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. Regards, Steve |