What is Video CD?

Vs.1.0, 940406
Andrew Davidson & Lucy Lediaev
Philips Interactive Media

THE TECHNOLOGY

Video CD is a new technology that allows you to play linear video material
from a compact disc. Compressed video and audio data are stored on a Video
CD and can be played back on several platforms. The technology is described
in detail in a functional specification commonly known as the 'White Book.'

One of the attractions of Video CD as a publishing format is the ability to
play these discs on multiple play-back platforms. This ensures a large
installed base of devices on which Video CD titles will play. Video CD lends
itself to delivery of motion pictures, music videos, and professional and
training applications. Video CD is independent of broadcast standards (NTSC
and PAL), and a single disc plays on any Video CD platform worldwide.

How Does Video CD Work?

Video CD uses an international standard for video compression-MPEG-1 (ISO
standard - IEC 11172).* This compression technology allows you to store over
70 minutes of VHS-quality video material on a standard compact disc. The
audio, although also compressed, is near the quality of Compact Disc-Digital
Audio.

A Video CD disc is a 'CD-i Bridge disc'. That means that it can play on a
number of devices but must always contain a CD-i application so that it can
be played on any CD-i player. The Bridge format assures multi-platform
compatibility. Target platforms for Video CD discs are dedicated Video CD
players, computer systems configured to support Video CD, and CD-i players
with Digital Video cartridges.

Video CD players. A dedicated Video CD player is designed to play only Video
CDs and is not a general purpose multimedia machine. This kind of player
might be likened to a VCR for compact discs. The play-back application for
Video CD is contained in the player hardware and allows play back of the
linear video material on the disc. Several hardware manufacturers will
release players later this year.

Computer systems. A computer system that features a CD-ROM/XA drive, an
MPEG-1 decoder, and a host play-back application can also be used to play
Video CD discs. The application can be included in the hardware (for
instance, on a special board) or as a software application on the compact
disc or on the host computer system's hard disk. This platform can provide
more functionality than a dedicated Video CD player.

CD-i player with Digital Video extension. Video CD discs can also be played
on any CD-i player that has the Digital Video extension. Philips players
currently offer this extension via a plug-in cartridge, but this capability
may be built into future CD-i players. Like computer systems, CD-i players
can allow the user to have more interactive control of play-back behavior
through the use of features like on-screen, graphical tables of contents and
supplemental material, such as behind-the-scenes interviews, commentaries,
discographies, etc.

Comparing CD-i DV and Video CD

A CD-i DV disc is a single platform format. It must be played on a dedicated
CD-i player or on computer hardware that fully emulates the capabilities of
a CD-i player. As mentioned earlier, Video CDs can be played on several
devices. CD-i allows up to 32 MPEG audio channels, which may be used for
multilingual and other applications, and only two audio channels are
possible with Video CD. There are also differences in the video frame size
and rate between the two formats, but the user of the disc is not aware of
these differences. It is possible to add features to a Video CD that can be
accessed by a CD-i player, but not by a dedicated Video CD player. For
example, a Video CD could contain on-screen graphics, menus, and other
features that would increase its interactivity and, thus, its perceived
value when played on a CD-i player.

Why publish in the Video CD format?

Because Video CD is an international, open standard, it is a desirable
delivery method for content publishers. The multi-platform compatibility
assures a large installed base of prospective customers. The publisher can
also offer a range of features on one compact disc; addressing several
platforms that allow a different sets of features related to the target
hardware platforms within one disc-based product. Production of a Video CD
is relatively simple and inexpensive, and replication and distribution costs
are much lower than those for VHS.

PRODUCTION

Specification

The first step in the Video CD production task is to specify the layout and
contents of the title. This includes defining the track/chapter breakdown,
planning the audio tracks, and defining platform-specific additions. The
producer must decide how to break a movie or other material into chapters
or, in the case of music videos, into tracks. For motion pictures, the
chapter organization is usually subject to approval by the film's original
director. Video CD allows use of one or two sound tracks; these tracks can
be used to provide a single stereo sound track or two mono sound tracks.
Based on the capabilities of the delivery platform, it may be possible, as
in the case of CD-i, to add chapter headings and other on-screen graphical
elements or reference material, such as discography or biographical
sketches.

Encoding

The video source material must be encoded, or converted, via compression
techniques into the MPEG 1 format. There are number of MPEG encoding systems
currently available, and, among those systems, there is a very wide range in
both quality and cost. In deciding which encoding system to use, you need to
ask yourself several questions. Should you install a desktop system at your
site or use a service bureau with a high-end encoding system? Will you get
better results from a real-time or non real-time system? Are you preparing
material for a consumer application or for a professional application. You
will need to examine the cost versus quality trade offs to make these
decisions. For example, you may be able to sacrifice quality to keep the
price down on MPEG data intended for a professional application, such as a
training disc. On the other hand, a movie studio that has made a
multi-million dollar investment in the production of a major motion picture
will demand the highest quality product of the encoding process.

