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1954-55 Theatre Catalog, 12th Edition, Page 266 (230)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 266
Page 266


1954-55 Theatre Catalog, 12th Edition, Page 266

system noise will exist in the auditorium at all times with or without signal, unless this noise is somehow suppressed between periods when there is signal. Therefore, a simple control device is being used for the initial CinemaScope releases. This control is operated by superposing a 12 kilocycle frequency on the effects tracks when recorded signal exists, i.e., there is 12 kc present only when the effects are to be reproduced and none at other times. Preceding the power amplifier there are simple separation circuits, control amplifiers and an audio amplifier or relay. All of these units are called "switchersff Kcontrol amplifiers" or ffsuppressor amplifiers." The separation circuits route the audio signal to the amplifiers and the 12 kc control signal to sharply tuned circuits. In some apparatus there are two such tuned circuits which might require adjustment in the theatre because of speed differences in the machines. The two tuned circuits should be stagger-tuned slightly, i.e., one tuned about 100 cycles below and the other 100 cycles above the optimum output tuning point. With proper adjustment slight speed variations will not cause erratic operation. Some equipments use only a single tuned circuit and this must be adjusted for optimum performance of the control signal. Also, there are units which have a tuned circuit in the audio branch which is intended to remove the 12 kc signal from the.audio circuit and this circuit should be adjusted so that it cuts off very sharply between 7 and 11 kc in order to remove substantially all the 12 kc from the surround speakers. When a control or suppressor amplifier is used the 12 kc signal derived from the separation circuits is amplified and rectified. The rectifier output is used, sometimes in conjunction with another tube, to vary the net grid bias on the audio amplifier section; this operates to change the gain of the amplifier. The circuits are specified to operate with a minimum 12 kc input voltage of .025 volts; an increase in the 12 kc input does

not appreciably increase the gain. As a safety factory the 12 kc control is being recorded to produce .05 volts input. When 12 kc is not present on the track the audio amplifier gain is low and the noise is suppressed. In those equipments using a relay instead of an amplider the presence of 12 kc operates the relay to close the audio circuit and when no 12 kc exists the relay opens the circuit. Thus, in either case, the control operates as a simple on-ofi' scheme.

It is obvious that the amplifiers preceding the switcher must pass sufficient 12 kc to provide satisfactory operation of the particular model used.

The gain of the effects channel should be adjusted to produce a surround level compatible with the screen horn levels so that the audience feels, or senses the sound from all directions.

Undoubtedly, additional control features and signals will be developed as the need arises. The system described appears to be the simplest and most economical scheme to meet the initial demand.

Noise

More attention must be paid to minimizing system noise since the noise signal from all three channels (film and amplifiers) is constant and adds, while the signal output from the three channels is seldom at maximum level simultaneously, Hence, the overall signal-tonoise ratio is sometimes less than that of a single system installation.

Response

The electrical frequency characteristic of each channel will eventually be standardized but for the time being it will be satisfactory to state that the overall characteristic from recording through reproduction shall be flat from 50 to 8000 cycles with tolerances as shown in Figure 9A.

The relative response of the channels is more important than absolute or average response. The tolerances or permissible variations between channels is

shown as Figure QB. This measurement must be made with a suitable test film because the recording characteristic will have pre-equalization of approximately 6 db at 60 cycles and 3 db at 8000 cycles, both of which are useful to reduce hum and noise difficulties. The amplifier system, if measured independently, will have a frequency characteristic with a slope of 6 db per octave (6 db decrease in signal for each doubling of frequency) except as this characteristic may be modified to take into account the low frequency pie-equalization and high frequency losses.

The mid-band channel gains must be adjusted to within 1 db between the lowest and highest channel.

Intimate contact between the magnetic track and the pickup head surface is very important for high frequency reproduction, and any factor, such as distorted film, low film tension or any accumulation of dirt or oxide on the head surface-all of which tend to destroy good contactemust be observed and minimized. Clean the surface with a soft rag or wooden stick. Azimuth should be adjusted for maximum output.

Subiecfive Tests

Since there is no simple way to determine the efiiciency of loudspeaker systems, a final check of the relative channel reproduction should be made by listening tests, comparing the output of each channel independently from a known test film having equal and identical signals on all three tracks. By this subjective test, the channel gains can then be adjusted to have equal output. An excellent method is to use warbled single frequencies, one below the loudspeaker crossover network frequency and one above. For example. 400 cps and 800 cps would be suitable.

Phasing

The overall phasing should be checked by making a common connection between one wire of each loudspeaker at the input to the dividing network and



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1954-55 Theatre Catalog, 12th Edition, Page 266