73
En
Techno Tidbits & Problem-solving
Dolby Digital
Digital
sound
2
4
6
8
10
12
14 MHz
Frequency
Analog
sound
(Monaural)
Video
Dolby Digital
sound
Dolby Digital is a compression format which records
the sound of 6 or 8 channels of the theater surround
system (Dolby Digital) on the movie film digital track.
Of the 6-8 channels, the sub woofer channel is
intended for bass only, and because the frequency
range is smaller than a main channel, the overall
soundtrack is expressed as 5.1 or 7.1 channels.
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, Dolby Digital
is used to compress the data.
In reproducing audio signals, the smaller the bit
number used, the lower the sound quality. With Dolby
Digital, 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 subwoofer channel can be used as desired to
enjoy strong bass sounds.
5.1 channel digital
surround track
4 channel analog sound
track (Matrix)
Dolby
Digital
encoding
Dolby Digital for movie films
MPEG Audio
MPEG is an audio encoding system which delivers high quality audio for music and movies. By removing
frequencies which are out of the human hearing range, the sound is then compressed to enable the
transmission of mono, stereo, or multichannel (5.1 and 7.1) audio in a single bitstream which is why it has
become a standard audio format for DVD and personal computers. The composition and versatility of MPEG
audio also ensures that the presentation of a multichannel soundtrack will sound appropriate on everything from
a 7.1 channel surround sound home theater setup to a mono television set. Above all, MPEG audio is designed
to be compatible with past and future MPEG audio versions, which means that future versions of MPEG audio
will be compatible with decoders currently being produced (the signal is merely reconfigured to fit the number of
channels available in a system).
1
