A speaker to microphone acoustic coupling would replace the hybrid in In case of acoustic environment, the echo would come from sound that comes out of a speaker and is reflected back to the microphone. The central part of any modern echo canceling system is a digital adaptive filter.įigure 1: Block diagram of an echo canceller.įigure 1 illustrates a simple general-purpose echo canceling system that is terminated with a network hybrid. An echo canceller (EC) is often implemented using DSP algorithms. To solve echo problems, designers typically implement an echo canceller in a system architecture (Figure 1). Miscellaneous: caller ID, telephony signaling support.Voice playout: synchronizes the voice packet arrivals with voice sampling rate.Voice packetization: implements the transport layer for voice data.Voice activity detection: minimizes bandwidth during silence.Tone generation: call progress tones, DTMF.Tone detection: DTMF and other signaling tones.Voice codecs: voice compression to facilitate packet network bandwidth savings.The echo cancellers must share space with the following algorithms: Among these challenges are the many different VoP algorithms which consume large amounts of computational and storage resources. The VoP environment presents unique challenges to the developer working to implement echo cancellation techniques. In addition to packet-induced echo, VoP systems must also take into account the presence of acoustic echo, such as that seen with handsfree phone operation. These long delays make echo readily apparent to listeners, and must be eliminated in order to provide viable telephony service. This form of echo is electrical and is produced by a network hybrid (a 2-wire to 4-wire converter).Įcho cancellation is critical to achieving high quality voice transmissions over packet networks, which typically face transmission delays above 30 to 40 ms. Line echo, which we'll discuss in part 2, is the second form. This form of echo is created by the acoustic properties of the areas where a VoP phone is used. The first, which we'll discuss below, is acoustic echo. In a broadband system architecture, two forms of echo cause problems for designers. To start our discussion, let's provide an overview of echo. Part 1 will focus on the basics of echo cancellation and acoustic echo canceller while part 2 will zero in on line echo cancellation techniques. In this two-part series, we'll explore the key echo cancellation elements/techniques that broadband designers need to worry about when developing VoP systems. In order to meet this requirement, dealing with echo is a must. In order to succeed, these data operators must deliver voice services with the same quality as existing landline networks. Now the challenge falls on broadband equipment developers implementing voice-over-packet (VoP) technology. And for years, equipment designers have turned to digital signal processing (DSP) and standalone board solutions as a means for curbing echo in carrier class equipment designs. Echo cancellation is a recurring problem in telecommunication and wireless networks.
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