Two-Wire to Four-Wire Conversion and Echo

Two-Wire to Four-Wire Conversion and Echo

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Telephone networks can experience two types of echo:

• Acoustic echo Acoustic echo frequently occurs with speakerphones, when the received voice on the speaker excites the microphone and travels back to the speaker.

• Electrical echo Electrical echo occurs when there is an electrical inconsistency in the telephony circuits. This electrical inconsistency is called an impedance mismatch.

If the lines have a good impedance match, the hybrid (that is, the two-wire to four-wire conversion circuit) is considered balanced, with little or no reflected energy. However, if the hybrid is inadequately balanced, and a portion of the transmit voice is reflected back toward the receive side, echo results.

Some form of echo is always present, as illustrated in Figure 2-21. However, echo can become a problem under the following conditions: