 for all samples. On the other hand, for low signal levels the quantizing error is small and SQR can be kept high enough over a wide range of signal levels. This principle is used, for example, in original GSM as part of the voice coding process. 3.7.2 Differential PCM (DPCM) In DPCM only the difference between a sample and the previous value is encoded as shown in Figure 3.17. Because the difference is typically much smaller than the overall value of the sample, we need fewer bits for the same accuracy as in ordinary PCM and the required bit rate is reduced [1]. In the example shown in Figure 3.17, PCM requires 5 bits (polarity and 4 bits for 16 quantum levels). DPCM, in which the only difference from the previous sample is encoded, 4 bits is clearly enough to describe the difference between subsequent samples. For better quality or to further reduce the data rate, DPCM may use several of the preceding samples to predict the next sample. The exam- ple in Figure 3.17 shows that if two previous samples are used for predic- tion the encoder and decoder assume that the next sample follows the same slope. Now, 3 bits would be enough for the encoder to describe the difference between the prediction and the actual sample value. The decoder performs the same prediction and only the difference between predictions shown in Figure 3.17 need to be transmitted. Further improve- ment can be achieved if three previous sample values are used for predic- tion, but more than three samples do not add much advantage [2]. Actually, the first simple form of DPCM in Figure 3.17, which encodes the difference between preceding sample values, uses prediction as well but that prediction is based on only one sample and it equals the previous sample value. Signals Carried over the Network Analog signal and 5-bit PCM samples -2...15 DPCM 4-bit values based on one previous sample DPCM 3-bit values based on two previous samples Figure 3.17 DPCM. These waveform coding methods do not introduce much delay because prediction is based on previous s