Echo Cancellation

Echo Cancellation

An echo might be amusing enough when we are hiking in the mountains, but it is certainly a

nuisance during a phone call and might even make communication impossible.

Hearing one's own voice within the handset is a normal effect that calms the speaker. But a

delay between the direct sound and the echo of more than 25 ms might cause interruptions

and disturb severely the rhythm of communication.

Legacy PSTNs have to cope with echoes produced by non-aligned impedances when the

signal leaves the 4-wire system of the switch to enter the local 2-wire trunk to the end user.

Very careful impedance supervision at the reflection points and the use of echo cancellers

helps to solve the problem.

In VoIP systems, echo cancellation is included with the codecs in the DSPs or digital signal

processors. Hence, a software-based solution is used for IP soft-phones and the gateways of

the network. For a certain period of time, it stores an inverse pattern of the sent speech

sample. Then the DSP listens to the sound that is reflected by the distant party and subtracts

the stored signal. This achieves a near-zero amplitude for the sound disturbance. The

majority of echo cancellers cope with 32 ms echoes. That is the maximum storage time for

the speech sample.