Reply by Maurice Givens August 5, 20032003-08-05
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 <> wrote in message news:<bgot71$hr1$>...
> 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