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...cho, and if possible, eliminate it at the source. The most common cause of echo is an unbalanced hybrid at the PSTN to TDM interface. This is where the 2-wire POTS line is divided into a 4-wire line (transmit and receive pairs). Under ideal conditions, the hybrid subtracts exactly the correct prorportion of the transmitted signal from the receive signal path to remove the echo caused by local loopback of the transmitted signal. Imbalance may be from poor design (common) or unpredictable impedance conditions on the POTS line (very common). The nature of this kind of echo is that only the ne...