Dolby Digital (formerly known as Dolby AC-3, where AC-3 is short for audio coding 3) is the de facto surround sound standard in today's home theaters. It is the surround sound format used in thousands of movie theaters today. And, since about the mid-1990's, it has become available for home theater use by consumers. Today, a large percentage of the DVD-Video titles come with Dolby Digital surround sound. Dolby Digital content first appeared on LaserDisc, since DVDs only emerged in the Spring of 1997. (Incidentally, Hi-Fi VHS still only supports up to Dolby Surround Pro-Logic.)
Not only is Dolby Digital the standard for DVD-Video, but it is also part of the new High Definition TV (HDTV) standard. It is used in pay-per-view movies and digital TV channels of digital satellite broadcasting (e.g., DIRECTV system). Dolby Digital is the successor to Dolby Surround Pro-Logic. The Dolby Digital surround sound format provides up to five discrete (independent) channels (center, left, right, surround left, surround right; giving it the "5" designation) of full frequency effects (from 20 Hz to 20,000 Hz), plus an optional sixth channel dedicated for low frequency effects (LFE), usually reserved for the subwoofer speaker. The low frequency effects channel gives Dolby Digital the ".1" designation. The ".1" signifies that the sixth channel is not full frequency, as it contains only deep bass frequencies (3 Hz to 120 Hz).
Dolby Digital soundtracks have 5.1 channels of audio. Those that are have the designation "Dolby Digital 5.1". Since Dolby Digital is a flexible surround sound format that supports up to 5.1 channels, Dolby Digital soundtracks could have one channel of audio (mono, designated as "Dolby Digital 1.0"), two channels of audio (stereo or Dolby Surround Pro-Logic, designated as "Dolby Digital 2.0"), or five channels of audio (designated as "Dolby Digital 5.0").
