DVD Demystified

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Deficient Pan and Scan
DVD uses a feature of MPEG-2 to store picture offset information that
allows anamorphic wide-screen video to be converted automatically to fullframe
pan and scan. However, the horizontal-only limitation of this feature
has caused it to be widely ignored. The full-frame conversion process in a
studio not only pans from side to side but also moves up and down and
zooms in and out and usually includes extra picture from above and below
the wide-screen area. Disc producers are not happy with DVD’s limited
automatic pan and scan feature, so they either create a wide screen-only
disc or they include a second full-frame pan and scan version of the movie
on the disc. The designers of DVD could have included the option for fullframe
video storage at high resolution to allow extraction of 4:3 pan and
scan, 4:3 letterbox, and 16:9 anamorphic with no loss of resolution. This
would require more pixels and thus more storage space (39 percent more for
720 � 666 anamorphic and 71 percent more for 888 � 666 full frame) but
would be more efficient than encoding two (or three) separate versions on
the disc. The added video processing complexity would have slightly
increased player cost in early generations.
Inefficient Multitrack Audio
TEAMFLY
Chapter 7
MPEG-2 audio is inefficient in MPEG-1—compatibility mode. Parts of the
audio signal must be duplicated to achieve full channel separation, thus
creating an overhead of about 10 percent (see Chapter 3 for details). MPEG-
2 provides a non-backward–compatible format (AAC), but since this system
was not finalized when DVD was introduced, and since MPEG-2 decoders
were not available, DVD-Video discs intended for PAL players are saddled
with a less-than-optimal algorithm on MPEG-2 multichannel tracks in
order to support MPEG-1 decoders.
Other ideas for improving the general efficiency of multiple audio tracks
were considered. Alternate language sound tracks contain the same music
and sound effects, differing only in the dialogue. It could be possible to store
all dialogue tracks in mono or stereo form and have the player use a twotrack
decoder to mix them together. The problem is that dialogue is not isolated
to one or two tracks. Using a mono or nondiscrete dialogue track could
detract from the audio presentation. Another possibility is differential
encoding, which allows additional speech tracks to be encoded relative to