Patent GB-1374206-A (GB)
Title: Electronic character generators
IPC: G09G5/24
Filed: 1972-05-05
Abstract: 1374206 Cathode-ray tube displays; television; pulse and ramp waveform generators ASTON ELECTRONIC DEVELOPMENTS Ltd 4 May 1973 [5 May 1972] 20983/72 Headings H4F H4T and H3T In an arrangement for generating video signals representing characters for television display ramp waveforms synchronized with the line and field scanning standards of the television system are produced and from these waveforms, further waveforms including, at least, substantially parabolic waveforms are derived, selected combinations of the derived waveforms being employed, directly, to produce outputs representing character elements from which the required video signals are produced. The invention is described in connection with an arrangement for superimposing characters, of adjustable size, such as captions &c., on a television picture display, the (white) characters being displayed within a black area which outlines each character. Each character is displayed on an area K (Fig. 1) notionally divided into quadrants 1-4 by selecting, for each quadrant, the outputs which produce the appropriate quadrant of one of the elements a to o shown in Fig. 2, together with an appropriate "modifier" (i.e. blanking signal). Figs. 3 and 4 show respectively the sequences employed in the production of the characters G and A. Fig. 5, shows a schematic arrangement of the complete system comprising chracter time base units 10, 11 (also Fig. 6) supplying processing units 14, 15 (also Fig. 7) which produce, as shown, ramps of opposite polarity together with triangular waveforms, some of the output ramps controlling parabolic generator units 16, 17 and 18 producing the parabolic waveforms shown. The various waveforms are connected, selectively, via busses 22 to fifteen element generating units 23-38 which respectively produce outputs corresponding to the fifteen elements a-o (Fig. 2) and also, selectively, to horizontal and vertical modifier units 86, 87 which are controlled by outputs from a character "shape" memory 78 to selectively supply appropriate blanking signals for combination with the outputs from the element generating units 23-38 in accordance with the elements to be displayed, the signals representative of which are selectively gated out to a mixer unit 79 by further outputs from the memory unit 78. The latter is addressed by character time base signals in respect of quadrant addresses and by outputs from a digital character storage unit 21, loaded by a keyboard 97, in respect of the particular "shapes" (elements) to be displayed. The mixer 97, which receives the outputs of the selected element generating unit 23-38, typically, waveform 71a, is also supplied with blanking inputs and privides two outputs 79a and its inverse 79b for supply to an "outline" generator unit 95. The latter then provides a "broadbase" negatively going pulse 95b, which, acts as a keying signal in a video processor unit 93, together with a positively going narrow peak waveform 95a the latter then causing the selected character to be reproduced within the "keyed outline" produced by waveform 95b in the clamped video signal input to the processor unit 93.  Fig. 6 shows the circuit of the character time base units 10, 11 (Fig. 5) the parameter values of which vary for horizontal or vertical operation as indicated by "H" and "V". As shown, a capacitor C charged by a current source VT2 is discharged by conduction of transistor VT4 under control of the output of an integrated circuit ÁA760 acting as a comparator and which compares the rising voltage of the ramp across the capacitor supplied to input 5 with a fixed reference voltage applied to terminal 4 and provides the output when the former input exceeds the latter. This produces the basic character ramp the production of which is inhibited during horizontal and vertical synchronizing periods by appropriate synchronizing signals from terminals 9 or 13 (Fig. 5) applied to an integrated circuit 74121 acting as a one-shot multivibrator and discharging capacitor C via transistor VT3. The size of the characters may be controlled by an input to VT1 and the position of the character may be shifted incrementally by means of an input to transistor VT3 which acts by bringing the initial voltage across capacitor C below zero. The pulse output signals from the integrated circuit ÁA760 provide some of the blanking signal inputs to mixer 79 (Fig. 5). Fig. 7 shows the basic circuit employed for each of the ramp processing units 14, 15 of Fig. 5. As shown, the input ramp is supplied, together with a reference voltage, to a differential amplifier VT3, VT6 across the collector loads of which opposite polarity ramps are produced and fed to the outputs via emitter followers VT9, VT10. To produce the required triangular waves the bases of transistors VT4, VT5, forming a second differential amplifier, are driven by VT3, VT6 and the triangular wave is developed across the common emitter load of VT4, VT5. This wave is also supplied to an integrated circuit ÁA760, acting as a comparator and providing output pulses which are integrated and stored on capacitor C1 and employed to drive a field effect transistor FET, the arrangement acting as a peak detector and controlling transistor VT7 which defines the current in the differential amplifier VT4, VT5 and serves to clamp the negatively-going peaks of the triangular waveform to zero and also defines the D.C. level of the ramp output waveforms. Fig. 8, shows the basic form of the parabola generator units 16, 17, 18 of Fig. 5. As shown the appropriate ramp waveform input is supplied to inputs 10 and 12 of an integrated circuit SG3402, acting as a double-balanced modulator or multiplier, thus squaring the inputs to produce the parabolic function waveform at terminal 4 and its inverse at terminal 11. In a similar manner to the arrangement of Fig. 7, an integrated circuit ÁA760, acting as a comparator, together with a storage capacitor C1 and a field effect transistor FET control the current source VT2, VT3 to clamp the negative peaks of the parabolic wave to zero. Fig. 12 shows the circuit arrangement of the video processing unit 93 of Fig. 5. In operation a normally non-conducting transistor VT3 is rendered conducting when the negatively going, "outline", pulse 95b (Fig. 5) is present at input 95b so that the emitter of transistor VT2 is prevented from going positive, thus holding the video input (and cursor, if present) at its black level and effectively gating a "hole" into which the character forming pulses 95a (Fig. 5) may be inserted. The latter, which are positively going, are supplied at terminal 95a and cause a normally non-conducting transistor VT8 to conduct thus causing the video signal to rise to its white level to provide the character forming video signal which is supplied via a buffer amplifier VT11, VT12 to the output 93a. Subsidiary outputs Key Out and Character Out are provided for the insertion of the characters into a video signal in remotely located equipment. Circuit details of the element generator units 23-38, the mixer unit 97, the outline generator unit 95 and the modifier units 86, 87 shown in Fig. 5, are described with reference to Figs. 9, 10, 11 and 13 respectively (not shown).