Narrow band energy and echo cancellation

Steve,
I can tell you a bit about the origin of CSS use in G.168.  CSS was
introduced to us in ITU SG-15/Q7 by Dr. Hans Gerlich of Germany.  It
is 44.1KHz because it was used with the artificial head and work done
on perception.  This should not be a big problem.  You can build the
signnal at 44.1KHz and then resample to 8KHz.

Your first question is a bit more sticky.  The solution is closely
held by most manufacturers (including me, at the time).  What I can
say is that there were several implementations.  Some were based on
performance, and some were based on narrowband detection.


Maurice Givens



Steve Underwood <steveu@dis.org> wrote in message news:<bgot71$hr1$1@hfc.pacific.net.hk>...
> Hi,
> 
> I am trying to implement a G.168 compliant echo canceller. I'm 
> conversant with most aspects of cancellation, including the problems of 
> mis-training on signal with strong continuous spectral lines. However, I 
> have never been forced to cure this problem before. :-)
> 
> It seems clear there is no way to get a canceller through the G.168 
> tests unless it freezes its adaption when the signal is too narrow band. 
> My question is how do people usually detect this condition? There is a 
> wealth of info on the web about most aspects of cancellers. However, on 
> the narrow band topic the only things are find are notes that you need 
> to deal with the problem.
> 
> I see several obvious robust solutions, but they require quite a lot of 
> computation. Just looking at the energy and declaring its a tone (or say 
> DTMF) if the energy is too constant seems to work, but I question its 
> robustness. Something makes me think I am missing an obvious 
> computationally lightweight solution. Can anyone enlighten me?
> 
> By the by. Does anyone know why the CSS in G.168 is specified at 44.1k 
> samples per second, when G.168 seems purely about 8k sample per second 
> telephony?
> 
> Regards,
> Steve