DVD Demystified

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audio compression takes a block of samples and divides them into frequency
bands of equal or varying widths. Bands of different widths are
designed to match the sensitivity ranges of the human ear. The intensity of
sound in each band is analyzed to determine two things: (1) how much
masking it causes in nearby frequencies and (2) how much noise the sound
can mask within the band. Analyzing the masking of nearby bands means
that the signal in bands that are completely masked can be ignored. Calculating
how much noise can be masked in each band determines how much
compression can be applied to the signal within the band. Compression uses
quantization, which involves dividing and rounding, and this can create
errors known as quantization noise. For example, the number 32 quantized
by 10 gives 3.2 rounded to 3. When reexpanded, the number is reconstructed
as 30, creating an error of 2. These errors can manifest themselves
as audible noise. After masked sounds are ignored, remaining sounds are
quantized as coarsely as possible so that quantization noise is either
masked or is below the threshold of hearing. The technique of noise masking
is related to noise shaping and is sometimes called frequency-domain
error confinement.
Another technique of audio compression is to compare each block of samples
with the preceding and following blocks to see if any can be ignored on
account of temporal masking—soft sounds near loud sounds—and how
much quantization noise will be temporally masked. This is sometimes
called temporal-domain error confinement.
Digital audio compression also can take advantage of the redundancies
and relationships between channels, especially when there are six or eight
channels. A strong sound in one channel can mask weak sounds in other
channels, information that is the same in more than one channel need only
be stored once, and extra bandwidth can be temporarily allocated to deal
with a complex signal in one channel by slightly sacrificing the sound of
other channels.
MPEG-1 Audio Coding
Chapter 3
MPEG-1 digital audio compression carries either monophonic or stereophonic
audio. It divides the signal into frequency bands (typically 32) of
equal widths. This is easier to implement than the slightly more accurate
variable widths.
MPEG-1 has three layers, or compression techniques, each more efficient
but more complicated than the last. Layer II is the most common and is the
only one allowed by DVD. Layer II compression typically uses a sample