Ever wanted to know exactly what a VCD or an SVCD was? Then this article is for you. Whilst these formats are a dying breed, there is still quite a legacy of programming available in these formats, so it is worth at least understanding the basics of what they are all about.
The Audio CD was introduced over 20 years ago. Whilst not quite the “perfect sound, forever” promised at its introduction, it was nonetheless a compelling advance over existing music formats, such as the LP, single and compact cassette and was thus a rip-roaring success.
Not all that long after the introduction of the Audio CD, the potential uses for the optical-based format expanded way beyond the simple reproduction of audio. Computer uses were the most obvious, with the CD-ROM format offering 650Mb of mass storage in a robust, compact and cheap package. It is worthwhile remembering that at the time of its introduction, the average computer had 20Mb – 40Mb of hard disc storage space in total, so 650Mb was a massively large increase in data capacity.
At around the same time, the home videotape market was exploding with the introduction of VHS and beta videocassettes. Great for time shifting, VCRs were not so great in very humid areas with the tape prone to sticking and rapid deterioration.
Video CD offered the advantages of a CD-based format
(robustness and high data capacity) without the disadvantages of tape-based
formats (deterioration and sticking). There were, however, other compromises
that were necessary.
650Mb sounds like quite a lot of data space until you
consider the tremendous amounts of data that video information takes up in
digital form. Uncompressed, you can only fit 30 seconds to 1 minute of PAL
resolution video into 650Mb – not very practical. The solution? Video
compression. At a basic level, video compression works by examining each frame
of a moving image and only storing the differences between them. Long, steady
shots of the distant landscape take up small amounts of room versus sports
action shots with motion all over the screen. Quite dramatic reductions in data
requirements can be made with the judicious use of video compression.
Video CDs store up to 73 minutes of compressed video and
audio. They utilize a video compression scheme known as MPEG-1 (DVD uses MPEG-2,
a more advanced version of this scheme). Other techniques reduce the data load
of the video stream; Video CD stores video at a resolution of 352x288 for PAL
video (versus 720x576 for PAL DVD video). This immediately cuts down the data
requirement by a large factor, and the MPEG-1 compression is then able to do the
rest. Audio is 2 channel and also compressed.
However, the compromises necessary to fit 73 minutes of video
onto a CD can be quite obvious image-wise at times, with high motion images
suffering the most. Compression artefacts are common on Video CDs, manifesting
as areas of increased blockiness in the image.
Another major compromise of the Video CD format is its
maximum 73 minute running time. Longer programs require 2 or 3 Video CDs.
Nonetheless, Video CD has proved to be a highly popular video
format, particularly in South East Asia and China where its robust nature and
ease and cheapness of production overwhelmingly overcome the deficiencies of the
format.
The newer MPEG-2 video compression offers a number of
advances over MPEG-1 video compression, not the least of which is Variable Bit
Rate video encoding. MPEG-1 video compression spits out its video data at the
same rate, regardless of whether a given scene needs it or not. This results in
low motion scenes being allocated data space that they don’t necessarily
require, whereas high motion scenes are not allocated enough data space to
render them without artefacts.
This was essential with Video CD as the data from a Video CD
player came off the disc at a constant rate, the same as it would for an Audio
CD.
MPEG-2 data compression addresses this deficiency by allowing
the overall data rate of the video to vary, so low motion scenes use less
overall data, saving valuable data space for high motion scenes.
The commonest use for MPEG-2 video data compression is DVD,
but it is also used on the successor to Video CD, Super Video CD.
Super Video CD increases the overall resolution of the stored
image from 352x288 to 480x576 for PAL Super Video CDs, and compresses the video
using MPEG-2 Variable Bit Rate compression. The nett result is a considerably
sharper image with less artefacts, but the trade-off is that a typical SVCD
running time is only 25 minutes. It has found a niche for storing episodes of TV
programming.
Note that these variants are not standard formats, but
rather extensions to the specifications which have been made over the years
which may or may not play back in all playback devices.
The Video CD was a compromise based on the available
technology of its time; MPEG-1 video encoding and single speed CD-ROM drives.
You could only get data off a CD at a specific rate (the same as Audio CD
playback). As the hardware progressed, CD-ROMs got faster and faster, so it was
almost inevitable that the opportunity was taken to enhance the humble Video CD
by increasing the data rate of the MPEG-1 video stream. A Video CD stores the
MPEG-1 video stream at a constant 1.1 megabits per second. Increasing this to
2.5 megabits per second gives us the extended variant of a Video CD, an XVCD (eXtended
Video CD). The quality is better than a video CD as the video data rate is
increased, but consequently the total time available per disc is decreased to
around 30 minutes.
The same principle applies to an Extended Super Video CD (XSVCD).
Again, the maximum video bit rate is increased over the Super Video CD
specification and again this results in increased image quality at the cost of
decreased playing time.
An intriguing format which has only limited support at
present is the miniDVD (or cDVD) format. In essence this is a CD which has been
created with a DVD’s file structure. It has all the benefits of DVD (complex
menus, high quality video and audio encoding) but has cheaper media costs
because it is based on CD rather than DVD. A miniDVD’s running time is limited
to around 30 minutes, and usually much less.
Unfortunately, not many DVD players will play back miniDVD
discs. This is due to the fact that when a player checks what sort of disc is in
the drive, the first choice it generally makes is whether the disc is a CD or a
DVD. If it detects a CD, it then does not look for DVD file structures and
consequently doesn’t know what to do with a miniDVD disc. Discs in this format
generally require a computer-based DVD player for successful playback.
© Michael Demtschyna
16th June 2004