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The China Video Disc (or CVD) standard is a CD-based MPEG-2 audio and video
format developed in 1997, intended as a successor to Video CD and an alternative
to DVD-Video. It is almost identical to the SVCD standard, the only technical
difference being a lower video resolution.
Technical specifications
On a technical basis the CVD standard is nearly identical to the SVCD standard.
The only difference between the two formats is the video resolution: SVCDs
require 480x576 (PAL/SECAM) or 480x480 (NTSC) resolution video, whereas CVDs use
352x576 (PAL/SECAM) or 352x480 (NTSC) resolution video. All other technical
aspects of the CVD standard are identical to those of the SVCD standard.
Video
Codec: MPEG-2
Resolution:
NTSC: 352x480
PAL/SECAM: 352x576
Aspect Ratio: 4:3
Framerate:
NTSC: 29.97 or 23.976 frames per second
PAL/SECAM: 25 frames per second
Bitrate: Up to 2.6 Megabits (2,600 kilobits) per second
Rate Control: Constant or variable bitrate
Interlaced video is supported (though not required) for CVD video, excepting any
video at 23.976 frames per second, as it must use 3:2 pulldown.
The CVD video standard is almost completely compatible with the DVD-Video
standard. However, the CVD standard does not specify any limits on GOP
structure, and as such it is possible to encode CVD video with a GOP size larger
than DVD-Video's 64 frame-per-GOP limit. In practice this is highly uncommon;
most commercially produced CVD video uses a 12 or 15 frame GOP structure, which
is compatible with the DVD-Video standard.
Audio
Codec: MPEG-1 Audio Layer II
Frequency: 44,100 hertz (44.1 kHz)
Output: monaural, dual channel, or stereo
Bitrate: Certain bitrates between 32 and 384 kilobits per second, inclusive
Rate Control: Constant bitrate
As with most CD-based video formats, CVD audio is incompatible with the
DVD-Video standard due to the difference in frequency; DVDs require 48 kHz,
whereas CVDs use 44.1 kHz.
Additional features
Support for features such as karaoke, selectable subtitles, two selectable audio
tracks, and DVD-quality slide shows.
Advantages/disadvantages to SVCD
Though the lower resolution of the CVD standard brings with it less detailed
video than the SVCD standard, it also provides certain advantages. Most obvious
are the space requirements for "decent quality" video, as the lower resolution
allows the bitrate to be reduced significantly more while keeping the number of
"compression errors", such as MPEG block artifacts, to a minimal level. Other
advantages primarily center around the DVD support for the resolution; while CVD
standard video is also DVD-compatible, SVCD standard video is not. Initially
this is only useful for transferring CVD content to DVD, as the video does not
have to be transcoded and can avoid the quality loss which would result. (Though
the audio must be transcoded, as DVDs require 48 kHz audio rather than the 44.1
kHz audio used on CVDs) Additionally, CVDs do not suffer from certain playback
issues faced by SVCDs due to limitations present in many DVD players. These
issues are mostly due to "foldover" or aliasing problems encountered when the
DVD player resizes the video as it plays it. While SVCDs are frequently affected
by this issue CVDs are not, as the resizing algorithms used by DVD players are
almost always optimized with valid DVD resolutions (such as the CVD resolution)
in mind.
The major disadvantages of the CVD standard, aside from the lower resolution,
are primarily compatibility issues. While almost all PAL DVD players which are
listed as "SVCD compatible" must also support the CVD standard, DVD players sold
in the United States have no such requirement. Though no solid figures exist, it
has been estimated that roughly a quarter of those DVD players sold in the
United States which are "SVCD compatible" do not support the CVD standard.
History of development
As the name indicates, the CVD standard originated in China. It was developed by
C-Cube Systems as part of a late 1990s Chinese government-sponsored effort to
create a next-generation CD-based video standard. It was the first of three
competing formats (the others being SVCD and HQ-VCD) to be released into the
Chinese market. This early release, which quickly established a significant CVD
customer base, eventually prompted the Chinese government to force a compromise
between the competing standards. Both the SVCD and CVD standards were combined
into one composite standard known as Chaoji Video CD, (though it is still often
called "SVCD") which was adopted by the Chinese government as the "official"
next-generation video disc standard.
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