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Techno Tidbits & Problem-solving
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 mono 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.
Digital
sound
2468101214 MHz
Frequency
Analog sound
(Monaural)
Video
Dolby Digital
sound
5.1 channel digital
surround track
4 channel analog
sound track (Matrix)
Dolby
Digital
encoding
Dolby Digital for movie films