 the signal is analyzed and the periodicity is noticed. The periodical component is subtracted by an analysis filter from the original signal and the amplitude of the voice signal level is considerably reduced (Figure 3.19). The periodical high-power component is transmitted as a set of parameters, and the low-level error or difference signal at the output of the analysis filter is waveform coded. This waveform coding does not require a high bit rate because the amplitude of the error signal is low. At the receiving end, a synthesis filter is used and, with the help of the transmitted coefficients, it adds the periodical component to the error signal, which is reproduced from waveform-coded samples. Signals Carried over the Network Transmitted speech Received speech Removal of the periodical components Error or difference signal Speech Analysis Synthesis Segmentation filtering filtering in digital form Reproduced (APCM) speech Filter Parameters coefficients (filter coefficients) Figure 3.19 The principle of GSM speech coding. 3.7.6 Summary of Speech-Coding Methods We have introduced some important standardized coding methods such as PCM, ADPCM, and GSM radio channel voice coding schemes that are widely used in public telecommunications networks. However, in private PABX networks more efficient coding schemes are sometimes attractive, because the charge for leased lines between office sites is based on the chosen bit rate capacity and we can accept worse quality than in public networks. One way to radically reduce the bit rate required is to use voice coding implemented by vocoders. The speech is first synthesized (or modeled) and the resulting parameters are then encoded for transmission instead of the actual signal. This method is also used for speech synthesis (speech genera- tion). These types of algorithms actually try to imitate the human vocal tract, utilizing codebooks of common phonetic sounds and transferring these codes between the encoder and decoder. The quality of vocoder is worse than the quality of