Developed for VCDs, MPEG-1
uses a pre-compression signal with 352x240-pixel image frames at a rate of 30
frames per second (fps) for NTSC (352x288, 25 fps for PAL).
This represents a medium-quality image, comparable with the quality of
VHS tape. MPEG-1 uses non-adaptive
compression which produces a fixed bit-rate of 1.15 Mbps, regardless of
how much motion there is in the scene. (Compression
algorithms developed after MPEG-1, such as MPEG-2, are adaptive and produce
bit-rates that vary with the amount of motion.)
The standard VCD MPEG-1 bit-rate of 1.15 Mbps includes a single stereo
audio stream. (The format does not
support multiple language soundtracks or Directors' commentaries.)
The audio is encoded using MPEG-1 Layer 3 audio compression (now commonly
known as “mp3”, which has been widely used in audio-only
applications). It is worth noting
that the definition of mp3 has been unchanged since its introduction for VCDs.
The same audio format is used on most PAL DVDs (although most NTSC DVDs
use Dolby Digital in its place).
The target bit-rate of 1.15
Mbps was chosen to match the bit-rate of a standard CD, allowing the use of
audio-CD/CD-ROM technology (i.e. the same disc-pressing plants and the same CD
drive technology). This gives 74
minutes of video on a standard CD (650 Mbytes = 5200 Mbits).
A full-length movie typically requires two discs.
MPEG-1 files can be readily
played and edited on a PC. When
files are copied to a PC from a VCD the “.dat” extension used on the VCD
disc should be changed to “.mpg”, so that they can be readily recognized by
the video player software. Most
video players will, in fact, play a “.dat”, either directly from a VCD disc
or from the hard-drive, if you open the file from the player.
However, you cannot launch the player by clicking on the “.dat”
filename. It is generally a bad
idea to place files with a “.dat” extension on your hard-drive because the
“.dat” extension is reserved for use by the operating system.
You should never attempt to redefine the “.dat” file type to
point to a video player!
On most VCDs are there is a
single large MPEG-1 file on each disc (plus some other indexing files).
For example, on the first disc of a two-disc set there will be a single
“.dat” file containing the first 74 minutes of the movie and the rest of the
movie will be in a single file on the second disc.
However, some VCDs split the material up into multiple files.
When transferred to a PC,
the two or more MPEG-1 files from a two-disc set can be edited together to form
a single file.
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