DVD Demystified

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The DTS Coherent Acoustics format used on DVDs is different from the
one used in theaters, which is Audio Processing Technology’s apt-X, a
straight ADPCM coder with no psychoacoustic modeling.
MLP Audio Encoding
In addition to the lossy perceptual coding formats from DVD-Video, the DVD-
Audio format includes a mathematical encoding technique called Meridian
lossless packing (MLP). MLP compresses audio data bit for bit, removing
redundancy without discarding any data. In addition to storing more data in
the same amount of space, thus giving longer playing times, MLP reduces the
maximum peak data rate. Since six channels of 96-kHz 24-bit audio have an
uncompressed data rate of 13.824 Mbps, reducing the peak data rate to fit
into the DVD-Audio maximum of 9.6 Mbps is important. MLP produces variable
data rates, providing longer playing times than a fixed-rate scheme.
MLP achieves a typical compression ratio of 2:1 by using a combination
of three techniques: lossless matrixing to compress interchannel redundancy,
lossless waveform prediction to compress intersample correlation,
and entropy coding of the remaining values. MLP also incorporates stream
buffering to help deal with transients and hard-to-compress segments so as
to limit the peak data rate. Unlike lossy encoding methods, MLP cannot
throw away more data to stay within a specified data rate, so in some case
it will be unable to sufficiently reduce the data rate. In such cases, the
encoder operator must use other options such as reducing the bit size of one
or more channels or filtering out high-frequency data.
MLP can encode a 2-channel downmix along with a multichannel mix. It
also can carry additional data such as dynamic range control profiles, copy
control information, time codes, and descriptive text. The format also has
built-in error detection to recover from transmission errors.
Effects of Audio Encoding
Chapter 3
Aside from lossless MLP encoding, which has no effect on the audio signal,
lossy perceptual audio compression can affect the quality of the audio signal.
Audio compression techniques result in a set of data that is processed
in a specific way by the decoder. The form of the data is flexible, so improvements
in the encoder can result in improved quality or efficiency without
changing the decoder. As understanding of psychoacoustic models improves,
perceptual encoding systems can be made better.