ADAPTIVE DIFFERENTIAL PULSE CODE MODULATION DECODING CIRCUIT

摘要

There is described an ADPCM decoding circuit of the type which receives a coded signal outputted from an ADPCM encoder which adaptively quantizes at each sampling time a residual signal which is a difference between a signal formed by linearizing an input nonlinear encoded PCM signal, and a prediction signal adaptively predicted, and which delivers a nonlinear PCM decoded signal. An adaptive inverse quantizer produces, from a quantized coded signal produced by the ADPCM encoder, a representative residual value signal, a lower limit residual signal and a higher limit residual in accordance with the residual signal on the side of the encoder, and which determines a quantizing characteristic at a next sampling time in accordance with the quantized code signal. An adder adds an adaptive prediction signal to each of the representative residual value signal, the lower limit residual signal and the higher limit residual signal to produce a representative decoded value signal, a lower limit decoded value signal and a higher limit decoded value signal. A nonlinear PCM converting circuit converts the representative decoded value signal into a nonlinear encoded PCM signal to form an output nonlinear PCM signal. A linear PCM converting circuit linearizes the output nonlinear PCM signal to produce an output linear PCM signal. A comparator compares the output linear PCM signal with the lower limit decoded value signal and with the higher limit decoded value signal to form a nonlinear PCM decoded signal which corresponds to the output nonlinear PCM signal, the output nonlinear PCM signal added with +1 or the output nonlinear PCM signal added with -1. An adaptive prediction circuit inputted with the representative decoded signal together with the representative residual value signal produces the adaptive prediction signal at a first sampling time and determines a prediction characteristic at a next sampling time.