2010318163752818

ISO/IEC 14496-3:2005(E)
Unit groups are collected ascending in spectral direction. For one long window, that gives the original
codeword order, but for eight short windows a unit based window interleaving has been applied.
Using this scheme, the codewords representing the lowest frequencies are the first codewords within
spectral data for both, long and short blocks.
Table 4.150 shows an example output of the first pre-sorting step for short blocks, assuming two-dimensional
codebooks for window 0, 1, 6, and 7 and four-dimensional codebooks for window 2, 3, 4, and 5.
Second pre-sorting step:
The more energy a spectral line contains, the more audible is its distortion. The energy within spectral lines
is related to the used codebook. Codebooks with low numbers can represent only low values and allow only
small errors, while codebooks with high numbers can represent high values and allow large errors.
Therefore, the codewords are pre-sorted depending on the used codebook. If the error resilient section data
is used, the order is 11, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 9/10, 7/8, 5/6, 3/4, 1/2. If
the normal section data is used, the order is 11, 9/10, 7/8, 5/6, 3/4, 1/2. This order is based on the largest
absolute value of the tables. This second pre-sorting step is done on the described unit by unit base used in
the first pre-sorting step. The output of the first pre-sorting step is scanned in a consecutive way for each
codebook.
Assigning numbers to units according the following metric can perform these two pre-sorting steps:
codbookPriority[32] =
{x,21,21,20,20,19,19,18,18,17,17,0,x,x,x,x,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1}
assignedUnitNr = ( codebookPriority[cb] * maxNrOfLinesInWindow
+ nrOfFirstLineInUnit ) * MaxNrOfWindows + window
with:
codebookPriority[cb] codebook priority according the second pre-sorting step.
maxNrOfLinesInWindow constant number: f in case of one long window and 128 in case of eight
short windows
nrOfFirstLineInUnit a number between 0 and 1020 in case of one long window and between 0
and 124 in case of eight short windows (this number is always a multiple of
four)
maxNrOfWindows constant number: 1 in case of one long window and 8 in case of eight short
windows
window always 0 in case of one long window, a number between 0 and 7 in case of
eight short windows and sort the units in ascending order using these
assigned unit numbers.
Encoder note: In order to reduce audible artifacts in case of errors within spectral data it is strongly recommended to use
codebook 11 only if necessary!
4.6.16.3.3.2 Segment width and segment instantiation
The segment widths depend on the Huffman codebook used. They are derived as the minimums of the (codebook
dependent) maximum codeword length and the transmitted longest codeword length:
segmentWidth = min (maxCwLen, length_of_longest_codeword)
Table 4.151 shows the values of maxCwLen depending on the Huffman codebook.
Segments are instantiated until the available buffer is exhausted, whereas the size of this buffer is given by the
data element length_of_reordered_spectral_data. The remaining bits at the end of the buffer increase the size of
the last segment.
4.6.16.3.3.3 Order of Huffman codewords in spectral data
Figure 4.34 shows the general scheme of the segmentation and the arrangement of the PCWs. In this Figure 4.34,
five segments can be provided to protect codewords from section 0 and section 1 against error propagation.
194 © ISO/IEC 2005 — All rights reserved