Dolby Digital, formerly known as AC-3, is a digital audio coding technique that reduces the amount of data needed to produce high quality sound. Dolby Digital takes advantage of how the human ear processes sound. When coding noise is close to the frequency of an audio signal, that audio signal masks the noise so that the human ear hears only the intended audio signal. Sometimes the coding noise is not in the same frequency of an audio signal and must be reduced or eliminated. By reducing, eliminating, or masking the noise, the amount of data is reduced to one tenth of the data on a compact disk (CD). Dolby Digital is used with digital versatile discs (DVDs), high definition television (HDTV), and digital cable and satellite transmissions. It has been selected as the audio standard for digital television (DTV). The European DVB standard does not use Dolby Digital for audio, but instead uses MPEG standard technology for both the audio and video signals.
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 (LFE) channel is included that provides the sound needed for special effects and action sequences in movies. The LFE channel is one-tenth of the bandwidth of the other channels and is sometimes erroneously called the subwoofer channel. This multichannel 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 downmixing 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 downmixes the 5.1 channel signal to a mono signal allowing the television to use the received audio signal. Because the playback device does the downmixing, producers do not have to create multiple audio signals for each playback device.
The Advanced Television Systems Committee (ATSC) selected Dolby Digital as a standard for DTV because of its popularity with film producers and consumers, its ability to use a single audio streaming video because of the downmixing feature, and its high quality sound. The U.S. cable television industry has also adopted Dolby Digital for DTV applications. Most television facilities are not equipped to produce 5.1 channel sound. For this reason, many DTV programs use two-channel sound. The 5.1 channel sound is used primarily for theatrical films on pay-per-view channels and at theaters.
To take advantage of Dolby Digital 5.1 channel sound for satellite broadcasts, a satellite receiver that provides a Dolby Digital output is necessary. For cable users, all digital set-top boxes are equipped with a Dolby Digital two-channel decoder. To use 5.1 channel sound, a 5.1 channel-compliant set-top box is needed or an external 5.1 channel decoder unit. The proper sound equipment is also necessary.