An audio commentary is an additional audio track, usually digital, consisting of a lecture or comments by one or more speakers, that plays in real time with a video. Commentaries can be serious or entertaining in nature, and can add information which otherwise would not be disclosed to audience members. Types of commentary The DVD medium allows multiple audio tracks for each video program. DVD players usually allow these to be selected by the viewer from the main menu of the DVD or using the remote. These tracks will contain dialogue and sound of the movie, often with alternative tracks featuring different language dialogue, or various types of audio encoding (such as Dolby Digital, DTS or PCM). Among them may be at least one commentary track. There are several different types of commentary. The two main types simply define the length of the commentary rather than the type of content. They are: Partial or scene-specific, which only covers selected scenes of the film. Sometimes these are recorded without the speaker viewing the film and thus the commentator may make more general comments than pointing out specific details. Feature-length or screen-specific, which is recorded in one session: the speakers watch the movie from beginning to end and give their thoughts directly based on what is happening on-screen. Typically a commentary track will include feature-length commentary from the film's director, cast members, or occasionally writers and producers. Occasionally actors will perform commentary in-character. (In recording sessions with multiple speakers, a designated moderator may encourage the discussion flow.) Some DVDs include outsider commentary performed by film critics, historians, scholars or fans. In more elaborate productions, multiple speakers from various recording sessions may be edited together for a single audio program. Some DVDs feature commentaries with on-screen video enhancements, such as telestrator prompts, (allowing the director or commentator to "draw" on the screen, pointing out specific details), or the Ghostbusters "video commentary", where one of the subtitle tracks is used to add silhouettes of the speakers in a manner where they seem to be in a theater commenting on the movie as it was screened for them in the style of Mystery Science Theater 3000. Less common are actual video commentaries, showing the speakers as they are recording the commentary, requiring separate video tracks. History The value of audio commentaries as a marketing tool was revealed during the heyday of Laserdisc, the laser-based video