Echo Cancellation Demystified

Let us now see a phone network connecting the distant abonents and doing signal amplification:

Figure 2. Long-distance phone network with amplifiers and 2-to-4-line conversion

Now, the interesting part is the hybrid performance because it is the hybrid where the echo can be and, in fact, is created. Ideally, the hybrid should just have a sum of the send and receive signals on the 2-wire side and these same signals separated on the 4-wire side. But in the reality, there are things like spread of equipment parameters and mismatch of line impedances, which all contribute to imperfect signal separation in the hybrid, which is the cause of the echo creation:

Figure 3. Hybrid echo

So part of the signal being sent to the hybrid on the 4-wire side is returned back as the echo superimposed on the signal being received from the hybrid on the 4-wire side. If, for example, the left hybrid of the Figure 2 has this kind of impairment, then the right talker will be hearing his own voice in the handset as an echo and the more the distance (and thus the signal delay) between the abonents' phones, the better this echo will be audible.

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