Specifications

5.1-Channel Production Guidelines Introduction
1-3
1.2 Dolby Digital and 5.1-Channel Audio
Dolby Digital (AC-3) is a perceptual audio coding system developed in 1992 to allow
35 mm theatrical film prints to carry multichannel digital audio in addition to the
standard analog optical soundtrack. The system has since been adopted for use with
laser disc, ATSC high definition (HDTV) and DVB/ATSC standard definition
(SDTV) digital television, digital cable television, digital satellite broadcast, DVD-
Video, DVD-Audio, DVD-ROM, and Internet audio distribution.
Dolby Digital divides the audio spectrum into narrow frequency bands using
mathematical models derived from the characteristics of the ear, and analyzes each
band to determine the audibility of those signals. To maximize data efficiency, the
greatest number of bits represent the most audible signals; fewer bits represent less
audible signals. In determining the audibility of signals, the system performs what is
known as masking. Masking refers to the phenomenon that the ear cannot detect low-
level sounds when there are higher level sounds at nearby frequencies. When this
occurs, the high level sound masks the low level one, rendering it inaudible.
Exploiting this phenomenon allows audio to be encoded more efficiently than in other
audio coding systems. This makes Dolby Digital an excellent choice for systems
where high audio quality is desired, but bandwidth or storage space is restricted. This
is especially true for multichannel soundtracks since Dolby Digital’s compact
bitstream allows 5.1-channel audio to occupy less space than a single channel of Red
Book PCM audio.