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Line echo cancellation - conditions for freezing adaptation

Started by Unknown March 10, 2007
Hi com.dsp-ers,

I know that there are some scenarios in which a line echo canceller
should suspend the process of adapting its coefficients:

1. Double talk (the most obvious case)
2. Narrow band reference signal detected
3. Numerical overload detected

I wanted to broadcast a call for ideas for detecting scenarios 2 and
3. For 2, I am currently thinking of putting in an 8 channel filter
bank and using the relation between a short term estimate of power
recorded at the output of the 8 channels to infer narrow band signals
and freeze adaptation. For numerical overload, short term estimate of
reference signal power is calculated and adaptation frozen in case a
defined upper limit is crossed. So, what I would like to ask is:

1. Are there any obvious deficiencies (that I am overlooking) in my
approach.
2. Is there a better/standard way of doing either of the two tasks? (I
could not find much in the several EC papers I went through, but these
were mainly pretty old papers by pioneers of EC).

Any help is greatly appreaciated.

Regards,
Ljubisa Bajic

eternal_nan@yahoo.com wrote:
> Hi com.dsp-ers, > > I know that there are some scenarios in which a line echo canceller > should suspend the process of adapting its coefficients: > > 1. Double talk (the most obvious case) > 2. Narrow band reference signal detected > 3. Numerical overload detected > > I wanted to broadcast a call for ideas for detecting scenarios 2 and > 3. For 2, I am currently thinking of putting in an 8 channel filter > bank and using the relation between a short term estimate of power > recorded at the output of the 8 channels to infer narrow band signals > and freeze adaptation. For numerical overload, short term estimate of > reference signal power is calculated and adaptation frozen in case a > defined upper limit is crossed. So, what I would like to ask is: > > 1. Are there any obvious deficiencies (that I am overlooking) in my > approach. > 2. Is there a better/standard way of doing either of the two tasks? (I > could not find much in the several EC papers I went through, but these > were mainly pretty old papers by pioneers of EC). > > Any help is greatly appreaciated.
There is very little information about anything important in echo cancellation on the internet. There is a lot about the blatantly obvious basics. There is also a lot about achieving the fastest convergence - e.g numerous papers on FAP. That is seldom make or break for a design, though. When it comes to the subtler things, like you are looking at now with narrowband energy, there is very little info. I don't think there is currently a "standard" way to look for narrowband energy in a canceller. An FFT might be one way. What happens if your spectral line signal is rich in harmonics, though? Looking for a peaky autocorrelation function certainly works, though it talks some compute. Low complexity things which directly look for narrowband energy, such as TKEO, might be more direct. Steve

eternal_nan@yahoo.com wrote:

> Hi com.dsp-ers, > > I know that there are some scenarios in which a line echo canceller > should suspend the process of adapting its coefficients: > > 1. Double talk (the most obvious case)
This is the most important and not so obvious case.
> 2. Narrow band reference signal detected
First option: you can trigger on the typical narrowband signals like DTMF, dialtone and other signaling tones. Second option: a narrowband signal can be approximately detected by the energy threshold and the average peak to RMS ratio.
> 3. Numerical overload detected
This one is relatively simple, however it depends on the hardware.
> > I wanted to broadcast a call for ideas for detecting scenarios 2 and > 3. For 2, I am currently thinking of putting in an 8 channel filter > bank and using the relation between a short term estimate of power > recorded at the output of the 8 channels to infer narrow band signals > and freeze adaptation. For numerical overload, short term estimate of > reference signal power is calculated and adaptation frozen in case a > defined upper limit is crossed. So, what I would like to ask is: > > 1. Are there any obvious deficiencies (that I am overlooking) in my > approach.
#2: It may work although it is complex and demands a lot of computation. #3: Not a good idea, since you don't know what is the coupling between the transmit and receive path.
> 2. Is there a better/standard way of doing either of the two tasks?
Keep it simple. The main advantage of the SG LMS adaptation is its simplicity and low computation requirements.
> could not find much in the several EC papers I went through, but these > were mainly pretty old papers by pioneers of EC).
I doubt if you can find any advanced information other then the common knowledge from the textbooks. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Thanks for the advice guys.

On Mar 11, 12:42 am, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
wrote:
> eternal_...@yahoo.com wrote: > > Hi com.dsp-ers, > > > I know that there are some scenarios in which a line echo canceller > > should suspend the process of adapting its coefficients: > > > 1. Double talk (the most obvious case) > > This is the most important and not so obvious case. > > > 2. Narrow band reference signal detected > > First option: you can trigger on the typical narrowband signals like > DTMF, dialtone and other signaling tones. > Second option: a narrowband signal can be approximately detected by the > energy threshold and the average peak to RMS ratio. > > > 3. Numerical overload detected > > This one is relatively simple, however it depends on the hardware. > > > > > I wanted to broadcast a call for ideas for detecting scenarios 2 and > > 3. For 2, I am currently thinking of putting in an 8 channel filter > > bank and using the relation between a short term estimate of power > > recorded at the output of the 8 channels to infer narrow band signals > > and freeze adaptation. For numerical overload, short term estimate of > > reference signal power is calculated and adaptation frozen in case a > > defined upper limit is crossed. So, what I would like to ask is: > > > 1. Are there any obvious deficiencies (that I am overlooking) in my > > approach. > > #2: It may work although it is complex and demands a lot of computation. > #3: Not a good idea, since you don't know what is the coupling between > the transmit and receive path. > > > 2. Is there a better/standard way of doing either of the two tasks? > > Keep it simple. The main advantage of the SG LMS adaptation is its > simplicity and low computation requirements. > > > could not find much in the several EC papers I went through, but these > > were mainly pretty old papers by pioneers of EC). > > I doubt if you can find any advanced information other then the common > knowledge from the textbooks. > > Vladimir Vassilevsky > > DSP and Mixed Signal Design Consultant > > http://www.abvolt.com