Echo Cancellation Demystified

By Alexey Frunze
Software Engineer

The Need for Echo Cancellation

People have been using phones as a means of distant voice communication for more than a century now. Using phones has become sort of a usual thing. We use the phones almost every day and just about everywhere: at home, at work, outside, in our cars and so on. A big thanks goes to the cellular phones, which set us free from wires!

Although we all enjoy this remarkable possibility to talk on the phone, there's always something we'd like to be better. This something is the speech quality in the phone conversations. The speech quality has always been an issue for both the phone network service providers and their subscribers. There are several reasons for the undesirable quality degradation and the appearance of audible echoes is one of them. This kind of quality degradation is inherent in the network equipment and the end-user phone devices.

Today it is easy to implement echo cancellation on DSPs and this is what engineers are doing in their devices. However, many of them face certain difficulties with achieving echo cancellation because of incomplete understanding of the echo cancellation principles and not meeting the requirements imposed by the echo cancellers. The purpose of this article is to demystify the topic of the echo cancellation by explaining its basics and providing useful information for those engineers, who need to implement the echo cancellation in their devices. We shall see where the echoes come from, how to fight them and what the known problems with the echo cancellation are. The information provided herein is based on the experience of developing echo cancellers and supporting echo canceller customers at SPIRIT Corp.

Where Does the Echo Come From?

There are generally two kinds of the echo, which can appear when talking on the phone. The two differ by the place where they are created and by their characteristics.

Hybrid Echo

The first kind is the line echo (also known as electric or hybrid echo) and it is created by the electrical circuitry connected to the wire lines.

Let us first see a simplified version of a network with two abonents.

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