DVD Demystified

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The hierarchical structure of MPEG-1 plus extension streams means
that a two-channel MPEG decoder need only decode a two-channel data
stream, and a six-channel MPEG decoder need only process a six-channel
data stream. The eight-channel MPEG decoder is the only one that must
decode the entire contents of an eight-channel stream. Therefore, an
MPEG-1 decoder “sees” only the MPEG-1—compatible data to produce
stereo audio, whereas an MPEG-2 decoder combines it with the first layer
of extension data to produce 5.1-channel audio or, with both layers of extension
data, to produce 7.1-channel audio. This clever technique provides an
advantage to MPEG-2 over other encoding schemes, since cheap MPEG-1
decoders can be used when necessary. However, the cost of MPEG-2
decoders dropped in a few years to about the same level as MPEG-1. Nevertheless,
there are several drawbacks to the backward-compatibility
scheme, in addition to the inefficiency of matrixed channel duplication. The
original two-channel MPEG-1 encoding process was not developed with
matrixed audio in mind and therefore may remove surround-sound detail.
The matrix-canceling process tends to expose coding artifacts; that is, when
the signal from a matrixed channel is removed, the remaining signal may
no longer mask the same neighboring frequencies or noise as the original
signal, thus unmasking the noise introduced by formerly appropriate levels
of quantization. This problem can be mitigated in part with special processing
by the encoder, but this makes the encoding process less efficient
with a resulting loss in quality at a given bit rate.
MPEG-2 includes a non-backward-compatible process, originally labeled
NBC but now known as advanced audio coding (AAC). This coding method
deals with all channels simultaneously and is thereby more efficient, but it
was not developed in time to be supported by DVD.
MPEG-2 allows a variable bit rate in order to handle momentary
increases in signal complexity. Unfortunately, this turns out to be difficult
to deal with in practice because audio and video are usually processed separately,
and with two variable rates there is a danger of simultaneous peaks
pushing the combined rate past the limit. This can be controlled by limiting
the peak rate, but this is done at the expense of possibly reducing audio
quality in difficult passages.
TEAMFLY
Dolby Digital Audio Coding
Chapter 3
Dolby Digital audio compression (known as AC-3 in standards documents)
provides for up to 5.1 channels of discrete audio. One of the advantages of
Dolby Digital is that it analyzes the audio signal to differentiate short, transient
signals from long, continuous signals. Short sample blocks are then