PDF of this tutorialAcoustic echo is generated with analog and digital handsets, with the degree of echo related to the type and quality of equipment used. This form of echo is produced by poor voice coupling between the earpiece and microphone in handsets and hands-free devices. Further voice degradation is caused as voice-compressing encoding/decoding devices (vocoders) process the voice paths within the handsets and in wireless networks. This results in returned echo signals with highly variable properties. When compounded with inherent digital transmission delays, call quality is greatly diminished for the wireline caller.
Acoustic echo was first encountered with the early video/audioconferencing studios and—as Figure 1 shows—now also occurs in typical mobile situations, such as when people are driving their cars. In this situation, sound from a loudspeaker is heard by a listener, as intended. However, this same sound also is picked up by the microphone, both directly and indirectly, after bouncing off the roof, windows, and seats of the car. The result of this reflection is the creation of multipath echo and multiple harmonics of echo, which, unless eliminated, are transmitted back to the distant end and are heard by the talker as echo. Predominant use of hands-free telephones in the office has exacerbated the acoustic echo problem.
Figure 1. Acoustic Echo on Digital Cellular
Acoustic echo cancellation is required in order to provide full duplex, fully interruptible speech. The acoustic echo canceller functions by modeling the speech being passed to the loudspeaker and removing any echoes picked up by the microphone. This type of operation necessitates a much more complex unit than is used in telephony in order to remove the many acoustic (multipath) echoes generated with each syllable of speech. The tail circuit requirement, or the amount of time the canceller has to hold the "model" of the echo in order to recognize it and cancel it, also is fundamentally greater and requires the echo canceller to contain much greater processing power.
Hybrid Echo
Hybrid echo is the primary source of echo generated from the public-switched telephone network (PSTN). This electrically generated echo is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire PSTN conversion points, reflecting electrical energy back to the speaker from the four-wire circuit.
Hybrid echo has been around almost since the advent of the telephone itself. The signal path between two telephones, involving a call other than a local one, requires amplification using a four-wire circuit. Although not a factor in itself on digital cellular networks, hybrid echo becomes a problem in PSTN–originated calls. The cost and cabling required rules out the idea of running a four-wire circuit out to the subscriber's premise from the local exchange. For this reason, an alternative solution had to be found. Hence, the four-wire trunk circuits were converted to two-wire local cabling, using a device called a "hybrid" (see Figure 2).
Figure 2. Hybrid Echo
Unfortunately, the hybrid is by nature a leaky device. As voice signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back on itself, creating the echoed speech. Provided that the total round-trip delay occurs within just a few milliseconds (i.e., within 28 ms), it generates a sense that the call is live by adding sidetone, which makes a positive contribution to the quality of the call.
In cases where the total network delay exceeds 36 ms, however, the positive benefits disappear, and intrusive echo results. The actual amount of signal that is reflected back depends on how well the balance circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of signal reflecting back. This is measured as echo return loss (ERL). The higher the ERL, the lower the reflected signal back to the talker, and vice versa.
