Motion Picture Herald (Oct-Dec 1951)

pump becomes part of the projection room equipment. UNITS OF SIMPLEX SYSTEM With the above brief review of the different types of theatre TV equipments now available or becoming available, the system to be described below falls readily into its proper category. It is a system of the first type, for direct projection from the face of a cathode ray tube. It is manufactured by General Precision Laboratory, an affiliate of National Theatre Supply, and is known as Simplex Direct Projection Television System, Model PB-600. The apparatus of Model PB-600 is subdivided into three components : the projector proper, the control board, and the high-voltage power supply. The control board preferably should be located in the projection room, alongside a viewing port. I'he projector in most theatres will have to be located in the auditorium, since distance between projector and screen should be approximately 60 feet, although some variation is possible. The high-voltage power supply may be located almost anywhere ; it has no external controls and no operating requirements. Its servicing requirements are limited to inspection and possible adjustment once every few months ; therefore its location and accessibility are not too critical. The projector of the Simplex Model PB-600 has an internal optical arrangement corresponding to those pictured in the center and right columns on page 20 of the October issue of Better Theatres. The picture tube, a 7N94, faces a spherically curved mirror, the curvature of which is concentric with that of the convex face of the tube. From the mirror the image is reflected to the theatre screen through a corrector plate which compensates for the spherical aberration, in an arrangement that closely resembles that of a Schmidt optical system ; however, the mathematical approach used in designing it was different and the result has been quite gratifying according to GPL engineers. As just noted, the curvature of the mirror must match that of the tube face. The projection throw is governed by screen size and mirror curvature. Hence, the shape of the face of the tube is the controlling factor in projection distance. The equipment produces a screen image of acceptable brightness, 15x20 feet in size, at a projection throw of 62 feet. A shorter throw will give a smaller but brighter picture. A longer throw is not practicable because it produces a larger picture and the brightness becomes unacceptably low. Only by manufacturing other and different optical elements would it become possible to increase the length of projection throw without reducing screen brightness, and such additional optics are considered at present economically impracticable. The optical barrel is both ventilated and sealed. Ventilation consists of internal circulation of air by means of a small blower, and cooling through the metal walls of the barrel. Sealing-in of the optical elements protects them against dust and dirt, avoids reduction of picture clarity because of accumulated dust, and minimizes the maintenance required. The optical barrel opens at the top for servicing. The projection tube mounting is designed for pre-focusing, and all electrical connections to the tube are of plugin type. Thus a spare tube and holder may be pre-focused in advance and kept ready to be substituted with minimum delay in case of symptoms of trouble in the tube being used. CONTROL PANELS The control board for the PB-600 system is shown in a drawing. Top left-hand panel is the picture monitor, by means of which the projectionist adjusts and perfects his image before projecting it to the large screen. Next panel down on the left-hand side is the TV receiver, used when programs are taken from the air instead of arriving at the theatre via coaxial cable. In the design of this receiver, the manufacturer states, no compromise whatever was made with costs — the number of tubes used is the number that gives best performance without regard to economics. The tv\m panels below the receiver on the left-hand side of the control board are the vertical and horizontal deflection chassis. These circuits create, shape and amplify the pulses of power which cause the cathode ray to scan the face of the cathode ray tube horizontally and vertically, thus “painting in” the television image. If the ray did not brush back and forth and up and down across the face of the tube there would be no image, but only a spot of light of fluctuating brightness at the screen center, and that would not be there long — the tube would melt. The blank panels at the lower left and right sides of the control board contain power supply circuits. Top panel on the right hand side is the projector control. All except one of the controls and switches needed in normal operation of the system are concentrated here. A multi-purpose meter and a waveform monitor are also mounted on this panel. Directly below the projector control panel is the program selector. Through this selector the operator chooses between pr(^ grams taken off the air via the receiver, and other programs that may be coming to the theatre by coaxial cable. Provision is made for a maximum of three incoming program lines, in addition to the receiver. Signal lamps on the control panel indicate when power has been turned on, when the low-voltage equipment is warmed up and ready for operation, and when the 80,000-volt high-potential has risen to its full value. These lamps also indicate that all safety interlocks are closed and circuits are performing normally. PROTECTIVE DEVICES Before starting his show, the projectionist checks his essential voltages at the multipurpose meter, then monitors his image by means of both picture monitor and waveform monitor. Operating the picture switch to “on” then puts the program on the big screen. The interlock system protects both personnel and equipment, and in addition prevents against imperfect performance. It will not allow an image to appear on the projection tube unless horizontal and vertical deflecting fields have at least 75% of their rated strength ; nor if the 80,000-volt operating jwtential is either too low or too high ; nor if any of the other operating voltages is not within correct and safe limits. The precautions used to assure proper operation and safety in the Model PB-600 go into rather unusual detail at some points. For example, in checking whether horizontal and vertical deflection fields are of proper strength the arrangement used is not the common and simple one of measuring the strength of the yoke currents that create those fields. Instead, small pickup coils suitably located measure the actual magnetic field itself. Throughout the entire system the safety provisions are elaborate. They include interlocks that keep the high voltage rectifier not only inactive but unenergized until all doors giving access to anode and focus voltages are closed. [Aaron Nad ell, radio and sound engineer, has been a 'icriter on those subjects for many years and has been a frequent contributor to Better Theatres on sound. He edited the Seventh Edition of “Richardson’s Bluebook of Projection.” — Ed.] 32 MOTION PICTURE HERALD, DECEMBER I, 1951