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En
Dolby Digital
Dolby Digital is a compression format which records
the sound of 6 channels of the theater surround
system (Dolby Digital) on the movie film digital track.
Of the 6 channels, the sub woofer channel is
intended for bass only, and because the frequency
range is smaller than the main channel, it is
expressed as 5.1 channel.
Dolby Digital is the name of the Dolby surround
multi-channel digital system that was developed
after the Dolby Surround System and Dolby Pro
Logic Surround System.
The number of movies made using Dolby Digital
since 1992 has exceeded 300 and continues to
increase. For compact disc players and laser disc
digital sounds, 16 bits are used to sample the original
analog audio waveform and sampling is carried out
44,100 times every second. However because an
enormous amount of recording signal data is
required for the multi channel system with this
method, AC-3 is used to compress the data.
In reproducing audio signals, the smaller the bit
number used, the lower the sound quality. With AC-
3, drop in auditory sound quality is prevented by
using masking technology and digital filtering
technology based on the human auditory
characteristics.
Laser disc format
Laser discs are now available on the market in large
numbers. This means that the recording of different
format audio signals on the laser disc raises the
important question of compatibility with existing
laser discs. Dolby Digital tracks on Laser discs record
signals using the space of one analog audio channel
so it will maintain compatibility with existing discs
and players.
As shown in the following figure, the digital audio
sounds of Dolby Digital discs can be played back as
currently done. Analog sounds are played back by
using the other channel without the Dolby Digital
signals for monaural audio signals.
Comparison with Dolby Pro Logic
Surround
Dolby Digital is also known as the 5.1 channel
system. It is equipped with 5 channels (front left,
front right, center, surround left, surround right) in
the frequency range from 20 Hz to 20 kHz and an
independent Low Frequency Effect (LFE) channel.
The sub woofer channel is also called Low
Frequency Effect (LFE).
The sub woofer channel can be used as desired to
enjoy strong bass sounds.
The table on the next page shows the comparison
with Dolby Pro Logic Surround effects.
Techno Tidbits and Problem-solving
5.1 channel digital
surround track
4 channel analog
sound track (Matrix)
Dolby
Digital
encoding
Dolby Digital for movie films
2
4
6
8
10
12
14 MHz
Video
Frequency
Digital
sound
Analog sound
(Monaural)
Dolby Digital
sound
