3.4 What are the video details?
Description
This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
3.4 What are the video details?
DVD-Video is an application of DVD-ROM. DVD-Video is also an application of
MPEG-2. This means the DVD format defines subsets of these standards to be
applied in practice as DVD-Video. 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 (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 is also allowed. 525/60 (NTSC, 29.97 interlaced
frames/sec) and 625/50 (PAL, 25 interlaced frames/sec) video display
systems are expressly supported. 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 60Hz (59.94) displays or 2-2 pulldown (with
resulting 4% speedup) for 50Hz displays. In other words, the player doesn't
really "know" what the encoded rate is, it simply follows the MPEG-2
encoder's instructions to produce the predetermined display rate of 25 fps
or 29.97 fps. (Very few players convert from PAL to NTSC or NTSC to PAL.
See 1.19.) 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. A computer can
mostly ignore the repeat_first_field flags and re-interleave (weave) the
video fields back into full-resolution progressive frames, which works
especially well at 72 Hz refresh rate (3x24). Computers can improve the
quality of interlaced source by doubling the lines in fields (bobbing) and
displaying them as progressive frames at twice the normal rate. Most film
source is encoded progressive (the inverse telecine process in the encoder
removes duplicate 2-3 pulldown fields from videotape source); 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 3.8 for an explanation of progressive and interlaced scanning. See 1.40
for progressive-scan players. See the MPEG page <http://www.mpeg.org> for
more information on MPEG-2 video.
Picture dimensions are max 720x480 (for 525/60 NTSC display) or 720x576
(for 625/50 PAL/SECAM display). Pictures are subsampled from 4:2:2 ITU-R
BT.601 down to 4:2:0, allocating an average of 12 bits/pixel in Y'CbCr
format. (Color depth is 24 bits, since color samples are shared across 4
pixels.) The uncompressed source is 124.416 Mbps for video source
(720x480x12x30 or 720x576x12x25), or either 99.533 or 119.439 Mbps for film
source (720x480x12x24 or 720x576x12x24). Using the traditional (and rather
subjective) television measurement of "lines of horizontal resolution" DVD
can theoretically produce 540 lines on a standard TV (720/1.33) and 405 on
a widescreen TV (720/1.78). In practice, most DVD players provide about 500
lines because of filtering in the players and in the optics of telecine
equipment. VHS has about 230 (172 widescreen) lines and laserdisc has about
425 (318 widescreen). Note that lines of horizontal resolution (visually
resolvable vertical lines per picture height) are not the same as total
horizontal pixels (720) or horizontal scan lines (480). In analog output
signal terms, typical luma frequency response maintains full amplitude to
between 5.0 and 5.5 MHz. This is below the 6.75 MHz native frequency of the
MPEG-2 digital signal (in other words, most players fall short of
reproducing the full quality of DVD). Chroma frequency response is half
that of luma.
Different players use different numbers of bits for the video
digital-to-analog converter. Current best-quality players use 10 bits. This
has nothing to do with the MPEG decoding process, since each original
component signal is limited to 8 bits per sample. More bits in the player
provide more "headroom" and more signal levels during digital-to-analog
conversion, which can help produce a better picture.
Maximum video bitrate is 9.8 Mbps. The "average" bitrate is 3.5 but depends
entirely on the length, quality, amount of audio, etc. This is a 36:1
reduction from uncompressed 124 Mbps (or a 28:1 reduction from 100 Mbps
film source). Raw channel data is read off the disc at a constant 26.16
Mbps. After 8/16 demodulation it's down to 13.08 Mbps. After error
correction the user data stream goes into the track buffer at a constant
11.08 Mbps. The track buffer feeds system stream data out at a variable
rate of up to 10.08 Mbps. After system overhead, the maximum rate of
combined elementary streams (audio + video + subpicture) is 10.08. MPEG-1
video rate is limited to 1.856 Mbps with a typical rate of 1.15 Mbps.
Still frames (encoded as MPEG-2 I-frames) are supported and can be
displayed for a specific amount of time or indefinitely. These are
generally used for menus. Still frames can be accompanied by audio.
A disc also can have up to 32 subpicture streams that overlay the video for
subtitles, captions for the hard of hearing, captions for children,
karaoke, menus, simple animation, etc. These are full-screen,
run-length-encoded bitmaps limited to four pixel types. For each group of
subpictures, four colors are selected from a palette of 16 (from the YCbCr
gamut), and four contrast values are selected out of 16 levels from
transparent to opaque. Subpicture display command sequences can be used to
create effects such as scroll, move, color/highlight, and fade. The maximum
subpicture data rate is 3.36 Mbps, with a maximum size per frame of 53220
bytes.
In addition to subtitles in subpicture streams, DVD also supports NTSC
Closed Captions. Closed Caption text is stored in the video stream as
MPEG-2 user data (in packet headers) and is regenerated by the player as a
line-21 analog waveform in the video signal, which then must be decoded by
a Closed Caption decoder in the television. Although the DVD-Video spec
mentions NTSC only, there is no technical reason PAL/SECAM DVD players
could not be made to output the Closed Caption text in World System
Teletext (WST) format; the only trick is to deal with frame rate
differences. Unfortunate note: DVD Closed Caption MPEG-2 storage format is
slightly different than the ATSC format. See the Closed Caption FAQ for
more about Closed Captions.
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