Dolby Laboratories CP500-300 Specifications download pdf (Page 53)

SECTION 6

A-CHAIN ALIGNMENT

The A-chain alignment involves adjustments made to the projector soundhead

optics, solar cell, and optical preamplifier card. 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 CP500 does not contain a magnetic A-chain but has facilities for switching

external magnetic preamplifiers into the B-chain.

6.1 Aligning the A-Chain

The A-Chain optics is first cleaned and mechanically adjusted, then calibrated by use

of the Cat. No. 69T Dolby Tone test film. This film is used to establish the correct

Dolby operating level within the CP500.

A 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 Cat. No. 661 optical preamplifier card.

The slit image must be correctly focused on the film and must be precisely at right

angles 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 Cat. No. 661 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 up to a minimum of 12 kHz can be achieved.

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Dolby Laboratories CP500-300 Specifications Page 53