"To the PBX or subscribers, with a small amount of electricity is not full conversion and along the way to return to, the formation of the echo, if the caller is not far from the PBX or switches, soon, the return of the echo to the human ear to listen to not to come out, in this case it doesn't matter, but when the return of the echo time more than 10 ms, the human ear can hear the echo of the obvious, in order to prevent the echo, generally need to echo cancellation technology, have a special software code in the processor, listening to the echo signal and remove it from the hearer's voice signal, the IP telephone equipment, echo cancellation technique is very important, because the general IP network delay is easy to reach 40 ~ 50 ms,"
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