Electronic Character Generating

Abstract

In an electronic character generating apparatus a display cathode ray tube is subjected to deflection in accordance with a television type raster. Some of the signals from a conventional character generating device which are applied to the tube during fixed time durations of successive television line periods so as to produce a character-simulating pattern of rectangular dots, are modified as to their time durations so that the dots of the pattern are modified in shape so as to be non-rectangular or cruciform.

Description

This invention relates to electronic character generating apparatus, i.e., to apparatus for electronically generating letters, numbers or other predetermined recognizable selectable patters -- the word "character" is herein employed to include all of these -- for display on the screen of a cathode ray display tube. Such electronic character generating apparatus is often required for such purposes as the presentation of alpha-numeric information on television displays.

Electronic character generating apparatus suitable for purposes as that just mentioned is, of course, well known. With known electronic character generating apparatus as at present in common use, characters are presented on the display tube by intensifying selected rectangular dots in a matrix of dot positions so that the intensified dots jointly provide a brightened pattern which approximates to the shape of the required character sufficiently closely to be recognizable as such. This customary present day practice is exemplified in FIG. 1 of the accompanying drawings. The dotted line rectangular areas in FIG. 1 represent a matrix of 35 rectangular dot positions of which 14 are shown brightened so as to present a recognizable approximation to the numeral 2. The brightened up dot positions are indicated in FIG. 1 by diagonal cross-shading. The brightening is obtained by means of brighten-up signals generated by suitable switchable signal generating means customarily of the counter type and suitably synchronized with relation to the line and field frequencies employed for the television raster. These signals are so timed as to brighten up the ray in the display tube over the fractions of the line and field periods necessary for producing intensification of the chosen dot positions. On FIG. 1 the horizontal full lines extending across the diagonally shaded areas diagrammatically represent brightened-up positions of television scanning lines. Thus, if, in the matrix represented in FIG. 1, each dot is 1/nth of a line wide and 1/mth of a field deep and if, for simplicity, it is assumed that the television raster is not interlaced (though, in practice, double interlacing is usually employed), the signal generating means would be switched into the condition in which brighten-up signals are produced during the second, third and fourth (counting horizontally to the right from the top left hand corner position in FIG. 1) 1/nth fractions of a line period in the first 1/mth fraction of a field period (counting vertically downwards from the top left hand corner position in FIG. 1); brighten-up signals are produced during the first and fifth 1/nth fractions of a line period in the second 1/mth fraction of a field period; brighten-up signals are produced during the fifth 1/nth fraction of a line period in the third fraction of a field period . . . . . . . and so on as requisite to produce the approximation to the numeral 2 shown by the shaded areas in FIG. 1. The lengths of the television lines in all brightened up dot positions are the same throughout and the number of brightened up line fractions, one under the other, in all brightened up dot positions is also the same throughout -- to quote a practical figure, this number could be 4 per dot position, this number of lines per dot position being that shown in the example of FIG. 1 by the aforesaid horizontal full lines. The switchable signal generating means may be keyboard controlled, with each key marked with a different one of the characters which can be generated, and by pressing any selected key, the necessary changes are made in the circuitry of the switchable generating means to cause it to produce brighten-up signals at the times and for the periods requisite to brighten-up the dot positions appropriate to the character selected. The means for generating the brighten-up signals -- the "character signal generator" -- may take any of a variety of known forms. Examples of known commercially available character signal generators are the one included in the device known as the Marconi Source Identity Generator and the one in the device manufactured by the Radio Corporation of America under the trade name "Divcon." Either will produce a character display like that in FIG. 1.

It will be apparent from the foregoing and from FIG. 1 that the known character signal generators, when employed in the known way such that their signals are used directly to brighten up the chosen dot positions, produce the result that each brightened up dot position is rectangular in shape. Accordingly, what may be termed the quality or legibility of the characters produced leaves much to be desired except in those cases in which the form of the character required is itself essentially rectangular in shape, e.g., in the case of the capital letter T. The larger the matrix used for each character and the greater the number of dot position it includes the better will be the legibility, but, obviously, improvement in legibility by increasing the number of dot positions in the matrix is accompanied by a corresponding increase in the cost and complexity of the character signal generator. The present invention seeks to improve (as compared with known apparatus) the legibility and quality of the characters obtained with a given size of matrix and a given number of dot positions in the matrix.

According to this invention an electronic character generating apparatus of the kind in which a display cathode ray tube which is subjected to deflection in accordance with a television type raster is caused to display an approximation to a selectable character by applying to said tube signals occurring during selected dot positions each occurring during a predetermined fraction of a television line period and a predetermined fraction of a television frame period said positions together constituting a rectangular matrix of such positions, comprises means for so varying the lengths of the television line fractions in each dot position during which said signals are applied that the shape of the display produced in each such dot position is not rectangular but substantially smaller in dimension in the line direction at the beginning and end of the aforesaid fraction of the television frame period than in the middle of said fraction.

