GB1299859A - Improvements in or relating to companding encoders and decoders - Google Patents

Abstract

1299859 Digital companding NIPPON ELECTRIC CO Ltd 31 Dec 1969 [24 March 1969] 61944/68 Heading G4H A digital companding PCM decoder comprises a memory circuit 26 for storing an input PCM binary code, a code conversion matrix 27 adapted to convert the input code into a modified digitally expanded output code having more bits than the input code, the modified output code consisting of a digitally expanded code of which a predetermined number of most significant zero bits are omitted when the most significant bit of the input code is 0 and of which the same number of least significant bits are omitted when the most significant bit of the input code is 1, a linear decoder 28 adapted to convert the modified output code into an analogue signal, and an adjustable attenuator or amplifier 29 controlled by the most significant bit of the input code to adjust the gain to which the analogue signal is subjected when this bit is 1 and U respectively to higher and lower values respectively whose ratio corresponds to the value of the omitted least significant bits. Besides being used as a PCM decoder, the circuit can also be used in the feedback loop of a PCM encoder (Fig. 4) in which the analogue feed-back signal from 29 is subtracted at summing amplifier 23 from a PAM input signal from 21 sampled and held at 22, the amplifier 23 output producing from comparator 24 a serial PCM signal at output 25 which also actuates memory 26. In operation a code 100000 is recorded by memory 26 and the signal compared with this to determine whether it is greater or less than it. The process is continued for successive bits until the complete PCM signal is built up. The feed back loop produces the compand characteristic of the encoder. This is curve 20-21 in Fig. 2. The output of matrix 27 is a characteristic comprising curve 21, 19 of Fig. 2. In this way the accuracy with which low values of the analogue signal are generated by linear decoder 28 is enhanced. Attenuator 29 converts the characteristic comprising curve 21, 19 of Fig. 2 back into the curve 20, 19. Thus the circuit behaves as a conventional compand circuit except insofar as the digital analogue conversion in the feedback loop is performed with improved accuracy for low amplitudes. Matrix 27 uses NAND circuits and performs the code conversion in Fig. 10 (not shown).