MPEG-2 is the standard used
for DVDs.
MPEG-2
was developed in the early 1990s, and the standard was frozen in 1995, ready for
the launch of DVDs and DVD players in 1997. MPEG-2 uses image frames of 720x480-pixels at a rate of 60 fps
(or 720x576 pixels at 50 fps for PAL), with adaptive compression that
produces a variable bit-rate (VBR) of up to a maximum of 9.8 Mbps.
The typical average bit-rate is about 5 Mbps (of which only about
3.8 Mbps is used for video, with the rest being reserved for audio and
subtitles). The audio component of
the bit stream can carry up to 8 audio tracks using (according to the original
DVD specification) one of three optional audio formats:
(a)
Any one of the six common linear PCM standards,
(b)
Dolby Digital 5.1, also known as AC-3, or
(c)
MPEG-2 Layer 3 Audio, which is virtually the same as MPEG-1 Layer 3,
commonly referred to as mp3.
For PAL DVDs option (c) is
the audio standard. For NTSC DVDs
option (b) is the standard. However,
most players will support all three options.
On DVDs, and on digital
cable and satellite systems, video material is encrypted with the CSS encryption
algorithm immediately after compression, so a “.vob” file is typically an
encrypted file. However, an
MPEG-2-format file can exist “out of captivity” if it is decrypted and then
saved in-the-clear. In this case it
would typically be saved as an “.avi” file.
MPEG-2 picture quality, in
terms of “pixels times frame-rate”, is roughly 8 times better than MPEG-1,
and would therefore be expected to produce a bit-rate close to 10 Mbps.
But because of the use of an adaptive compression algorithm, the average
bit-rate for a movie is kept down to about 5 Mbps (although during
fast-moving scenes the instantaneous bit-rate may rise to the 9.8 Mbps maximum
supported by the DVD medium). This
5 Mbps average includes about 3.8Mbps for video, 1.152 Mbps for audio (8
channels), and 40 kbps for subtitles.
As a result of the good
compression performance of MPEG-2, a single-layer DVD (4.7 GBytes = 37,600 Mbits,
or 7.23 times the capacity of a CD) can hold about 125 minutes of video and a
dual-layer disc (8.5 GB capacity) can hold about 225 minutes.
MiniDVD:
A “miniDVD” is a standard CD containing DVD-style MPEG-2 video and using the file names and file structure of a true DVD. In other words, it is logically a DVD but physically a standard CD. It can only hold about 15 minutes of video. These discs can be played on a PC’s CD drive. Some of the newer DVD players can play miniDVDs.
The SVCD was introduced just before the launch of DVDs in order to try to extend the life of VCD technology. An SVCD is a standard CD using the file names and file structure of a normal VCD, in combination with a reduced-quality implementation of MPEG-2 in place of MPEG-1. This version of MPEG-2 uses a pre-compression signal with 480x480 pixels at 30 fps. The result is a disc that gives significantly better quality than a VCD, but not as good as a DVD, and with a maximum playing time of only 35-40 minutes. An SVCD, unlike a miniDVD, is logically a VCD, but with “tweaked” video using a variant of MPEG-2.
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