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Richard C. Prather, of Motion Picture Specialties, has provided this RCA Service Company document on installation procedures of the four channel magnetic sound system used with Perspecta encoding for the 35mm Cinestage version of Around the World in 80 Days. This document makes a nice companion piece to the Michael Todd Company materials provided by SabuCat Productions.

A Division of Radio Corporation of America

CLASSIFICATION: Technical - Theatre - Special Systems
DATE: 4/3/58

SUBJECT, Todd-AO "Around the World in 80 Days" 4-Track Print, 35mm

For RCA Service Company Employees Only

1. The 35mm, 4-track magnetic, Technicolor release print of "Around The World in 80 Days" embodies a special type of fourth sound track. To obtain more flexible control of the sound effects, the fourth track is recorded in "Perspecta Sound", having 30, 35, and 40-cycle control modulation in addition to the desired sound effects modulation.

2. In the early model Todd-AO 4-track sound reproducing system, the sound signal from the fourth sound track is fed into a "Perspecta Sound" integrator. Here the 30, 35, or 40-cycle signal causes the recorded intelligence to be "Switched" to the corresponding one of the three integrator output circuits. Whenever a combination of two or three of these control frequencies is reproduced at the same time, the output of the integrator is simultaneously fed to the corresponding output channels. A low-pass filter (5000 cycle cutoff) is to be used to eliminate unwanted film noise.

3. The three outputs of the integrator are fed into three separate amplifier channels. One of these feeds the left side wall effects speakers; the second feeds the right side wall effects speakers; and the third feeds the rear wall effects speakers.

4. Thus, the sound output from the fourth channel is automatically switched to the left, right, or rear wall surround speakers, or to any combination of them, by the presence of the 30, 35, or 40-cycle control tracks singly or in combination. The three control track tones do not appear in the loudspeaker sound because a high-pass (60 cycle cutoff) filter in the suppressor unit eliminates them from the output circuits.

5. When the integrator is not furnished:

a. If surround speakers are used, the fourth track usually feeds all three groups of surround speakers simultaneously.

b. Where no surround speakers exist, but three stage speaker channels are used, it will be necessary to feed the fourth track signal into the center stage channel, as the fourth track carries essential information which must not be omitted. Because the recording level of the fourth track is higher than that of the other three tracks, its output must be padded down (by trial) if a separate volume control is not included in the system.

c. Where no surround speakers exist, and only one stage speaker is used, the outputs of all four tracks must be mixed together and fed into this speaker. If only one preamplifier is used, soundhead outputs #1 and #3 are to be connected in series aiding to the amplifier input. Output #2 and output #4 are to be connected in series aiding and paralleled with #1 and #3 into the amplifier input. It will probably be necessary to pad down output #4 by trial.

In any of the above three situations, it will be necessary to attenuate the high and low frequency amplifier response to eliminate the high frequency film noise (above 5000 cycles), and any possibility of thumps, intermodulation, or other extraneous sounds from the low frequency control tracks. if the amplifier incorporates HF and LF compensation, (such as found in the MI-9335) these may be used. If not, external filters must be obtained and inserted into the circuit between the preamplifier and the power amplifier. Such filters must: -

a. Have cutoff frequencies of 60 (HP) and 5000 (LP) cycles per second, respectively;

b. Have input and output impedances which match the preamplifier output and power amplifier input, or intermediate link circuit impedance, as the case may be.

6. Suitable filters, AM-1154, (National Theatre Supply) may be supplied with the equipment. (See drawing and schematic, page 3.)

This unit is actually a band-pass filter which passes frequencies between 50 and 5000 cycles. It is used in those installations for the reproduction of "Around The World in Eighty Days" printed on 35mm film, where a Perspecta Sound unit is not installed in the fourth channel. The 30, 35, and 40 cps control frequencies are blocked or filtered out of the reproduced sound. Frequencies above 5000 cycles per second are also filtered from the energy fed to the auditorium (surround) speakers, since the units commonly used for this purpose are occasionally not suitable for the reproduction of the frequencies above the 5000 cps range.

The AM-1154 plugs into the receptacle in the AM-1089 system control cabinet which normally receives the NTS (Simplex) AM-1064 control unit when the system is a Simplex XL500 Series. In the XL200 or XL400 Series, the AM-1064 plugs into a receptacle in the PU-1012 or PU-1014 pre-amplifier power supply cabinet. In these systems the AM-1154 would plug into these receptacles; two AM-1154's being required here, as against one in the XL500 Series System.

When a Perspecta Sound integrator is supplied, and so far it appears that the Fairchild integrator modified by Westrex will probably be furnished by Simplex, a low-pass filter has been incorporated during the modification, and this band-pass filter will not be required. The AM-1154 may be received with or without the filters. In the former instance, the filter should be disconnected from the plug and terminals 6 and 10 on the plug strapped. In the latter instance these terminals should be strapped when the filter is received.

The above information is a direct reproduction of a procedure submitted by the National Theatre Supply Company of New York.

7. Alternatively, the exhibitor may obtain the two filters* directly from Pulse Techniques, Inc., 1411 Palisade Avenue, West Englewood, New Jersey. Note that these are for use in a 500-ohm circuit. When the preamplifier output - power amplifier input circuits are not of 500-ohm impedance, matching transformers, as specified in Todd Dwg. 110457-SL, A.L.-A-435-A, are available from Altec-Lansing. Two RCA MI-11712 matching transformers may be used since they are readily available. Equivalent transformers are also available from other manufacturers.

*RCA does not stock these filters. Consult the local Theatre Equipment Supply Dealer.

Paul V. Smith, 203-3
Electronic Engineering
Tech. Prod. Service


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