Adaptive differential pulse code modulation

DETAILED DESCRIPTION FIG.
1 depicts an ADPCM system known to the prior art as exemplified by the cited Cummiskey et al.
paper.
In the known system an analog signal, such as human speech, is first sampled at a sufficient rate, typically at 8 kiloHertz (kHz) and then the difference between the present sample and the reconstruction of one or more previous quantized samples is itself quantized against a plurality of uniformly spaced discrete signal levels.
In order to compensate for differences in adjacent sample levels provision is made for adaptive variation of the uniform spacing of the quantization levels for the present sample in accordance with the level occupied by one or more previous samples.
The purpose for the adaptive step sizing is to increase the step size for potential overload distortion and to decrease it for low-level granular distortion.
The ADPCM system of FIG.
1 broadly comprises analog signal source 1, encoder 10, transmission line 18, decoder 20 and utilization circuit 2.
Encoder 10 further comprises subtractor 11, adaptive quantizer 12, binary encoder 13, integrator 14 and logic circuit 15.
Decoder 20 further comprises binary decoder 23, logic circuit 25, adder 21, integrator 24 and low-pass filter 22.
An analog signal, such as a speech signal, is sampled at a rate exceeding the Nyquist rate and presented at the output of source 1 to coder 10.
At the input to coder 10 subtractor 11 acts on the samples before application to quantizer 12 to generate difference vaues.
Quantizer 12 has a characteristic of the type diagrammed in FIG.
2.
The eight evenly spaced discrete levels are shown along staircase 30.
For this number of levels a minimum of three bits is required to describe each level uniquely.
A uniform number R of bits is needed to define each level, where the number of levels is 2 raised to the Rth power.
For the eight levels shown in FIG.
2 the value of R is 3.
The abscissa, in FIG.
2 representing levels of input samples applied to quantizer 12, is laid off in even increments