same algorithm to estimate the values of the following samples with help of the preceding samples, and only the error to this estimate is transmitted as in DPCM in Figure 3.17. To further reduce the number of bits per sample, ADPCM adapts quantizing levels to the characteristics of the analog signal. Figure 3.18 shows a simplified example in which the prediction error is ini- tially small and all bits can be used for half of the full quantizing error scale. Then the prediction error increases and the quantizing step size is doubled to describe higher values of prediction error. When the prediction error decreases, the quantizing step size is reduced again to describe properly small errors. Adaptation information is transmitted from encoder to decoder in addition to the prediction error. In the original 32-Kbps ADPCM method, the difference between the predicted and actual sample value is coded with four bits, that is, into 15 quantum levels, and the data rate is half that of conventional PCM. If several subsequent samples vary widely, the quantizing steps are adapted to that change SO that four bits are enough for prediction error. If prediction errors tend to increase, quantizing steps are increased and vice versa. Prediction error Small quantizing Large quantizing Small prediction steps, prediction steps, prediction error, small error increases error decreases quantizing steps Figure 3.18 ADPCM principle. Signals Carried over the Network According to the ADPCM standard, commercial voice quality is coded into 32 Kbps or even a lower (24 or 16 Kbps) bit rate. Samples are still taken at 8 kHz but transmitted with four bits (in the case of 32-Kbps ADPCM) and the quality is equal, or at least close, to the quality of ordinary PCM. Recommendation G.728 for 16/24/32/46-Kbps ADPCM was approved by the ITU-T in 1990 and has been adopted worldwide for digital voice transmission between countries or within a country. It can partly resolve the current compatibility problem between North America's and Europe's PCM formats, du