Waveform Quantization and Compression - Digital Signal Processing: Fundamentals and Applications - page 506

1.

Segment +7 64:1 compression ratio

Seg m en t +6 32 : 1 co m press i on ratio

Seg m ent +5 16:1 co m p r ession ra ti o

Segment +4 8:1 compression ratio

Segment +3 4:1 compression ratio

Seg m ent +2 2:1 co m p r ession r at i o

Seg m ent +1 1:1 no co m press i on

Segment +0 1:1 no compression

0.

1.

Relative input amplitude

FIGURE 11.9

m 255 compression characteristics (for positive portion only).

The format of the 12-bit linear PCM code is in the sign-magnitude form with the most significant bit
(MSB) as the sign bit (1 ¼ positive value and 0 ¼ negative value) plus 11 magnitude bits. The
compressed 8-bit code has the format shown in Table 11.2 , where it consists of a sign bit, a 3-bit
segment identifier, and a 4-bit quantization interval within the specified segment. Encoding and
decoding procedures are very simple, as illustrated in Tables 11.3 and 11.4 , respectively.
As shown in those two tables, the prefix “S” is used to indicate the sign bit, which could be either
1 or 0; A, B, C, and D, are transmitted bits; and the bit position with an “X” is the truncated bit during
the compression and hence would be lost during decompression. For the 8-bit compressed PCM code
in Table 11.3 , the 3 bits between “S” and “ABCD” indicate the segment number that is obtained by
subtracting the number of consecutive zeros (less than or equal to 7) after the “S” bit in the original
12-bit PCM code from 7. Similarly, to recover the 12-bit linear code in Table 11.4 , the number of
consecutive zeros after the “S” bit can be determined by subtracting the segment number in the 8-bit
compressed code from 7. We will illustrate the encoding and decoding processes in Examples 11.3
and 11.4.

Table 11.2 The Format of 8-Bit Compressed PCM Code

Sign bit:
1

3-bit segment
identifier: 000 to 111

4-bit
quantization
interval:
ABCD
0000 to 1111

¼þ
0¼