Patent GB-1328830-A (GB)
Title: Film frame displaying apparatus and method
IPC: G03B21/10
Filed: 1970-12-18
Abstract: 1328830 Displaying selected film frames SAAB-SCANIA AB 18 Dec 1970 [22 Dec 1969] 60215/70 Heading G4H An apparatus for displaying single film frames from a series of frames on a strip includes a means for comparing the number of frames to be passed before the selected frame is reached with a predetermined number, and a control and drive means for feeding the strip past the display point at a fixed speed when the number of frames to be fed exceeds the predetermined number, for reducing the feeding speed when the number of frames to be fed is less than the predetermined number, the decreasing velocity being dependent on the number of frames remaining to be fed, and for holding the selected frame at the display point. Two motors each connected to a spool on which the film strip is mounted are provided, the motors being operative to drive the strip in different directions. A computer presets the contents of counter 30 to the number of frames to be passed and sets one or other of flip-flops 67, 68 in accordance with the direction of feed. Each frame is marked and as the strip is transported two photoelectric detectors detect the marks and generate a train of pulses S 1  and S 2  which are gated to the counter 30 to decrement the count. The upper stages of the counter are coupled to an OR gate 49 which produces an output if the count is greater than 32. This output 50 is passed to a summing point 35 and thence to the drive control (FET's 69-72) which, in accordance with the direction signals from flip-flops 67, 68 activate the appropriate motor, the other motor being short circuited to provide a brake and keep the strip under tension. When the count drops below 32 the signal 50 is absent and the speed of the motor is determined by the digital/analogue converter 37, the speed being proportional to the count. In both cases the pulses S 1  and S 2  from the photocells are differentiated (at 56-59) to produce a pulse train the frequency of which is indicative of the actual speed of the strip. These pulses, via a monostable 61 and a converter 62, produce a signal which is passed to the summing point 35 to provide a feedback control in order to match the speed of the strip to the required speed. The braking motor is supplied with a braking voltage when the counter 30 is registering less than 32. The signal from the OR gate 49 indicating a count of more than 32 is inverted at AND gate 63 to which the differentiated pulses S 3  are also fed. Thus flip-flop 65 is activated via monostable 64 in phase with the pulses S 3  when the counter contents are less than 32. The resulting signal is converted at 66 and applied to the drive control circuit (69-72) to provide a braking signal for the appropriate motor. Thus as the selected frame nears the display point the speed of the strip is reduced. A further pair of photoelectric cells 27 and 28 are provided which centre on a mark on the stationary selected frame and provide holding signals to the drive control in order to ensure that the motors supply equal and opposite driving forces and the selected frame remains at the display point.