Patent GB-1329591-A (GB)
Title: Apparatus for reproducing video information
IPC: H04N5/84
Filed: 1970-11-06
Abstract: 1329591 Television; sawtooth wave generating circuits MITSUBISHI ELECTRIC CORP 6 Nov 1970 [7 Nov 1969] 52931/70 Headings H3T and H4F  A continuously moving record medium such as a cine film, is scanned by a raster the vertical component of which comprises a combination of two sawtooth waveforms, the first waveform having the same frequency as that at which the frames are to be reproduced, and the frequency of the second waveform being adjustable together with the speed of the record medium to allow slow motion reproduction by scanning each frame a plurality of times. The system is shown in Fig. 1 and comprises a flying spot scanner 1 which provides a raster on a film 3 which is continuously moved by means of capstan 6. The transmitted light is picked-up by detector 9 and the signal is applied to a monitor 11 via amplifier 10. The film is provided with a pattern of vertical lines between adjacent frames to provide a synchronizing signal which is selected by sync. separator 12 and triggers a pulse generator 13 which provides an output pulse B of constant width which in turn triggers a second pulse generator 13 which provides a narrow flyback pulse C at the trailing edge of the pulse B. The pulses C control the flyback of a first sawtooth generator 17 and are also passed through a pulse divider 15 the output of which controls the flyback of a second sawtooth generator 18 the output amplitude of which is made the same as that of generator 17 by means of a control circuit 19. The two sawtooth waveforms are combined in scanning generator 20 to provide the vertical scan for the flying spot scanner 1 the horizontal scan being provided by a further generator 2. For Normal Operation the divider 15 is arranged to give a division ratio of unity and the capstan drive 7 moves the film at normal speed. The second sawtooth wave has the same frequency as the first sawtooth wave so that the vertical scan comprises a sawtooth of the same frequency but double the amplitude and each frame of the film is scanned once while it moves down one frame spacing.  For Slow Motion Operation the capstan drive speed is reduced e.g. to one third of its normal value and the divider 15 is arranged to give the same division ratio, i.e. to divide by three. The second sawtooth waveform now has a frequency one third that of the first sawtooth so that the vertical scan comprises three cycles of the first sawtooth frequency but of larger amplitude posi - tioned in overlapping relationship down the screen corresponding to the movement of the film so that each frame is scanned three times while it moves down one frame spacing. The direction of motion of the film may be reversed. The sawtooth wave generator circuits are shown in Fig. 5, the frame frequency generator 17 comprising an integrator utilizing a capacitor connected in the feedback path of an operational amplifier 41, the capacitor being short circuited when a parallel connected transistor is driven into conduction by the synchronizing pulses C to provide the flyback. The second sawtooth generator 18 is similar to the first but instead of utilizing a constant voltage source to supply the integrator it is connected to the output of a differential amplifier 47 to the inputs of which are connected circuits which detect the amplitudes of the outputs of generators 17 and 18. This ensures that the amplitude of generator 18 is equal to that of generator 17. The outputs of the generators 17 and 18 are added and amplified to provide the vertical scanning waveform.