Nearly Instantaneous Companding for Nonuniformly Quantized PCM

The technique of nearly instantaneous companding (NIC) that we describe processes
n
-bit μ-law or A-law encoded pulse-code modulation (PCM) to a reduced bit rate. A block of
N
samples (typically
N \cong 10
) is searched for the sample having the largest magnitude, and each sample in the block is then reencoded to a nearly uniform quantization having (
n - 2
) bits and an overload point at the top of the chord of the maximum sample. Since an encoding of this chord must be sent to the receiver along with the uniform reencoding, the resulting bit rate is
f_{s}(n -2 + 3/N)
bits/s where f
s
is the sampling rate. The algorithm can be viewed as an adaptive PCM algorithm that is compatible with the widely used μ-law and
A
-law companded PCM. Theoretical and empirical evidence is presented which indicates a performance slightly better than (
n - 1
) bit companded PCM (the bit rate is close to that of (
n - 2
) bit PCM). A feature which distinguishes NIC from most other bit-rate reduction techniques is a performance that is largely insensitive to the statistics of the input signal.