Following are the steps involved in the encoding process. If you are
undertaking this part of the produciton yourself, you will need to be aware
of these issues. If you are using a service bureau, you will want to be
conversant with them, at the least.

   * Grabbing is the process of capturing the data from the videotape and
     moving it to a computer hard disk for processing. In general, digital
     source gives better results than analog source material.

   * Some preprocessing of the video material may be required to reduce
     electronic noise and to correct aspect ratios to assure the best
     possible images on the target play-back system.

   * Filtering processes may also be performed that automatically detect the
     cuts from one scene to another and that adjust the frame rate from
     videotape to one of the three choices for frame rates in MPEG-24, 25,
     or 30 frames per second. One of these methods is known as 'inverse 3:2
     pull down.'

   * There are fewer pixels in a Video CD image than on a standard videotape
     frame (D1, for instance), so the image must be reduced in size, or
     subsampled. The way this is done depends on a number of factors
     including the picture rate (film or video), broadcast standard (NTSC or
     PAL), and play-back system.

   * The actual process of compressing data successfully is as much an art
     as it is a science. Years of experience and an aesthetic sense are
     required to make sure that all of the appropriate 'knobs' in the
     process are fine tuned. For the best results, it is extremely important
     to have the services of an experienced video post-production house or a
     person with extensive post-production experience. Using all of the
     techniques available, it is possible for a skilled video technician to
     optimize the results of the encoding process.

There are some basic rules of thumb for getting the best results from the
encoding process in conjunction with MPEG:

   * Stay as close to the original source as you can. Each additional
     (analog) generation degrades the quality. This is axiomatic.

   * Component source is better than composite. Digital source is better
     than analog. Component versus composite is a more important factor in
     quality than is the digital versus analog factor. Thus, the order of
     preference among common formats is D1, Beta SP, D2, and then any other
     format. Avoid letting anyone talk you into using D2 instead of Beta SP
     (if the Beta system is a high quality component one.) Here's a summary
     chart:

                               Digital               Analog

     Component                    D1
                           Digital Betacam           Beta SP

     Composite                    D2              1', 3/4', VHS

   * Garbage in-worse compressed garbage out! Remember that VHS, Umatic,
     Beta, etc., are all forms of compression when the original medium was
     film. Compression magnifies any artifacts in the source; it doesn't
     conceal them. If you have a compressed intermediate source, you are
     compressing your data two (or more) times.

   * Avoid standards conversions. Conversion of source material from NTSC to
     PAL, or vice versa, automatically induces artifacts that just get worse
     during compression.

Following these guidelines and adhering to standard high-quality
post-production practices will help in preserving the quality of your source
material during MPEG encoding.

Graphics

The services of a graphic artist are required to create the table of
contents, menus, and any additional material desired on the disc. The artist
needs to be aware of the constraints of the graphical display on each of the
target hardware platforms and to take those in to account. The amount of
graphical material accessible from a dedicated Video CD player is limited to
a rudimentary table of contents and controls. The producer can, if desired,
take advantage of the features of the CD-i player or of a computer
application and provide additional material for play on those platforms.

Assembly

The disc building process consists of assembly of all of the elements and
creating a script that generates the disc image for the title. Philips and
others have developed software tools that automate the assembly and building
processes. Once the disc image is made, it is sent on tape to a disc
manufacturing plant for mastering and replication. It is also common
practice to create one-off discs on a WORM burner so that the disc can be
tested internally before replication.

FOR MORE INFORMATION

The contents of a Video CD disc are described completely in the
specification. Adherence to the standard is the only way to ensure that a
Video CD disc will play on any of the target platforms. If any of the
required files is missing or contains incorrect data, the disc may not play
on some players.

The Video CD Specification (and other Philips-licensed optical disc
specifications) may be obtained from:

Bert Gall
Philips Consumer Electronics
Coordination Office Optical & Magnetic
Media Systems
PO Box 80002
5600 JB Eindhoven
The Netherlands
Phone:  +31 40 736409
FAX:            +31 40 732113

For information on development tools or on obtaining the CD-i application
provided by Philips, write to Philips Interactive Media Systems at the
address above or call:

Phone: +31 40 735932
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* MPEG stands for Motion Picture Expert Group.

Copyright 1994 Philips Interactive Media. All rights reserved.
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