APPENDIX C: DOLBY AC-3 SURROUND
Dolby Laboratories spearheaded home theater with the introduction of Dolby Surround, and then defined it with the follow-up format Dolby Pro-Logic Surround. They have continued the evolution of home theater with the introduction of Dolby AC-3 Pro-Logic, a Multi- Channel Digital delivery system. Just as Dolby Surround found it’s beginnings in the cinema, so does AC-3. This advanced multi-channel digital sound, although originally conceived for and later chosen for HDTV, made it practical to implement in the cinema because it enabled the film to retain the analog soundtrack. With backwards compatibility a priority so the film could be played in any cinema, it was decided to place the new digital optical track between the sprocket holes, which was a key factor in defining its maximum practical bit rate (see illustration).
5.1– Channel Digital Soundtrack
3
4 – Channel Analog
Soundtrack (Matrixed)
3
AC-3
ENCODING
AC-3 IN THE CINEMA
In 1992 Dolby Surround Digital, the film sound format with AC-3 as its keystone, made its debut in the cinema. In less than three years, more than 300 feature films had been released in this new format, and nearly 1,500 cinemas had been equipped for playback of the digital soundtrack. Furthermore, AC-3 can now be brought to the consumer’s home today via the LaserDisc.
Traditional Pulse Code Modulation (PCM), which is utilized on compact discs and LD’s 4-channel matrixed digital audio tracks, uses a 16-bit word to “describe” the sampled point in the original analog wave. By sampling this analog wave 44,100 times per second for each channel, the system will yield a dynamic range of 96 dB from the loudest sound to the noise floor. However, this process creates an immense amount of data which is too large for multi-channel systems. Thus, a form of data reduction must be employed to reduce the total amount of information without serious degradation of the sound quality — hence the introduction of AC-3.
The premise of AC-3 is simple, it reproduces the full bit resolution (16-bit) when a lower resolution will not result in the same sound quality. However, when a lower resolution is acceptable, it will utilize this lower resolution opting instead for psychoacoustical theories and digital noise filtering technologies to help insure acceptable sound quality.
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