Preferably the variation of the lengths of the said television line fractions is such that said shape of the display is cruciform.

In the simplest way of carrying out the invention the aforesaid predetermined fraction of a television frame period is such that and the applied signals are such that, the display in any dot position is formed by four television line fractions one over the other and of which the middle two occupy substantially the whole dimension, in the line direction, of said dot position and the other two occupy each substantially a centally situated half of said whole dimension. In this case the display shape will be cruciform, the lengths of the line fractions above and below the horizontal cross bar of the cross being substantially one-half the lengths of the line fractions in the said cross bar. However, as is possible if the display in a dot position is formed by more than four television line fractions, one over the other -- for example six -- there may be more than two steps of change equally divided between steps of opposite directions of change and occurring symmetrically with respect to the middle of the fraction of the television frame period. Thus, for example, with six line fractions, one over the other, the middle two may occupy each substantially the whole dimension, in the line direction, of the dot position, the fractions immediately adjacent said middle two (one above and below) may occupy each substantially a centrally situated two-thirds of said whole dimension and the remaining two may occupy each substantially a centrally situated one-third of said whole dimension. Other things being equal, the more the number of line fractions, one over the other, in a dot position display and the more the number of steps of line fraction variation, the better the quality of the display but, of course, this is obtained at the cost of more complex and expensive apparatus. Because of this the simple four television line fractions per dot position display is at present preferred; as will be seen later it produces a quite considerable improvement in legibility and character quality as compared with comparable known equipment as at present in common use.

The invention is equally applicable to the case in which the television raster in the tube is non-interlaced and to the case in which the raster is interlaced. In the non-interlaced case the television field and frame frequencies are of course the same and (assuming four line fractions, one over the other, in each dot position display) the four line fractions per dot position display will be produced, counting downwards in the frame direction, in direct time sequence. In the double interlaced case (the usual practical case), the television field frequency is, of course, twice the frame frequency and (again assuming four line fractions, one over the other, in each dot position display and again counting downwards in the frame direction) the four line fractions will be produced in the time sequence 1, 3, 2, 4, the first and last being, as already explained, half the length of each of the other two.

The signals applied to the display tube may be obtained by a combination of a known character signal generator adapted to produce signals which, if directly applied to the tube, would produce rectangular dot position displays and, in cascade therewith a signal converting circuit arrangement time controlled by the television line and frame frequencies and adapted to convert the constant length line fraction signals from said known generator into the required varying length line fraction signals. In a preferred embodiment, suitable for use in the case of an interlaced television raster, said signal converting circuit arrangement comprises a first pair of positive NAND gates (a positive NAND gate is one which provides an AND function in response to positive inputs, producing a negative output and an OR function in response to negative inputs producing a positive output) each having two inputs of which one is fed from said signal generator with substantially rectangular pulses at the frame frequency and the other is fed from said signal generator with substantially rectangular pulses at half the line frequency and such pair of gates performing an AND function in response to positive inputs thereto; a possitive NAND gate having three inputs of which one is fed with negative output from one of the first pair of gates, another is fed with negative output from the other of said pair of gates and the third is fed rectangular waves of dot frequency derived from a master oscillator and delayed by one quarter of the dot period and such gate performing an OR function in response to negative inputs thereto; and a further positive NAND gate having two inputs of which one is fed with output from said third gate and the other is fed with character signals (which if directly applied to the display tube, would produce rectangular dot position displays) from said character signal generator, output from said further gate, which is adapted, when positive output signals from said OR function gate are fed thereto, to perform an AND function in response to the positive inputs thereto and pass character signals, being applied via a polarity inverter as brighten-up signals to the display tube.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a diagram illustrating display of the numeral 2 according to the prior art;

FIG. 2 is a block diagram illustrating a preferred embodiment of the invention;

FIG. 3A is a timing diagram showing the Q outputs of the two bistables in FIG. 2;

FIG. 3B is a diagram illustrating the output waveforms of the prior art circuit which produces the display of FIG. 1;

FIG. 3C is a diagram illustrating the output waveforms of the circuit of FIG. 2 which produces the display of FIG. 4;

FIG. 4 is a diagram similar to FIG. 1 but showing the display achieved by the circuit of FIG. 2; and

