PDF of this tutorialTo deal with hybrid echo created by vocoder processing delays, it is mandatory for digital cellular mobile calls to have a group echo canceller installed—even for local calls. As a result, all calls on to the PSTN must pass through an echo canceller to remove what would otherwise be a noticeable and annoying echo, as shown in Figure 3.
Figure 3. Digital Cellular Network
For example, consider a digital cellular mobile user who makes a call to the PSTN without an echo canceller in place. The user would hear his or her own speech being echoed back 180 ms or more later, even if the called person is in the same locality. The mobile user will either be using a hands-free system installed in his or her vehicle or a hand portable. In either case, these units will involve the occurrence of direct and indirect coupling between the microphone and the speaker, creating acoustic echo. In this situation, however, it is the PSTN user who suffers by experiencing poor speech quality. Hence, the echo canceller installed in the digital cellular network must be capable of handling both sources of echoes.
