Dolby
®
Model CP650 Installation Manual
5-1
Chapter 5
A-Chain
Alignment
5.1 Overview
The A-chain alignment involves adjustments made to the projector soundhead optics,
solar cell, and optical preamplifier board. It is the part of the sound system that covers
the film path, analog soundtrack solar cell, optical preamplifier, slit-loss equalizer,
digital soundtrack reader, associated digital signal processing and decoding circuitry,
and Dolby
®
processing circuits.
The A-chain optical level is calibrated with the Cat. No. 69T Dolby Tone test film.
This film is used to establish the correct Dolby operating level within the CP650.
A Cat. No. 69P pink-noise test film is used for A-chain frequency response and
soundhead alignment. Pink noise has a constant amount of energy per 1/3-octave
band, creating a flat response on a real-time analyzer display.
The optical slit is the key element in the A-chain because it imposes the initial
limitation on the high-frequency response of the system. Light from the exciter lamp
passes through the optical slit and is focused on the optical soundtracks on the film.
The light that passes through the soundtracks falls on the stereo solar cell, which
generates an electrical signal proportional to the audio signal recorded on the optical
soundtracks. The slit introduces high-frequency loss, which must be compensated by
circuitry in the CP650.
The slit image must be correctly focused on the film and must be set at precisely a
right angle to the direction of film movement in order to maintain the correct phase
relationships between the two optical tracks. Any azimuth error will show as a loss of
high frequency in the front channels and potentially excessive crosstalk in the
surround channels.
Each channel in the CP650 optical preamplifier is equipped with a slit-loss equalizer
control. Adjustment of this control shifts a fixed amount of boost upward or
downward in frequency, but the shape of the curve remains constant. A flat response
to 16 kHz can be achieved.
