Methods of Encoding the Analog Signal

Real-world requirements may make it impossible to handle the full bit stream of, for example, CD-quality audio. One solution (delta modulation is to encode not the value of each sample, but the difference between one sample and the next. In most cases, fewer bits per sample need to be transmitted, but delta modulation has problems. A more practical variant is called adaptive delta pulse code modulation, or ADPCM. In order to handle both signals that change quickly as well as signals that change slowly, the step size encoded between adjacent samples varies according to the signal itself. In other words, if the waveform is changing rapidly, large steps can be quantised. This is the method used in CD-I (compact disc-interactive), discussed below.
For speech signals, a widely used system works with a quantisation step size that increases logarithmically with the level of the signal. This means that the quantisation levels are closest together when the signal is quiet and spaced further apart when the signal is louder. CCITT Recommendation G.711 codifies the A-law and u-law encoding scheme whereby speech is transmitted at 8 kHz.