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108 Everything You Ever Wanted to Know About DVD<br />
This is why the line level from DVD players is lower than from almost all<br />
other sources. So far, unlike on CDs and laserdiscs, the level is much more<br />
consistent between discs. If the change in volume when switching between<br />
DVDs and other audio sources is annoying, you can adjust the output signal<br />
level on some players, or the input signal level on some receivers, but<br />
other than that, you don’t have many other options.<br />
How Do the Interactive Features Work?<br />
DVD-Video players (and software DVD-Video navigators) support a command<br />
set that provides rudimentary interactivity. The main feature is menus,<br />
which are present on almost all discs to allow content selection and feature<br />
control. Each menu has a still or motion background and up to 36 highlightable,<br />
rectangular buttons (only 12 if widescreen, letterbox, and pan and<br />
scan modes are used). Remote control units have up/down and left/right<br />
arrow keys for selecting onscreen buttons, along with numeric keys, a<br />
select key, a menu key, a top menu (title) key, and a return key. Additional<br />
remote functions may include freeze, step, slow, fast, scan, next, previous,<br />
audio select, subtitle select, camera angle select, play mode select, search<br />
to program, search to part of title (chapter), search to time, and search to<br />
camera angle. Any of these features can be disabled by the producer of the<br />
disc, an act which is called user operation control (UOP). It’s commonly<br />
used to lock you into the copyright warning or movie previews at the beginning<br />
of the disc, or to keep you from changing audio or subtitle tracks during<br />
the movie.<br />
Additional features of the command set include simple math (add, subtract,<br />
multiply, divide, modulo, and random); bitwise and; bitwise or; bitwise<br />
xor; plus comparisons (equal, greater than, and so on); and register<br />
loading, moving, and swapping. Twenty-four system registers exist for<br />
information such as the language code, audio and subpicture settings, and<br />
the parental level. Sixteen general registers are available for command use.<br />
A countdown timer is also provided. Commands can branch or jump to<br />
other commands. Commands can also control player settings; jump to different<br />
parts of the disc; and control the presentation of audio, video, subpicture,<br />
camera angles, and so on. The command set enables the creation<br />
of relatively sophisticated discs, such as games or interactive educational<br />
programs.<br />
DVD-V content is broken into titles (movies or albums) and parts of titles<br />
(chapters or songs). Titles are made up of cells grouped into programs and<br />
are linked together by one or more program chains (PGC). A PGC can be<br />
one of three types: sequential play, random play (may repeat), or shuffle<br />
play (random order but no repeats). Individual cells may be used by more<br />
than one PGC, which is how parental management and seamless branch-
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