Shaw, David G (David) wrote:|
>The network generally has ECs for voice. These are
>very seldom co-located with the central office equipment.
The network generally does *not* have EC for voice. A phone to phone
call across town is not cancelled at all. The only suppression of echo
is by the hybrids at the 2-4 wire transitions. These seldom give more
than 20dB rejection (often its just 12dB, which is the typical spec for
minimum performance), which is fine for short delay calls. You actually
want some feedback to your ear, or the call sounds dead. With the short
delay for a call across town it sounds pleaant and reverb-like. Thus, in
the days when the traditional phone network evolved there was no
incentive to make a super-duper hybrid, even if such a thing were
possible. The hybrid basically just needed to keep enough control of the
echo to prevent howling.
Only when the delay is longer (usually due to a cellphone or VoIP phone
being one of the parties, but an international call would extend the
delay too) does the returned energy become an annoying echo. Here you
need to remove the echos due to hybrids, and you have limited
information about where those echos might be. You often end up needing
to use quite a long cancellor to be sure you catch the whole echo.