this information to reconstruct the speech the redundancy contained in voiced sounds, thereby reducing the block from the model. magnitude of the difference signal to be encoded. This scheme is Because of the small amount of information transmitted, LPC known as adaptive differential pulse code modulation (ADPCM). It coders can use digit rates as low as 4.8kbit/s, although error protec- enables a SQNR of about 40dB to be obtained with 4-bit encoding. tion of the code is essential. However, LPC coders have the severe Telephone transmission can therefore use only 32kbit/s instead of disadvantage of introducing a coding delay of two or three times the 64kbit/s. length of the speech block. Consequently, echo suppressors or echo The CCITT has specified an algorithm for 32kbit/s ADPCM cancellers must be used with LPC. (Recommendation G.721). This is embodied in a transcoder, whose input is a normal 64kbit/s PCM signal (with A-law or u-law com- The Group Speciale Mobile (GSM), which was formed by the panding) and whose output is 32kbit/s ADPCM. Block diagrams of European PTTs, had the task of specifying the speech encoding this encoder and the corresponding decoder are shown in Figures algorithm for a pan European digital cellular radio system. It has 19.22 and 19.23. chosen a variant of LPC, known as regular pulse excitation long Alternatively, ADPCM can be used to enable a 64kbit/s digital term prediction (RPE-LTP) (ETSI, 1989). This will use a speech channel to transmit speech having a bandwidth of 7kHz instead of block of 20ms and will operate at 13kbit/s, with an additional 3kbit/s only 3.4kHz (CCITT Recommendation G.722). for error protection. 64kbit/s Difference A-law or law Convert to Input signal signal Adaptive 32kbit/s PCM input uniform PCM quantiser output Signal estimate Reconstructed signal Quantised Adaptive difference Inverse predictor signal adaptive quantiser Figure 19.22 ADPCM encoder Delta modulation 19/19 Quantised difference Reconstructed 32kbit/s Inverse signal Convert to 64kbit/s signal Synchrono