Chapter 1

94 Everything You Ever Wanted to Know About DVD
Picture dimensions are at maximum 720�480 (for 525/60 NTSC display)
or 720�576 (for 625/50 PAL/SECAM display). Pictures are subsampled
from 4:2:2 ITU-R BT.601 down to 4:2:0 before encoding, allocating an average
of 12 bits per pixel in Y'CbCr format. (Color depth is 24 bits, because
color samples are shared across 4 pixels.) DVD pixels are not square (see
“What’s Widescreen? How Do the Aspect Ratios Work?”).
The uncompressed source is 124.416 Mbps for video source
(720�480�12�30 or 720�576�12�25) or 99.533 or 119.439 Mbps for
film source (720�480�12�24 or 720�576�12�24). In analog output
terms, lines of horizontal resolution are usually around 500, but they can go
up to 540 (see “What Do Lines of Resolution Mean?”). The typical luma frequency
response maintains full amplitude 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 a DVD).
Chroma frequency response is half that of luma.
Allowable picture resolutions are as follows:
MPEG-2, 525/60 (NTSC) 720�480, 704�480, 352�480, 352�240
MPEG-2, 625/50 (PAL) 720�576, 704�576, 352�576, 352�240
MPEG-1, 525/60 (NTSC) 352�240
MPEG-1, 625/50 (PAL) 352�288
Different players use different numbers of bits for the video digital-toanalog
converter, with the best-quality players using 10 or 12 bits. This has
nothing to do with the MPEG decoding process, because each original
component signal is limited to 8 bits per sample. More bits in the player
provide more “headroom” and more signal levels during the digital-toanalog
conversion, which can help produce a better picture.
The maximum video bit rate is 9.8 Mbps. The average video bit rate is
around 4 Mbps but depends entirely on the length, quality, amount of audio,
and so on. This is a 31:1 reduction from an uncompressed 124 Mbps video
source (or a 25:1 reduction from a 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 I-frames) are supported and can be displayed
for a specific amount of time or indefinitely. These are used for
menus or slideshows. Still frames can be accompanied by audio.