What Causes Echo?

Echo is the reflection or return of the speaker’s voice to the speaker. It has an analog source, and it usually occurs at the far end of a conversation. Cisco explains that the main two types of echo have different sources:

Hybrid echo. Caused by an impedance mismatch in a hybrid circuit, such as a two-wire to four-wire interface, which allows the Tx signal to appear on the Rx path.
Acoustic echo. Caused by poor insulation between the ear piece and the microphone in telephone handsets and hands-free devices.

At several places along a phone circuit, your voice can get into the return channel and come back to you. The first interface where echo may occur is at the transition between a 4-wire and a 2-wire interface. Analog telephone handsets are 2-wire devices. At some point in the path, perhaps in a local PBX, there is a hybrid interface that converts the network 4-wire interface to the 2-wire interface. Impedance mismatches here reflect some of the energy back into the network, creating a potential source of hybrid echo.

Another common source of echo is the basic hardware. The mouthpiece of the phone at the far end may be too close to the ear piece, or it may be poorly insulated, so that your voice is heard and forwarded on the same return channel as the one on which the person at the far end is speaking. Therefore, the analog phone itself is a possible source of acoustic echo. But even more suspect these days is the speaker phone function of the phone at the far end of the call. Speaker phones broadcast the voice and simultaneously listen to the voices of the speakers in the room. Despite advanced DSP functions, it is very easy for speaker phones (especially cheap ones) to send back some of the far end voice as part of what they are “hearing.”

Because delay is a necessary condition for echo, it is rare for components on the speaker’s side of the call to cause echo. Even if part of the transmitted signal is reflected back to the speaker by means of the return channel, the propagation delays are so brief that they are never heard as echo. And because echo is rarely caused by a local component and has an analog source, the main suspects when an echo problem crops up are usually part of the “tail circuit” connecting the far end speaker to the PSTN. See Figure 1 for an illustration.