always give roughly the same output, but I think that
> may not be happening right now.
> 
> Is this issue addressed?  Perhaps PCs can set their sampler to run at an
> exact clock speed to avoid nibbling, on the same sound card as output,
> but that doesn't seem to be available easily on the Mac.  Because of all
> the latencies and nibbling that can happen in various sound
> engines/hardware, I think matched input/output is a pipe dream.
> 
> Off the top of my head, there are other methods, like finding the
> maximum covariance, that can determine the phase of 2 sounds fairly
> easily.  Also, it might work to keep track of 4 phases in the echo
> canceler instead of 1, to account for the 4 types of lag seen on each
> input/output buffer pair:
> 
> input    output
> ---------------
> 0    0
> 0    nibbled
> nibbled    0
> nibbled    nibbled
> 
> However, this will not work when the nibbling is a non-even multiple. 
> For example nibbling 160 sample speex buffers from 256 sample hardware
> buffers.  Which makes me think that each buffer should be analyzed with
> something like covariance to find the phase, or else it should adapt
> somehow to find what type of nibbling is occurring, if it's going to
> keep a running average for phase.  It could also queue up several
> buffers to do this more accurately (which introduces lag), perhaps it
> already does this.
> 
> I have not yet tried the new 1.2 beta.
> 
> Thanx,
> 
> --Zack
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