This book is about the specific use of DVD discs to hold DVD-Video content. Information on the lower-level structure of a DVD disc can be found elsewhere; here we will concentrate only on the directory- and file-level structures necessary for DVD-Video.
The titles (content) on a DVD-Video disc are stored in MPEG format. In addition to this, a disc will typically have one or more menu screens with buttons marked on them, linked to each other and to the titles. You can navigate around the buttons in a menu by using the up, down, left and right arrow buttons on your player remote, and select a menu button by pressing the button marked “OK” or “Enter” on the remote, normally located in the middle of the arrow buttons.
There will usually be a “main menu” which automatically comes up when you put the disc in the player (after initial company logos, copyright warnings and other guff that the distributors insist on inflicting on us), and that you can return to with the “Top Menu” button on the remote. Other remote buttons labelled “Menu”, “Subtitle”, “Audio”, “Angle” or “Chapter” may bring up other special menus, depending on the disc.
Disc Structure Versus Title Structure
One thing that will strike you about the DVD-Video data structures is that they tend to offer a variety of ways of doing what amounts to the same thing. For example, how menus and titles are grouped on disc is one thing, whereas how they appear to the user (linked to each other by the buttons in the menus) can be very different.
DVD-Video also allows for subtitles. These are called subpictures, because they are represented, not as text, but as bitmaps overlaid on the video. They can thus represent arbitrary fonts, languages, writing systems etc. And they don’t even have to be text. However they are stored as two bits per pixel, thus allowing for only four different colours (transparent areas where the video shows through also count as a colour).
Subpictures are also used to implement highlighting of buttons in menus. In this situation, their display is normally “forced” (i.e. unconditional), so the user doesn’t have to turn them on. In fact it doesn’t make sense for the user not to see the buttons on the menus.
Menu buttons can do more than just bring up a title or another menu; there is a whole virtual machine language that is understood by an interpreter built into the DVD player, that can implement quite complicated interactive behaviours.
Commercial DVD-Video discs are commonly region-coded, while the consumer players are region-locked to only play discs coded with the same region. For example, North America is Region 1, Europe is Region 2, etc. This was to allow the distributors to release discs first into their most important markets, while taking their time over making them available elsewhere, without the worry that somebody might ship the discs to the latter markets and bypass the “official” distributors in the process.
Nevertheless, region-unlocked players are commonly available in many parts of the world, and in some places it is actually illegal to prevent customers from unlocking their players.
Commercial DVD-Video discs are commonly encrypted with the Content-Scrambling System (CSS). The idea was that makers of player hardware and software would only be given access to the encryption keys if they signed an agreement to ensure that the decrypted video could not be easily recorded or copied by other devices or software.
Unfortunately, the CSS encryption was badly designed, and many people who did not sign such agreements were soon able to break it. Regretfully there are still legal restrictions in some jurisdictions on the distribution of CSS-decryption software, but all the good Free Software DVD-Video players are able to use the decryption library once you find it and install it (wink, wink).
CSS-encrypted discs have a special area containing decryption keys, which is only accessible via a special handshake between the DVD drive and the player software, details of which are not publicly known. Writable DVD blanks that ordinary consumers can buy do not have provision for this special area (and in any case off-the-shelf DVD writers cannot write it). So at this time Free Software cannot create such discs.
Prohibited User Operations
Remember I mentioned above the stuff that commonly starts playing on commercial discs as soon as you put them in the player? What makes them doubly annoying is that you often cannot skip or fast-forward over them.
This is done by specifying that certain “user operations” (skip, fast-forward, pause/still etc) are prohibited over certain areas of the disc.