Note: Zones are not regions. If you're looking for a discussion of geographical playback restrictions, this isn't the document for you!
The DVD-Video (and DVD-Audio, ...) specifications impose certain restrictions upon the file structure and layout of a DVD disc. This document provides a very quick high-level overview of the top-level of this imposed structure.
Each sector on a media DVD will contain information for one of the following purposes:
The zoning system on a DVD requires that all sectors intended for a particular purpose be packed together into a contiguous set of sectors on the DVD disc. Each contiguous set of sectors is known as a zone. The zone for the primary media format of the disc will appear first. For a DVD-Audio disc, the DVD-Audio zone will be first, since this is the primary data accessed by a DVD-Audio player. On a DVD-Video disc, the DVD-Audio zone is empty. Extra files will always be placed last on the disc. A graphical representation is shown below:
| DVD Data Area | ||||||||
| DVD-Audio zone | DVD-Video zone | Extra data files | ||||||
| n sectors | m sectors | p sectors | ||||||
| 0 | ... | n | n + 1 | ... | ((n + 1) + m) | ((n + 1) + m + 1) | ... | (((n + 1) + m + 1) + p) |
The DVD specification impose further restrictions upon the packing of files into the respective zones. See other documents for more information.
On pure DVD-Video discs, the DVD-Audio and "extra data" zones will be empty. However, the AUDIO_TS directory must still exist in the filesystem; but there will simply be no files present.
Discs that contain files in the "extra data" zone are often referred to as "Hybrid" DVDs. Such discs are often used to distribute data intended for consumption by a computer application, such as web pages, or custom executables providing additional content (e.g. PCFriendly). Some authoring tools may also save project files onto DVDs, to allow easy re-authoring in the future (this is seen in some consumer authoring tools).
Zones help pack related data as close together as possible. This tends to reduce the "seek time" when accessing non-seamless data. Seek time is the time taken for a DVD drive to move its head from one part of the disc to another. Seek time is a partial contributor to the pause you'll see between selecting a button on a menu and seeing the new menu, or title, begin to playback.
Another reason for zones is disc layout simplicity. DVDs were originally designed for standalone hardware players. Typically, such players do not have CPUs as complex as desktop computers, and hence do not allow such complex processing, nor data buffering, as regular computers.
The concept of zones allows standalone players to bootstrap their file reading code from a small subset of the information in the filesystem, but subsequently access data by specifying raw sector addresses to the drive.
A typical PC-based player will simply read all data using standard C library calls; fopen, fread, fseek, fclose, or semantically equivalent operations. All such accesses use the ISO or UDF filesystem to determine the location of the data to be processed.
a typical standalone player will use the location of the VIDEO_TS directory, or VIDEO_TS.IFO file to determine the starting sector number of the DVD-Video zone is located on the DVD media. This operation alone uses the UDF filesystem. Subsequent accesses use the sector numbers contained in the .IFO file(s) to calculate the absolute LBA to read data from; no filesystem information is required.
On a DVD-Video disc, the DVD-Audio zone is empty, and the DVD-Video zone is placed first, for easy access. There is no data in the DVD-Audio zone.
DVD-Audio discs will typically be purchased by people owning DVD-Audio players, although these discs may include DVD-Video compatible content. Hence, the DVD-Audio data is the primary data on the disc, and is placed first, for easy access. This is followed by any DVD-Video compatible data.
Future formats may add new data types. If such data is intended for desktop PC playback, it will probably be added to the extra data zone. However, the new format is intended for consumption by standalone players, as a new primary data format, it will probably be placed in a new zone at the start of the disc.
Finally, the "extra data" is always packed last, since it is only accessed by desktop computers, which can typically cope with all the nuances of UDF filesystem based access to files. Computers have the most horsepower, and standalone players the least - hence, computers are given the (perhaps marginally) more complex access task.
Note that the DVD-Video specifications are dataed 1996/1997, and developed prior to that. In those times, standalone players (AKA embedded devices) had much slower CPUs and fewer resources than is common today. The DVD specifications were essentially designed to guarantee that standalone devices of that time could be successful at DVD playback. Modern DVD players (especially combo devices) may often have multi-100MHz CPUs and buckets of RAM, and hence could stand a more complex spec. Still, there is always a trend towards cost reduction, which still limits CPU power and RAM capacity in embedded devices. Hence, simplicity of data access is still of paramount importance.
This simplicity also helps the software quality in standalone players - simpler access means less code, which typically translates to fewer bugs i.e. higher quality (or at least one aspect of it). How often does your standalone DVD player crash or hang up (on correctly authored discs!) compared to your desktop PC?