Dolby Laboratories CP65 manual

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and with a broad range of speakers. Accurate equalization requires the use of standardized acoustic measurement procedures.

A pink noise generator provides a continuous random noise signal that covers the total bandwidth and is used to measure and adjust the response of the loudspeakers. The use of random noise eliminates the problems inherent with tones (standing wave patterns in the theatres) and enables the frequency response of the entire system to be observed. Each channel can be measured and adjusted independently of the other channels.

A calibrated microphone is placed in the auditorium to receive the pink noise reproduced by the loudspeaker; the output of the microphone is fed to a real time analyzer (RTA). The RTA displays the complete audio spectrum received by the microphone in the form of a frequency response curve. Pure pink noise would yield a “flat” horizontal line on the RTA. Thus, the effect of adjustments to the equalizers is quickly and easily seen.

One of the problems inherent in equalization is the nature of the environment. In an open space, a perfect loudspeaker, radiating a perfectly flat response in all directions, placed in front of a perfectly flat microphone, producing perfectly flat response to sounds arriving from all directions, will produce a perfectly flat response on the RTA from pink noise. In an enclosed space such as a theatre, the results are different. When the pink noise generator is first turned on, all of the sound that initially reaches the microphone comes directly from the loudspeaker; the response is flat–for a few milliseconds. Then reflected sound from the walls, ceiling, floor, seats, etc. starts to arrive at the microphone together with the direct sound from the loudspeaker. This indirect or reflected sound reinforces the direct sound. The system soon settles into an equilibrium condition. As much energy is being absorbed at the walls, ceiling, etc. as is fed into the room. Since high and mid frequency energy is absorbed when sound is reflected, the displayed response appears to have a rising bass and a falling treble characteristic. At first glance, rolling off the bass and boosting the high frequencies may appear to be the logical approach for a flat steady-state response, but such an arrangement works only on sustained sounds. Dialogue contains short, impulsive sounds and will yield a much-too-bright result because there is no time for reverberation to build and add to the original sound. What is required is a curve that favors such impulsive “first arrival” sound and implies the same gently falling response that is observed when the output of an ideal loudspeaker is measured with a perfect microphone in the theatre.

The amount of reverberation varies with frequency and the higher the frequency the more the treble will be absorbed rather than being reflected. A typical reverberation curve in a theatre rolls off at about 3 dB per octave above 2 kHz. This characteristic is used to define the standard steady-state response curve for all dubbing theatres in which Dolby stereo films are mixed and for all Dolby stereo-equipped cinemas.

The size of the theatre affects the reverberation time and, therefore, the measurement of frequency response. After alignment to this standard curve, some slight adjustment of high frequency slope may be found necessary for extremely large or small theatres. The treble control on the Cat. No. 64 Equalizer card can be adjusted to reduce the output on the response curve by approximately 1 dB at 8 kHz for very large theatres; an increase of 1 dB at 8 kHz may be in order for a very small theatre. Any such adjustment should be based on an evaluation by ear of actual known films rather than as a rule of thumb.

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Contents Model CP65 Cinema Processor Dolby Laboratories Inc Headquarters Table of Contents Section Reference Block Diagrams Introduction Page Section Equipment Required Jiffy Test Film Cat. No 2 Listen . . . Film Cat. No Smpte Buzz Track Stereo Optical Surround Level Cat. NoSection Initial SET-UP and Installation Voltage Acceptable Setting Voltage Range Fuse Type Next, check the jumpers in the following locations Cat. No , B-Chain Card Cat. No , Power Supply ModuleJ2 NON-SYNC Cat. No , Control Logic CardStandard CP65 or CP65A CP65 BackplaneCP65-300 High Low Subwoofer subwoofer Output output These Modules are Necessary for Bypass Operation Two-channel Noise Slot Card Function Cat. NoReduction Module Channel DecoderContains a semi-regulated power supply with International Voltage Setting Fuse TypeIEC Notices Room lighting dimmer controls SCR-TYPE HUM ProblemsPage Aligning the B-Chain Section AN Overview of the Alignment ProcedurePage Page Section Chain Alignment Action Indication Preliminary ProceduresLamp Voltage Optical Preamplifier Adjustments Chain Alignment Procedures Optical Preamplifier Adjustments 222 CAT. no Test Film Oscilloscope Traces Repeat Repeat 1 → CAT noMagnetic Preamplifier Adjustments Magnetic Alignment 100mV Section Chain Alignment Bypass Setting Room EqualizationLoudspeakers and Crossovers General AmplifiersOff Chain Alignment ProceduresAction RepeatChain Alignment Procedures Setting Room Equalization Do not adjust Paul 14 → 21 R 14 → Setting Room Equalization b. Adjusting L,C,R Gain79 dBC Setting Mono GainChain Alignment Procedures Subwoofer Alignment Cat. No 85 dBC 90 dBCStep No FromSelect format 04 Dolby Stereo A-Type 441 Same ASSurround Equalization Alignment Cat. No Cat. No 441 Surround Equalization Alignment Cat. No NotesDesired 88 dBC Bypass Thumbwheel Switch Adjustment of Surround DelayMsec Setting Xx dBC Chain Alignment Procedures Adjustment of Bypass GainXx dBC -3 dB Yy dBC FromPress the format 60 non sync switch Adjustment of Non-sync GainAuto Freq Repeat page 5-20 Step Adjustment of Mono Gain and Equalization NotesPress the format 04 Dolby Stereo A-type switch Chain Alignment Procedures Final ChecksSection Reference Diagrams Cat. No Dual Noise Reduction Module Inputs Cat. No Optical Bass Extension and Surround Equalizer Card Cat. No B-Chain Facilities Card Cat. No Power Supply Card Cat. No Transformer Module Inputs Cat. No Control Logic Card 15V +12B Cat. No +24V +15V+15M Appendix a Checking Phasing of Speakers Appendix B Effect of Changes in Slit Height on Slit Losses Operation Appendix C CP65M Mono InstructionsCat. No a Circuit Description Schematic Cat. No a Terminal Block TB3 Appendix D Backplane Connections ListConnector J18
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