FIG. 5 is a timing diagram illustrating waveforms at various points of the circuit of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, blocks 1 and 2 represent bistable elements of the J-K type, block 2 being part of a known character signal generator (not otherwise shown). Block 1 provides, on lead 1L1, rectangular pulses which are shown by line 1L1 of FIG. 3 and appear at the frame frequency, which is half the field frequency. Similar pulses, but at 180.degree. displacement appear on lead 1L2. Blocks 1 and 2 have indicated thereon in conventional manner their four terminals referenced J, K, Q, and Q, block 1 having also indicated thereon preset terminal P. Block 1 is "clocked" by field frequency pulses to produce at Q and Q the complementary frame frequency square waves (see lines 1L1 and 1L2 of FIG. 3). Applied over lead F' to the preset terminal P are frame frequency pulses which ensure the correct phasing on the output wave forms from Q and Q with respect to the field sequence (odd, even, odd........ and so on). Block 2 is the first stage of the vertical (field) counter of the known character generator.

Block 2 provides on lead 2L1 rectangular pulses as shown by line 2L1 appearing at half the line frequency. Similar pulses, displaced by 180.degree., appear on lead 2L2. Leads 1L1 and 2L1 provide the two inputs to a positive NAND gate G1 and leads 1L2 and 2L2 provide the input to a positive NAND gate G2. The first of these NAND gates gives a useful output in one of two successive interlaced fields and the second gives a useful output in the other, in each case the output being a waveform as in 1L1 but "slotted" during alternate fields by 2L1. The outputs from gates G1 and G2 provide two of the inputs to a three input OR gate G3 the third input to which is provided by a dot period full rectangular wave, obtained, for example, from the master oscillator (not shown) of the television equipment, which is fed in on lead DF and delayed by a quarter of a dot period (the dot period corresponds with the width, in the line direction of one dot position) by a suitable delay device D. Output from the OR gate G3 provides one input to a fourth gate G4 having two inputs the other of which is fed in over lead CS and consists of character signals which are taken from the known character signal generator and are such as to provide, if fed unchanged to the display tube (not shown) rectangular dot positions, each with four identical television line fractions, one under the other, as shown in FIG. 1. When gate G4 receives positive signals from gate G3 it passes these character signals to the display tube through a polarity inverter INV.

The output from gate G4 after inversion by INV appears on lead OUT and constitutes the video signals which are applied as brighten-up signals to the display tube (not shown). These signals are represented to the right of the chain line X-X in the lower part of FIG. 3, the signals to the left of that line, provided for the purpose of comparison, being those (provided by the known character signal generator) on lead CS. In this part of FIG. 3 the references F1L2 mean field No. 1 line No. 2; the reference F2L2 means field No. 2 line 2; the references F1L3 mean field 1 line 3; and the reference F2L3 means field 2 line 3. Below the left hand half of the position of FIG. 3 is a pattern referenced CS of four equal line fractions such as would be produced in a dot position display by the signals on lead CS: below the right hand half of this position is a pattern referenced OUT of four raised line fractions of which the top and bottom are shortened in relation to the two middle ones and which will be produced by the signals on lead OUT. As will be seen pattern CS is rectangular; pattern OUT is cruciform. FIG. 4 shows the numeral 2 as produced by such cruciform dot position displays and it will be at once seen from a comparison of FIGS. 4 and 1, that marked improvement in quality or legibility is obtained by use of the invention.

Briefly to describe the functional operation of the circuitry of FIG. 2 it will be seen that gates G1 and G2 select, one for each field, the lines in which the line fractions as produced by the character generator are to remain of unchanged length. At the times at which outputs occur from these gates (see FIG. 3) the outputs will be negative. These negative outputs are OR'd by the positive NAND gate G3 the resultant positive output from which causes the complete character signal to be passed by the further positive NAND gate G4. In alternate fields in which signals are obtained from G1 those from G2 are positive and in other alternate fields in which outputs are obtained from G2 those from G1 are positive. When neither G1 nor G2 provides an output, i.e., when neither is AND'ing gate G3 will be controlled by the delayed master oscillator waveform during its negative excursions. These negative inputs are OR'd by gate G3 to produce therefrom a positive output signal which allows gate G4 to pass only the central half of the character signal to pass to INV and the output.

To appreciate the function of the FIG. 2 circuitry, reference to FIG. 5 should be had. As stated, the high or positive Q outputs of the two bistables 1 and 2 and AND'ed by the gate G1 whereas the high Q outputs of the two bistables 1 and 2 are AND'ed by the gate G2. Because the bistable 1 is being clocked at field frequency F while the bistable 2 is being clocked at the line scan frequency, the Q output at 1L1 will be high to enable the gate G1 only during that half of each frame during which the unprimed line scans occur, whereas during that half of each frame during which the primed line scans occur, the gate G1 is not enabled by the output at 1L1. Thus, low or negative outputs from the gate G1 will occur at half line frequency only during those periods in which the gate G1 is enabled by the output 1L1. The half line frequency high inputs from the Q output of the gate 2 of course provide these low outputs from the gate G1 at this time.

