What's different from VCD and MPEG-1 system ? I'd like to know the system layer of VCD.

-- Takashi Kaneko (takashi@nsg.sgi.com), December 13, 2000

Answers

The MPEG videos on a Video CD must meet certain requirements which are outlined in the White Book. To create MPEG videos for your Video CD there is a range of hard and software solutions available from third-party vendors. When making MPEGs it is important to encode to White Book specifications. MPEG video can either be stored as a separate track on the Video CD (MPEG track) or as a play item inside the Video CD data track (MPEG file). MPEG tracks must be encoded to MPEG-1 standard at 1150 kbit/s video and 224 kbit/s audio, and must be multiplexed according to White Book standard. Most encoding software will have a standard setting called White Book or Video CD to produce appropriate MPEG streams.

MPEG files can have a data rate of up to 1,37 Mbit/s split at different ratios between the video and the audio data rate. For audio encoding, three data rates are allowed for single channel mode (64, 96, and 192 kbit/s), and four data rates for stereo, intensity stereo and dual channel mode (128, 192, 224, and 384 kbit/s). The rest of the bandwidth is available for video.

CDs are physically divided into tracks. In the case of Video CDs, all tracks are recorded in a single session, after which the CD cannot be written to again. The beginning of the session has a session lead-in, and the end of the session has a lead-out. The Video CD layout is shown in the following diagram:

Video CD Layout: LEAD-IN TRACK 1 (VIDEO-CD TRACK) TRACK 2,3,4... (MPEG TRACKS) TRACK N, NN... (CD-DA TRACKS) LEAD-OUT

The lead-in is followed by track 1 of the Video CD, which is the Video CD data track. The MPEG tracks consecutively follow track 1. The MPEG tracks are followed by the CD-DA (Compact Disc-Digital Audio) tracks. The session ends with the lead-out. The maximum number of tracks allowed on any CD is 99. The Video CD data track contains data needed for the Video CD or CD-i player to play back the Video CD. The following diagram shows the layout of track 1:

Data Track Layout: 00:02:16 PVD - PRIMARY VOLUME DESCRIPTOR RESERVED AREA (OPTIONAL) SEGMENT PLAY - PLAY ITEMS MPEG STILL, AUDIO AND FILES. CDI APLICATION PROGRAM OTHER FILES

The file system of the data track is ISO9660. The track starts with a two second gap, followed by 16 empty sectors, according to standard ISO9660. The PVD (Primary Volume Descriptor) is located at time-code 00:00:02:16. The PVD contains basic information about the CD. The PVD is followed by the reserved area, which contains the Karaoke basic information area and the Video CD information area. The Karaoke basic information area contains information relevant for Karaoke CDs. It is not used in VideoPack because Karaoke CDs cannot be created using VideoPack.

This area is followed by the Video CD information area, which contains the structure of the Video CD, i.e. the links to and from the different nodes within a Video CD, and how and when they can be activated. This area is used by playback devices like the Video CD player. Optionally the reserved area is followed by the segment play item area, which contains some types of play items available on the Video CD, i.e. still MPEG streams and MPEG files. They have a different function to the MPEG tracks, and are therefore contained in the data track rather than being located on a separate track.

The CD-i application program is also part of the first track. It is the software necessary for playing a Video CD on a CD-i player. The Video CD is a bridge disc. Therefore, some of the information in the data track is represented in an ISO 9660 file system. This means that this data track can be accessed by most computer platforms. If, for example, a Video CD is inserted into the Video CD-ready CD-ROM drive of a PC it is possible to access the first track of the CD, and it will appear as if it were a normal CD-ROM. The Video CD contains a directory structure and files in this ISO9660 file system structure. A Video CD must always contain certain directories and files as specified in the White Book, and these directories and files are automatically created by VideoPack. They contain information about the content of the Video CD. A typical directory structure of a Video CD may look like this:

The Directory Structure: ROOT-> It is possible to add other files and directories to this structure as needed. Examples of the use of this feature are the inclusion of an MPEG player into a computer platform so that the MPEG videos of Video CDs can be played, or the inclusion of a complete application program.

-- Carlyle Zamith (carlyle@internext.com.br), December 13, 2000.