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.
DolbyAnalogACMono-3 | Dolby AC-3 |
Analog Mono |
DigitalAudio | Digital Audio |
Audio |
| | | | | | Analog Video |
2 | 4 | 6 | 8 | 10 | 12 | 14 MHz |
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:
| | DOLBY SURROUND DIGITAL (AC-3) | DOLBY SURROUND PRO-LOGIC |
| | | |
| Available Channels on Disc | 5.1 Channels | 2 channels |
| | | |
| Playback Channels | 5.1 Channels | 4 channels |
| | | |
| Description of Playback Channels | Left, Right, Center, Left Surround, Right | Left, Right, Center, and Surround |
| Surround, and Sub-woofer |
| | |
| | | |
| Sound Processing | Discrete Digital AC-3 | Analog Matrix Processing for Dolby Surround |
| Encoding & Decoding |
| | |
| | | |
| Surround Channel Maximum | 20,000 Hz | 7,000 Hz |
| Frequency Response | | |
| | | |
| Other Information | - 5 Discrete Channels | |
| | - Fully Backward Compatible with | |
| | Dolby Surround | |
| | - High Dynamic Range | |
| | - Extremely Accurate and Natural | |
| | Sound for Vocals and Effects Imaging | |
| | | |
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