International projectionist (Jan-Dec 1954)

used without sacrificing compatibility or introducing intermodulation distortion. Standards for Low-Frequency Control Track The first series of tests comprised low frequencycontrol signals in varying amplitudes. First, singular signals, and then varying combinations of the three basic control signals for observation of any in-phase additive conditions that might cause either amplitude modulation of the complex waveform or an undue amount of harmonic distortion. After a survey in a number of theatres, the standards of control signal levels were set as they now exist. The standards are that: The maximum amplitude of any control frequency shall not exceed 16 db below a 100% constant tone level based on area film recording with a 76-mil peak-to-peak voltage. There is a test reel that incorporates a series of test signals to align the Integrator equipment, consisting of combinations of control signal and 100% tone modulation at a 16 db differential. This calibration represents the maximum reproduction volume level control that would be apparent in the theatre when the control signal reaches this 16 db point. In mixing or scoring the film, to allow for the normal dynamic range of the film track, it was found desirable to set the average mixing control signal amplitude 5 db below its maximum level or at approximately minus 21 db with reference to 100% tone. This allows leeway for further dynamic enhancement beyond the limitations of the optical recording for effects and musical sequences. "Perspecta" Not a Switching System It should be emphasized that Perspecta stereo sound is not a switching system. Continuously varying amplitudes of control govern the output level of each of the three horn channels, thus giving an extremely smooth directional effect without jump between channels. In the final release track, then, the average amplitude of any of the control signals does not exceed 21 db below 100% tone, except for effects or music sequences, thus insuring a greater compatability of the Perspecta release track. The generation for the control signal, as installed at M-G-M, Warners and Paramount, consists of three master oscillators, the frequency of which is set FIG. 2. Rear exposed view of components of the Integrator Unit of the Perspecta sound system. with the use of precise observations on an oscilliscope against the 60-cycle line frequency. One set of master oscillators can supply all the re-recording and mixing channels through the use of appropriate bridging amplifiers or pad networks. The Integrator, the heart of the system to which we have been referring (Fig. 2) works as follows: It incorporates a pre-amplifier with enough gain to accommodate any type of output level from any type of projector. The output of this pre-amplifier is fed into two filters. On the audio side, a 63-cycle, high-pass filter system and booster amplifier feeds a buss connecting to the inputs of the three controlled push-pull stages, so that the same audio signal is available at the input of each of the three controlled amplifiers. A 63-cycle, low-pass filter system with another booster amplifier feeds the inputs of the three band-pass filters. This low-pass filtering removes the largest component of complex audio program, and primarily the only intelligence at this point is a composite of the control signals. Each of the band-pass filters is unique in the following features : Physical Size: A great deal of engineering has been done to reduce the size of these low-frequency units. Electrical Response: They have a 2-cycle bandwidth, to allow for a variation up to 4% in the speed of the projector. At the same time, the filters produce a 20 db rejection at the 5-cycle point between each of the channels to give adequate separation between control channels. Each of the low-frequency control signals after acceptance by its corresponding band-pass filter is again amplified and sent through an appropriate coupling to a diode rectifier system. The amount of rectified voltage appearing on the output of the diode rectifier sections is linearly equivalent to the amplitude of the low-frequency control signal that was introduced to the input of the band pass filter. It is obvious, therefore, that the unit operates on a 1-to-l ratio through its controlled range. Simply expressed, this means a 1 db change in output level of the controlled push-pull amplifier section. This insures a smooth change in volume when changing directional characteristics by variance of control signal amplitude from one band pass section to another. Novel Automatic Switching Device The unit also incorporates an auto-switching device, which is necessary to complete the pattern of compatability. This auto-switching circuit performs as follows: The presence of any control signal of normal amplitude for a period in excess of 7 seconds activates a relay system which connects the output of each of the rectified components of the low-frequency controlling signals, to control the bias of its corresponding push-pull controlled amplifier stage. Absence of all low-frequency control signal for 1^2 seconds causes the auto-switching unit to deactivate the relay system. Under this condition, a bias equal to the cut-off value of the push-pull stages is applied to the two (Continued on page 41) I. A. CONVENTION EDITION • July 1954 33