gnal may change SO rapidly that the correspond respectively, to the A-law with A=86.7 and the u-law difference between it and the integrator output exceeds the maxi- with u = 255. The A-law is used across Europe, and the u-law is mum voltage that can be encoded. This is known as slope overload. used across America and Japan. Both schemes use codewords which There is then an error in the output voltage from the receiver. bear a remarkable similarity to 8-bit binary floating point numbers. However, this rapid rate of change is unlikely to persist, SO the One bit is reserved for the sign, three bits for the exponent, and four receiver output will be corrected over the next few samples. 19/18 Digital modulation Sample and Coder Decoder Baseband Transmission input Baseband output Figure 19.21 Block diagram of a differential PCM system 19.5.2 Adaptive differential PCM 19.5.3 Linear predictive coding The basic DPCM system of Figure 19.21 may be thought of as predicting that each sample will not change from the previous one and transmitting the difference between each sample and this pre- Linear predictive coding (LPC) takes the idea of using a model of dicted value. Thus, the integrators at the sending and receiving ends the vocal tract one stage further (Atal and Schroeder, 1970). A short are a simple form of predictor. The performance of the system can block of speech, typically 20ms in duration, is processed to extract be improved, or the digit rate reduced, by using a more sophisticated pitch period and formant information, which describe respectively form of predictor based on the a priori knowledge that the baseband the activity of the vocal chords and the resonances of the vocal tract. signal to be encoded is a speech signal. Then, rather than transmitting speech samples, the LPC coder sends The predictor can be a self adaptive filter based on a model of the the pitch period and formant model coefficients for the block of vocal tract (Wallace et al., 1991). This removes a large amount of speech. The decoder uses 