Many years ago, about the late 70s or early 80s, when the deployment
of echo cancelers began in the telephone network, there were a few
engineers at Bell Labs experimenting with echo canceler based full-
duplex modems.
One of them was the late J. J. Werner who extended the echo canceler
work of one of either Muller or Mueller (I don't remember which one).
Previously, modems were either half-duplex or split band because of
the echo problem within the modem and network far end temination.
Early network cancelers employed a technique designed by Geigel, that
is still used, to detect the double talking condition for voice
calls. Duplex modems are by defintion double talking devices.
Unfortunately, Geigel's algorithm, and some others that have been
tried, can misbehave in the presence of modems signals, causing the
cancelers to update during data signalling. If the network echo
canceler updates at all during a call, this is catastrophic for full-
duplex modems.
It took several years before the modem community in Bell Labs finally
convinced the network echo canceller community that it would just be
better if the network echo canceller could be disabled in the presence
of modem signal. The modems would then take care of the network
produced echoes themselves. Eventually, all network echo cancelers
were updated or replaced with ones that could be disabled.
The disabling technique is/was a modification to the echo supressor
disabling that already worked for existing modems that was simply the
V.25 2100 Hz answer tone sent by the called modem. The tone disabling
was modified to apply a 180 degree phase reversal in the tone every
450 msec. Since the full-duplex modems, starting with V.32 needed to
see about a second of the answer tone before responding anyway, there
would be at least two phase reversals seen in the network without any
transmission from the calling modem.
Today we still want the network echo-canceler disabled when a modem
call is initiated, so that the modem can train a "far" canceler to
deal with the remote network echo/echoes. The types of modems that
still use this technique include V.32, V.34, V.90 and V.92. A network
echo canceler without tone disabling is unusable in the telephone
network.
So, what is needed for the network echo canceler is a detector that is
looking for the 2100 Hz tone (in either direction of tranmission) and
thence looking for phase reversals every 450 msec. Obviously, there
are tolerances on the frequency of the tone, duration between
reversals, actual amount of phase change, signal level of the tone,
and SNR of the tone. Search through ITU standard V.25 for information
about the tone.
Regards,
David Shaw
--- In e..., mseyedhossainy@... wrote:
>
> dear all Hi
> i want to know what is disable tone detector and why is it use in
echo canceller.
> thanks
>