 waveform coders. Vocoders sound synthetic. They do not meet the quality requirements of the telephone network, one of which is speaker recognition, but they can be used in private networks. This principle is also used as a part of the GSM speech- coding scheme together with waveform coding as we saw in the previous section. The service quality of a telephone channel is governed by many factors, including volume, distortion, background noise, round-trip delay, and echo loss. We have many ways in which to measure quality. The results in Figure 3.20 are based on a so-called mean opinion score (MOS) measurement, in which many people have given their opinions about the quality. Introduction to Telecommunications Network Engineering The interactive nature of human conversation places a demand on the coder in terms of an acceptable path delay. Subjectively, noticeable deteriora- tion is perceived in channel quality once the round-trip delay exceeds 180 ms. Note that one-way delay via a geosynchronous satellite is approxi- mately 250 ms. For high-quality voice, the round-trip delay should be less than 150 ms. Another problem is an echo, which is noticeable if the delay is more than 30 to 50 ms. Delays of 10 to 20 ms are generally undetectable. Some echo is always produced at the far end by a 4W/2W hybrid of the far-end subscriber loop described in Chapter 2 because of the nonideal return loss of the hybrid. This is why, in the case of a long delay (e.g., on satellite chan- nels), echo cancellers are needed. The long coding delay (e.g., GSM) also requires echo cancellers. Figure 3.20 provides a comparison of the coding schemes discussed in this chapter. The measure of quality is the MOS, which indicates the average opinion expressed by a number of people about the quality of each coding scheme. We have introduced just some of the available speech-coding methods; many other standardized speech-coding schemes are in use at different data rates. ITU standards cover constant-bit-rate coders at data rates down to 5 Kbps. For 