Chapter 1

DVD Technical Details 93
Dolby Digital, DTS, and other formats. This means the DVD-Video format
defines subsets of these standards and formats to be applied in practice to
make discs intended for DVD-Video players. DVD-ROM can contain any
desired digital information, but DVD-Video is limited to certain data types
designed for television reproduction.
A disc has one track (or stream) of MPEG-2 constant bit rate (CBR) or
variable bit rate (VBR) compressed digital video. A restricted version of
MPEG-2 Main Profile at Main Level (MP@ML) is used. SP@ML is also supported.
MPEG-1 CBR and VBR video are also allowed, along with 525/60
(NTSC 29.97 interlaced frames per second [fps]) and 625/50 (PAL/SECAM
25 interlaced frames/sec) video display systems. Coded frame rates of 24
fps progressive from film, 25 fps interlaced from PAL video, and 29.97 fps
interlaced from NTSC video are typical.
MPEG-2 progressive_sequence is not allowed, but interlaced sequences
can contain progressive pictures and progressive macroblocks. In the case
of 24 fps source, the encoder embeds MPEG-2 repeat first field flags into
the video stream to make the decoder either perform 2-3 pulldown for 60
Hz NTSC displays (actually 59.94 Hz) or 2-2 pulldown (with a resulting 4
percent speedup) for 50 Hz PAL/SECAM displays. In other words, the
player doesn’t know what the encoded rate is; it simply follows the MPEG-2
encoder’s instructions to produce the predetermined display rate of 25 or
29.97 fps. This is one of the main reasons two kinds of discs are available,
one for NTSC and one for PAL. (Very few players convert from PAL to NTSC
or NTSC to PAL. Refer to Chapter 1’s “Is DVD-Video a Worldwide Standard?
Does It Work with NTSC, PAL, and SECAM?”)
Because film transfers for NTSC and PAL typically use the same coded
picture rate (24 fps) even though PAL resolution is higher, the PAL version
takes more space on the disc. The raw increase before encoding is 20 percent
(480 to 576), but the final result is closer to 15 percent, depending on
encoder efficiency. This translates to an increase of 600 to 700 MB on PAL
discs compared to NTSC discs.
It’s interesting to note that even interlaced source video is often encoded
as progressive-structured MPEG pictures, with interlaced field-encoded
macroblocks used only when needed for motion. Most film source is
encoded progressive (the inverse telecine process during encoding removes
duplicate 2-3 pulldown fields from videotape source), and most
video sources are encoded interlaced. These may be mixed on the same
disc, such as an interlaced logo followed by a progressive movie.
See “What Is the Difference Between Interlaced and Progressive Video?”
for an explanation of progressive and interlaced scanning. See Chapter 1’s
“What’s a Progressive DVD Player?” for progressive-scan players. See the
MPEG page at www.mpeg.org for more information on MPEG-2 video.