ith fewer bits since the range of the signal is much smaller than the overall signal range (since it does not vary largely between two consecutive sampling intervals). Broadband Telecommunications Technologies and Management Figure 5.15 Differential encoding The difference between two consecutive samples of voice can be entropy coded as the range of possible values is smaller and well understood. The ADPCM standard samples are repre- sented by 4 bits, and therefore the total coding rate is equal to 8000 X 4 = 32 kbps. Both PCM an ADPCM coding use A to D conversion and are lossless. Synthesis Coding An alternative class of voice coding is through voice analysis into its components. These methods are lossy in that some information is lost in the source coding process. Synthesis coding uses the fact that human voice generation occurs because of the sound created by our vocal cords which is then formed into speech as it travels through our vocal tract (throat and mouth). Our speech consists of a 'buzzer' sound generated by the vibrating vocal cords, and 'hissing' and 'popping' sounds as air passes through stationary vocal cords and is shaped by our tongue, lips and throat. The voice decoder finds the intensity and pitch of the buzzer sound and the residue of the signal when the buzzer sound is filtered out [6]. The calculation of filter values is done through a synthesis process as illustrated in Figure 5.16. Here a codebook representing a range of possible pitches, intensity and residue is used to synthesise a sound similar to the input voice every 20 ms. A feedback loop adjusts the filter parameter until the synthesised and actual sounds differ minimally. The numbers representing the pitch, intensity and filter parameters are transmitted to a receiver which then synthesises the received voice. From Analogue to Digital Original speech signal Codebook Speech filters To next code word Perceptual Calculate weighing error factor Figure 5.16 Voice analysis coding Table 5.3 A number of commonly used voice codecs and their