443C_Simion_Alexandru
代码说明：
The (a) section of Figure 1 illustrates the Line Echo Canceller (LEC) residing in the access network where echoes occur. Other voice processing modules, such as Automatic Level Control (ALC) and Noise Reduction (NR), are shown merely for illustration purposes. On the terminal side, both acoustic and electrical echo can occur. If the voice terminal is a speaker telephone set with an open acoustical path from the speaker to the microphone, the effect of acoustical echo is combined with the effect of the network echo. Because there is only a miniscule delay between the electrical signal and its echo, the speaker/listener is exposed to an effect known as side tone. Typically, instead of mounting a LEC at the terminal, side tone circuitry is placed to control the amount of the returned energy. The (b) section of Figure 1 shows examples of echo canceller signals representing the following:
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