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APPENDIX C: DOLBY AC-3 SURROUND
THE LASER DISC FORMAT
The one major concern when the idea of incluiding another form of audio on a laser disc was .... Backwards Compatibility. It would not
do the laser disc format any justice, especially in a time of growing acceptability, to change the audio system without considering those
consumers that had already purchased a laser disc player. Thus to maintain backward compatibility, the AC-3 encoding was deemed
to be stored in one of the existing analog tracks.
The above diagram shows the format of the AC-3 Pro-Logic laser disc. The PCM audio track, containing the 4-channel matrixed digital
soundtrack, was fully maintained for continued use in laser disc players with digital capabilities. As mentioned above, one of the analog
channels will now contain the AC-3 encoding, while the other analog channel will contain a mono analog soundtrack, which still provides
backwards compatibility with those pre-1984 laser disc players (1984 was the year we introduced an LD player with digital capabilities).
THE ADVANTAGES OF AC-3 OVER PRO-LOGIC
AC-3 is called a 5.1-channel system because it provides 5 independent channels (left, right, center, left surround, and right surround),
all reproducing a 20 – 20,000 Hz range. In addition, the system provides for an independent sub-woofer channel, which is called a low
frequency effects channel. Use of the low frequency effect channel is optional, since the 5 channels provide a full frequency bandwidth,
but available for those consumer wishing to include a sub-woofer for thunderous bass reproduction. Furthermore, AC-3 provides
improved channels separation, multi-directional discrete capability, and increased sound-design creative flexibility (for the movie
makers). Below highlights some of the other advantages of Dolby AC-3:
Available Channels on Disc
Playback Channels
Description of Playback Channels
Sound Processing
Surround Channel Maximum
Frequency Response
Other Information
DOLBY SURROUND DIGITAL (AC-3)
5.1 Channels
5.1 Channels
Left, Right, Center, Left Surround, Right
Surround, and Sub-woofer
Discrete Digital AC-3
Encoding & Decoding
20,000 Hz
-5 Discrete Channels
-Fully Backward Compatible with
Dolby Surround
-High Dynamic Range
-Extremely Accurate and Natural
Sound for Vocals and Effects Imaging
DOLBY SURROUND PRO-LOGIC
2 channels
4 channels
Left, Right, Center, and Surround
Analog Matrix Processing for Dolby Surround
7,000 Hz
2 4 6 8 10 12 14 MHz
Digital (PCM)
Audio
Dolby AC-3
Digital Audio
Analog Mono
Audio
Analog Video
Dolby AC-3
Digital Audio
Analog Mono
Audio
Digital (PCM)
Audio
Analog Video