Audio compression - The MPEG - page 123

Figure 4.32 (b) shows the Layer II elementary stream structure. Following the sync pattern the bit-allocation data
are sent. The requantizing process of Layer II is more complex than in Layer I. The sub-bands are categorized into
three frequency ranges, low, medium and high, and the requantizing in each range is different. Low-frequency
samples can be quantized into 15 different wordlengths, mid-frequencies into seven different wordlengths and high
frequencies into only three different wordlengths. Accordingly the bit-allocation data uses words of four, three and
two bits depending on the sub-band concerned. This reduces the amount of allocation data to be sent. In each
case one extra combination exists in the allocation code. This is used to indicate that no data are being sent for
that sub-band.
The 1152 sample block of Layer II is divided into three blocks of 384 samples so that the same companding
structure as Layer I can be used. The 2 dB step size in the scale factors is retained. However, not all the scale
factors are transmitted, because they contain a degree of redundancy. In real program material, the difference
between scale factors in successive blocks in the same band exceeds 2 dB less than 10 per cent of the time. Layer
II coders analyse the set of three successive scale factors in each sub-band. On a stationary program, these will be
the same and only one scale factor out of three is sent. As the transient content increases in a given sub-band, two
or three scale factors will be sent. A two-bit code known as SCFSI (scale factor select information) must be sent to
allow the decoder to determine which of the three possible scale factors have been sent for each sub-band. This
technique effectively halves the scale factor bit rate.
As for Layer I, the requantizing process always uses an odd number of steps to allow a true centre zero step. In
long-wordlength codes this is not a problem, but when three, five or nine quantizing intervals are used, binary is
inefficient because some combinations are not used. For example, five intervals need a three-bit code having eight
combinations leaving three unused. The solution is that when three-, five- or nine-level coding is used in a sub-
band, sets of three samples are encoded into a granule . Figure 4.34 shows how granules work. Continuing the
example of five quantizing intervals, each sample could have five different values, therefore all combinations of
three samples could have 125 different values. As 128 values can be sent with a seven-bit code, it will be seen that
this is more efficient than coding the samples separately as three five- level codes would need nine bits. The three
requantized samples are used to address a look-up table which outputs the granule code. The decoder can
establish that granule coding has been used by examining the bitallocation data.
Figure 4.34: Codes having ranges smaller than a power of two are inefficient. Here three codes with a range of five
values which 3 bits can be carried in a single eight-bit × would ordinarily need 3 word.
The requantized samples/granules in each sub-band, bit allocation data, scale factors and scale factor select codes
are multiplexed into the output bit stream.
The Layer II decoder is shown in Figure 4.35 . This is not much more complex than the Layer I decoder. The
demultiplexing will separate the sample data from the side information. The bit-allocation data will specify the
wordlength or granule size used so that the sample block can be deserialized and the granules decoded. The scale
factor select information will be used to decode the compressed scale factors to produce one scale factor per block
of 384 samples. Inverse quantizing and inverse sub-band filtering takes place as for Layer I.
Figure 4.35: A Layer II decoder is slightly more complex than the Layer I decoder because of the need to decode
granules and scale factors.