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This article describes a coding scheme referred to hereafter as LOG-DPCM, for digital compression of speech at a medium bit rate (around 32 Kbps) with good communications quality.
English (United States)
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Block Compared Differential Coding of a Law Coded Signals
This article describes a coding scheme referred to hereafter as LOG-DPCM,
for digital compression of speech at a medium bit rate (around 32 Kbps) with
good communications quality.
A general block diagram of a LOG-DPCM coder/decoder is shown above.
The speech signal is sampled at 8 KHz, and each sample is quantized with 8 bits
according to the standard A-Law. For convenience, the 8-bit sample X(n) is
denoted: (1) X(n) = (sabcxyzt) where (s) is the sign bit, (abc) is the 3-bit
representation of the segment K (0 </- K </- 7), and (xyzt) is the 4-bit
representation of the level L (0 </- L </- 15) on that segment. The corresponding
decoded levels (See Original).
One can check that these decoded levels must be coded with 11 bits plus
sign. However, the dynamic range can be reduced by using a block companding
technique, as explained hereafter.
The 8-bit A-Law coded samples are bufferized while a forward adaptation of
the quantizer is made so as to take into account the energy variations of the
speech signal. This adaptation is based on the simple determination of the
maximum segment, KMAX, of the bufferized block of N samples: (3) KMAX =
Max ( K(S(n) ) n = 1,N. (A typical value of N is 8, corresponding to 1 ms
blocks.) The block information is sent to a scaling logic in which an 8-bit linear
representation of input samples is derived according to (1), (3) and (4): (See
Original). Taking into account the range of K and L, one can check that the
decoded value of S(n) is bounded by: (5) absolute value of S(n) </- 31.
and therefore it...