Patent GB-1350692-A (GB)
Title: Image transfer devices
IPC: G03G15/32
Filed: 1971-04-28
Abstract: 1350692 Cathode-ray tube displays; sawtooth generators COMPUTER OPTICS Inc 28 April 1971 [1 May 1970] 11855/71 Headings H4T and H3T [Also in Division G2] In an arrangement in which data displayed on the screen of a cathode-ray tube by intensity modulation of a line scanned raster is recorded on a moving record medium, position sensing means provides a signal indicative of the instantaneous position of the medium, which moves in the direction of the vertical (frame) scanning, and operates to shift the vertical sweep waveform in time such that relative motion between the displayed data and the record medium is zero. Thus, as shown in Fig. 1, the " normal" display 11 produced by vertical sweep 10 would assume successively, the forms shown at 13, 17, 21, 25 by introducing successive delays td 1 ; td 2 ; td 3  and td 4  in the start of the sweep 10 to produce the sweeps 12, 16, 20 and 24 respectively and by limiting such sweeps between the limits +i and -i the successive lines of data may be caused to appear in a " window " 15 outside of which the other data lines " pile-up " and may be blanked. Since the recording medium is in continuous motion the position sensing signal causes a continuous progressive delay in the start of the sweep and therefore the line of data move " through " the window at the same speed as the recording medium. Fig. 2, shows the constructional arrangement of the recording apparatus in which the position sensing means, comprising a potentiometer 44 (see also Figs. 3 and 6), is rotated by movement of the recording medium 43, consisting of a paper coated with a photoconductor such as zinc oxide, as it is driven via a corona discharge chamber including electrodes 48, 49 which charge it negatively, past a fibre optic 50 (which images the " window " displayed on tube 30 on the medium) the recorded medium then passing through a toner and heater (not shown) which provide a permanent image and then via a cutter 55 to a tray 56. The screen of tube 30 preferably has a width greater than the normal 8 inches of the recording medium and a height sufficient to produce the " window " portion of the display. Shields 33 and 34 prevent inadvertent contact of the tube by operators. Fig. 3, shows the vertical sweep generator the waveforms present at the points A to F being shown in Fig. 4. The generator comprises a first ramp generating circuit including capacitor 70 which is charged via a constant current source including transistor 60 and discharged via transistor 61 when the latter is rendered conductive by pulses A of field period T supplied at terminal 79. The ramp waveform B thus generated is fed to a comparator circuit including transistor 62 which is back biased whilst the amplitude of ramp B is less than the potential supplied by the position sensing potentiometer 44 but conducts when the former becomes equal to (and exceeds) such potential the resulting output waveform C, the part td of which (see Fig. 4) determines the delay, is amplified, inverted and limited between limits 0 and +5,V in stage 63 and after shaping in network 107 to provide waveform E is employed to hold transistor 65 conductive which forms the discharge path for capacitor 105 in the constant current charging circuit including transistor 64, until the end of the delay period td to provide the ramp waveform F. This is then limited between +i and - i to provide the " window " display ramp waveform G (Fig. 4). Fig. 6 shows a computer terminal at which a display tube 120 displays data derived from a keyboard 126 or from a remote computer 125 the horizontal sweep generator 117 controlling deflection in tube 120 being also employed to produce the horizontal deflection on the recording display tube 30 the vertical sweep of which is produced by the modified generator 113 described above in connection with Fig. 3. In order to produce the " window " display, the waveform E (Figs. 3 and 4) is supplied to a fixed delay circuit 119 (this waveform being designated A in Fig. 6 and shown as waveform A in Fig. 5) to control a one-shot multivibrator 114 which supplies a gating pulse C (Fig. 5) to a blanking generator 115 during part of the delayed ramp waveform B (Figs. 5 and 6) = G (Fig. 4) so that only the character video from generator 118 which occurs during the occurrence of the gating pulse C is effective to intensity modulate the beam of tube 30.