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 theatres.
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.
