Dolby Laboratories CP500-300 Operations Instructions download pdf (Page 45)

APPENDIX B

DOLBY TEST AND DEMONSTRATION FILMS

Several test and demonstration films produced by Dolby Laboratories are available

from your equipment supplier. For proper system maintenance, the Cat. No. 69T

test film is required and the Cat. No. 251 Jiffy Test Film is strongly recommended.

The Cat. No. 69T test film should be formed into endless loops for ease of use.

Other test films (Cat. Nos. 69P, 97, 151, 566) are for use only by a trained engineer

with special test equipment. They are mentioned here so you know what they are

should you ever come across them.

B.1 Cat. No.69T: Dolby Tone

This film is required to maintain your theater system. The Dolby Tone recorded on

the film is for automatic calibration of the operating level in the CP500. This

simple procedure must be performed whenever an exciter lamp is replaced and

should also be performed from time to time to compensate for the normal aging

(and reduced light output) of exciter lamps.

B.1.1 Why Adjusting Dolby Level Is Important

The accurate reproduction of Dolby soundtracks requires that the decoders in the

theater act as precise mirror images of the encoders used when the tracks were

recorded. This precision is ensured in two ways. First, Dolby system circuits are

manufactured to very close tolerances. Second, a reference Dolby Level has been

established so that any Dolby-encoded recording can be accurately decoded by any

Dolby decoder of the same type.

Remember that Dolby circuit action in both recording and playback is level-

dependent. Loud signals are untouched, while low-level signals are boosted during

recording and attenuated during playback, by an amount that depends on their

level. How does the playback Dolby decoder know when, and by how much, to

attenuate the previously-boosted signals? The answer is that it doesn’t know. When

the decoder sees any particular playback voltage, it assumes what happened when

the recording was originally encoded. For that assumption to be accurate, however,

it was necessary when Dolby noise reduction was first developed to establish a

standard reference level to which all encoded recordings and all decoding circuits

could be calibrated.

1 2 ... 40 41 42 43 44 45 46 47 48 49 50 ... 53 54

No comments

Dolby Laboratories CP500-300 Operations Instructions Page 45