September 2002|Authoring may be the aspect of production that sets DVD apart from its forbearers such as CD and laserdisc, but in the making of many DVD titles, asset preparation actually turns out to be a bigger, more time-consuming job. You not only have to create or obtain all the individual media elements that are specified on the project's asset list, but also to convert those elements into the form needed to integrate them into a DVD.
While video may be the focus of the DVD-Video format, the basic requirements for preparation of DVD-compliant video assets are actually quite straightforward. In contrast, the subject of the audio that accompanies DVD-Video content can be a bit more complicated because it covers a greater variety of supported formats, multiple channel configurations, and multiple audio streams per video program. To clarify the issues involved, we'll first look at the DVD-Video format's audio support, and then at the production processes typically used in audio preparation. The use of audio in DVD-Audio content, meanwhile, is a distinct story unto itself, one that we'll save for another day.
Supported Audio Formats
Before we look at the specifics of audio-for-DVD production, let's review both the possible uses of audio in DVD-Video and the specific requirements for source assets. The first thing to know is that the DVD-Video specification allows each movie—either a video clip or a series of still images (a slideshow or stillshow)—to be accompanied by up to eight independent mono, stereo, or surround sound audio streams. DVD-Video players are required to support seamless switching between these streams (using the player's remote control) during playback.
The most popular uses for these multiple audio streams are to provide different language versions of a soundtrack, or commentary tracks by the director, actors, or special effects crew. They may also be used to deliver the same soundtrack in different audio formats. Or there may be some combination of all the above. It might make sense, for instance, for a title released for North American markets to feature a 5.1-channel soundtrack in English, a stereo English commentary track, and also stereo soundtracks in Spanish and French.
A variety of formats are available for audio streams. To ensure that every player will support some form of audio playback from every disc, the specification mandates audio format requirements related to both players and discs. For NTSC, players must support playback of both Linear PCM and Dolby Digital (sometimes referred to as AC-3), and at least one of the audio streams accompanying every movie (video or stills) must be in one of those two formats. For PAL, players must support not only PCM and Dolby Digital, but also MPEG audio (MPEG-1, Layer 2, and MPEG-2). Similarly, on a PAL disc, at least one audio stream per movie must be Linear PCM, Dolby Digital, or MPEG.
As for additional audio streams in a movie, if any, they may be in any of the required formats or in an optional format, which is a format that players are allowed, but not required to support. These optional formats include DTS (Digital Theater Systems) and SDDS (Sony Dynamic Digital Sound). Of the two, DTS has been more successful in obtaining support from consumer electronics manufacturers, some of whom include DTS decoders in their DVD players or A/V processors.