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From: maury001 on 8 Sep 2009 17:36
On Sep 8, 4:22 pm, maury...(a)core.com wrote:
> On Sep 8, 11:19 am, "steveu" <ste...(a)coppice.org> wrote:
>
>
>
>
>
> > >On Sep 4, 8:32=A0am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
> > >> createdon2003 wrote:
> > >> > may be it will be more clear by a diagram.
>
> > >> > --far_end-------------------|ENC|-------|DEC|--------//
> > >> > =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
> > =
> > >=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | speaker
> > >> > =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
> > =
> > >=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 \\
> > >> > =A0 AEC
> > >> > =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
> > =
> > >=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0LEM with
> > >> > T_60~60ms
> > >> > =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
> > =
> > >=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
> > >> > --to far end----------------|DEC|-------|ENC|--------O microphone
> > >> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
> > =A0=
> > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> > >> > Now that i have given the diagram (hope the situation is clearer now)
> > c=
> > >an
> > >> > you suggest any good material which can help me design a AEC for
> > this
> > >> > condition.
>
> > >> You can not. The AEC is not going to work since you have vocoders in
> > the
> > >> loop.
>
> > >> Vladimir Vassilevsky
> > >> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
>
> > >If that blanket statement were true, then echo cancellers with G.729
> > >vocoders would not work.  Nor would channels with ADPCM enc/dec.  I
> > >would think that you are obviously restricting your statements to the
> > >high-compression codecs/vocoders.  The echo canceller would still
> > >work, just not very well.
>
> > That's an odd thing for someone like you to say.
>
> > The non linearities in G.711 limit an echo canceller to about 30 dB or so
> > of ERLE, with some nasty quirks at low levels that demand NLP.
>
> > ADPCM (e.g. 32kbps G.726) reduces the potential ERLE considerably, and NLP
> > has to be used quite aggressively. EC with ADPCM is not all that useful..
>
> > You're lucky if you can even make an EC converge through an G.729 channel,
> > and the results are not useful. The quirks are just too severe. You should
> > know that as  well as anyone.
>
> > Steve- Hide quoted text -
>
> > - Show quoted text -
>
> The original statement was that they don't work.  They work, but not
> very well.  Because they don't converge very well to the desired, is
> not to say they don't converge. I'm being very theoretical because I
> have a "thing" about blanket statements.  There is a lot of
> misconception about convergence and stability.  You can have the
> algorithm converge and not be stable.  You can have it be stable and
> not converge very well to the desired.  You can actually get a LMS-
> based algorithm to converge, with small error, to an unstable system.
> Analysis of LMS filters typically looks at the weight difference
> between the desired and the estimated.  If that difference is
> relatively large, it is said the filter doesn't converge.  It may very
> well converge, just not to want you want.  LMS is derived from minimum
> squared error, not weight difference.  This subtlety is often lost
> when talking about LMS-based algorithms.  When a LMS-based algorithm
> produces a relativly stable estimate, it is working, and working the
> way designed - minimizing the squared error.
>
> One analysis, using weight difference, looked at adaptive filters in
> tandem and came to a conclusion that was arguably not supported by the
> data.  I did an analysis of the same configuration, but using minimum
> squared error analysis to show the result consistent with the data.
>
> Yes, the NLP becomes very "useful" under these conditions. u-law will
> converge to about 30 dB ERLE, and a-law to about 34dB.  I have had
> echo cancellers converge on ADPCM.  Not to the 34 dB of a-law, but
> they do converge.  The reason for mentioning Volterra kernel is that
> this is a non-linear model.  I have always been interested in how well
> a Volterra-based echo canceller would work on these signals (and not
> just a 2nd-order Volterra), just never had the time.
>
> Finally, I admit I was nit-picking.
>
> Maurice Givens- Hide quoted text -
>
> - Show quoted text -


Oops, G.728 is the code-excited linear prediction. This is what I
meant originally, the 16-bit coder based on code-excited LP, not G.
729.

Maurice
From: Vladimir Vassilevsky on 8 Sep 2009 18:12


maury001(a)core.com wrote:


>>The reason for mentioning Volterra kernel is that
>>this is a non-linear model. I have always been interested in how well
>>a Volterra-based echo canceller would work on these signals (and not
>>just a 2nd-order Volterra), just never had the time.

Besides the numerous difficulties such as getting stuck in the local
minima, it would not work because you are dealing with the loss of
information rather then with the dynamic nonlinearity.

>>Finally, I admit I was nit-picking.
> Oops, G.728 is the code-excited linear prediction. This is what I
> meant originally, the 16-bit coder based on code-excited LP, not G.
> 729.

There you go :)))) G.728 is a high bitrate engine which behaves similar
to the ADPCM.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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