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Dolby Digital, formerly known as AC-3, is a digital audio coding technique for home theater. Dolby Digital is used with digital versatile discs (DVD’s), high definition television, and digital cable and satellite transmissions. It has been selected as the audio standard for digital television (HDTV).
Dolby Digital provides five full bandwidth channels, front left, front right, center, surround left, and surround right, for true surround sound quality. A low frequency effect channel is included that provides the sound needed for special effects and action sequences in movies. This channel is one-tenth of the bandwidth of the other channels and is sometimes erroneously called the subwoofer channel. This multi-channel scheme is known as 5.1 channel.
Because not everyone has the equipment needed to take advantage of Dolby Digital's 5.1 channel sound, developers included a down-mixing feature that ensures compatibility with any playback device. The decoder in the playback device delivers the audio signal specific to that particular device's ability. For example, a 5.1 channel audio signal is delivered to a mono television. The playback device's decoder down-mixes the 5.1 channel signal to a mono signal allowing the television to use the received audio signal. Because the playback device does the down-mixing, producers do not have to create multiple audio signals for each playback device.
The ATSC (American Television Standards Committee) selected Dolby Digital as a standard for HDTV because of its popularity with film producers and consumers, its down-mixing feature, and its high quality sound. The U.S. cable television industry has also adopted Dolby Digital for HDTV applications. Most television facilities are not equipped to produce 5.1 channel sound. For this reason, many HDTV programs use two-channel sound. The 5.1 channel sound is used primarily for theatrical films on pay-per-view channels and at theaters.
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