Operating instructions

F-1

APPENDIX F

OPTICAL BASS EXTENSION

Dolby Laboratories developed the optical bass extension (OBE) circuitry for

use with subwoofers — special bass loudspeaker units designed to provide the

very low-frequency bass performance lacking in most conventional theatre

speaker systems. (If your theatre is not equipped with subwoofers, consult

your theatre equipment supplier for further information on how to take full

advantage of optical bass extension.)

At first glance, it would appear appropriate just to add extra amplifiers and

subwoofers to reproduce the lowest bass frequencies recorded on a soundtrack.

Such an approach is insufficient and can even be detrimental to achieving

realistic, natural sound. One problem is that at very low frequencies, there may

not only be desirable music and effects information on the soundtrack, but

there may also be hum and low-frequency print noise. Furthermore,

reproducing very low bass in the theatre can excite resonances which add an

artificial, boomy quality.

The optical bass extension circuitry is the vital link between the very low bass

music and effects often recorded on wide-range Dolby Stereo prints and an

installation that incorporates subwoofers and their associated amplifiers. The

circuit is not merely a crossover network or a bass synthesizer; it extracts

whatever low-frequency music or effects information is already recorded on

the release print and, at the same time, rejects low-level hum and low-

frequency optical noise. In addition, a sophisticated filter system (adjusted by

the trained installer) assures natural low bass response without the boomy

quality which would otherwise result from the interaction between low-

frequency bass speakers and the theatre acoustics. The result is smooth, deep

bass response which complements the improved mid and upper range response

already achieved in your theatre by means of the equalization in the CP45.

CP45

POWER AMPS

BASS

EXTENSION

SURROUND

BASS

EXTENSION

SUBWOOFER

Figure F-1. Bass Extension Installation.