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ISO/IEC 10918-1 : 1993(E)
D.2.7
Initialization of the decoder
The Initdec procedure is used to start the arithmetic decoder. The basic steps are shown in Figure D.22.
TISO1220-93/d060
Initdec
Byte_in
C = SLL C 8
Done
Byte_in
Initialize statistics areas
BP = BPST 1
A = X'0000'
A =
(see Note below)
C = 0
C = SLL C 8
CT = 0
Figure D.22 Initialization of the decoder
Figure D.22 [D60], = 16 cm = 625.%
The estimation tables are defined by Table D.3. The statistics areas are initialized to an MPS sense of 0 and a Qe index of
zero as defined by Table D.3. BP, the pointer to the entropy-coded segment, is then initialized to point to the byte before
the start of the entropy-coded segment at BPST, and the interval register is set to the same starting value as in the encoder.
The first byte of compressed data is fetched and shifted into Cx. The second byte is then fetched and shifted into Cx. The
count is set to zero, so that a new byte of data will be fetched by Renorm_d.
NOTE Although the probability interval is initialized to X'10000' in both Initenc and Initdec, the precision of
the probability interval register can still be limited to 16 bits. When the precision of the interval register is 16 bits, it is initialized to
zero.
D.3
Bit ordering within bytes
The arithmetically encoded entropy-coded segment is an integer of variable length. Therefore, the ordering of bytes and
the bit ordering within bytes is the same as for parameters (see B.1.1.1).
76
CCITT Rec. T.81 (1992 E)
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