This paper provides an introduction into the basics of echo cancellation. The paper is kept general but it emphasizes on the emerging Voice over IP (VoIP) market. However, most aspects can also applied to Asynchronous Transfer Mode (ATM) or wireless networks. This paper shows why and where echo cancellation is required, what echo is, and what causes it. It also gives an overview of the standards involved in echo cancellation. The digital echo canceller, meets the requirements of the VoIP market. This paper shows how it can be integrated into a VoIP system and the advantages the solution offers.
What is echo and where does it come from ?
A simple example from everyday experience is a good introduction to the basics of echo cancellation. Most people have heard an echo when speaking loudly in the mountains. The echo you hear is your own voice, reflected back to you from distant surfaces. Figure 1 shows an example. John and Little-John are yelling toward the mountain and the signal is reflected from certain areas (reflection points). The voice and the echo are visualized as waves. Bryan can’t hear any echo because he is behind the reflection point.The time it takes for the voice to travel from John and Little-John of Figure 1 to the reflection point is called one-way delay. The time it takes to travel to the reflection point and back to the person making the sounds is called the total round-trip delay. In this illustration, the one-way delay is either 80 ms or 10 ms, depending how far John and Little-John are away from the reflection point.
Let’s assume two people are having a conversation over a network. The network can be IP, satellite, or another network type with long delay. In Figure 2 John on the telephone is 80 ms or more (one-way) delay away from the reflection point. This is an equivalent situation to Figure 1. Bryan on the mountain in Figure 1 is in the house in Figure 2, and John who is 80 ms away from the reflection point in Figure 1 is the person with the telephone in Figure 2.
John would hear two different kind of echoes of his own voice. One echo is the electrical echo that occurs at the transition from 2- to 4-wire cable, called the hybrid (H). The hybrid itself is typically located in the Central Office (CO) other location are possible, e.g. Private Branch Exchange (PBX). The hybrid is the reflection point of the electrical echo. The reflection point of the voice in the house could be the wall, the furniture, etc. This echo is called acoustic echo. The voice coming out of the loudspeaker bounces back from the wall to the microphone. The wall is the reflection point of the acoustic echo. In both cases electrical and acoustic echo, John can hear his own voice. It may be nice to hear one’s own echo in the mountains, but not during a telephone call, when echoes will definitely interfere with the conversation.