The same general relationship prevails for the gate G2 except that its negative or low outputs occur only during the time periods in which the primed line scans occur and due to the half line frequency Q outputs of the bistable 2.

Thus, while the gate G1 is providing a sustained high input to the gate G3, the gate G2 provides periodic high pulse outputs to the gate G3 at half line frequency. This occurs during the alternate half frames during which the primed lines scans occur. When the gate G2 provides a sustained high input to the gate G3, the gate G1 provides periodic high pulse outputs to the gate G3, this occurring during the other alternate half frames during which the unprimed line scans occur.

To assure proper timing and synchronization, both bistables 1 and 2 are reset at the frame frequency F'.

Insofar as operation during sustained high output from the gate G2 is concerned, same may be seen from the expanded section A of FIG. 5. The signal DF, as explained above, provides a dot period full rectangular wave. However, the signal DF is delayed 90.degree. and appears as an input to the gate G3 as the signal D, as shown. Thus, during the sustained high output from the gate G2 and while a high output also is occurring from the gate G1, the low outputs at D are OR'd by the gate G3 to produce the periodic high outputs therefrom, as shown at the left of the "A expanded" portion of FIG. 5. The gate G4 provides an ANDing function for the high outputs from CS and the gate G3 so that the brightening pulse F1L2 (see FIG. 3C also) occurs only during the central half of the dot signal CS during line 2. During line 3, the brightening signal F1L3 is coincidental with the signal CS, as shown at the right-hand portion of "A expanded" in FIG. 5. Both of these brightening signals occur in conjunction with the unprimed line scans of FIG. 5 and produce the brightened line portions a and b in FIG. 3C.

During the interlaced primed line scans of FIG. 5, exactly the opposite situation prevails, as may be seen in the "B expanded" section. That is to say, for line 2', the brightening pulse F2L2 is coincidental with the signal CS whereas the brightening pulse F2L3, for line 3', occurs only during the central half of the signal CS, these two brightening pulses producing the brightened line portions c and d of FIG. 3C.

Thus, it will be appreciated that the system according to this invention is of the type which includes gate means receiving the conventional brightening pulses CS and, as well, signals from the output gate G3 of control means which modifies the normal dot produced by these prior art brightening pulses.

Claims

I claim:

1. An electronic character generating apparatus of the kind in which a display cathode ray tube which is subjected to deflection in accordance with a television type raster is caused to display an approximation to a selectable character by applying to said tube signals occurring during selected dot positions each occurring during a predetermined fraction of a television line period and a predetermined fraction of a television frame period, said positions together constituting a rectangular matrix of such positions, and comprising means for so varying the lengths of the television line fractions in each dot position during which said signals are applied that the shape of the display produced in each such dot position is not rectangular but substantially smaller in dimension, in the line direction at the beginning and end of the aforesaid fraction of the television frame period than in the middle of said fraction the signals applied to the display tube being obtained by a combination of a character signal generator adapted to produce signals, which, if directly applied to the tube, would produce rectangular dot position displays and, in cascade therewith a signal converting circuit arrangement time controlled by the television line and frame frequencies and adapted to convert the constant length line fraction signals from said known generator into the required varying length line fraction signals, the display being by means of an interlaced raster and said signal converting circuit arrangement comprising a first pair of positive NAND gates each having two inputs of which one is fed from said signal generator with substantially rectangular pulses at the frame frequency and the other is fed from said signal generator with substantially rectangular pulses at half the line frequency; and OR gate having three inputs of which one is fed with output from one of the first pair of gates, another is fed with output from the other of said pair of gates and the third is fed rectangular waves of dot frequency derived from a master oscillator and delayed by one quarter of the dot period; and a further gate having two inputs of which one is fed with output from said third gate and the other is fed with character signals (which if directly applied to the display tube, would produce rectangular dot position displays) from said character signal generator, output from said further gate, which is adapted, when positive output signals from said OR gate are fed thereto, to pass character signals, being applied via a polarity inverter as brighten-up signals to the display tube.

2. In an electronic character generating system of the type having signal-conveying means for providing brightening signals of fixed time durations during successive television line periods so as normally to create a pattern of rectangular dot displays to generate the relevant character on an associated television screen, the combination of:

gate means for passing said brightening signals during selected portions only of at least some of said time durations to produce a pattern of modified dot displays in which the modified dot displays are of non-rectangular shape, said gate means having an enabling input, an input connected to said signal-conveying means and an output at which said pattern of modified dot displays appears; and

control means connected to said enabling input of said gate means for causing the output thereof to produce said modified dot displays.

3. In an electronic character generating system as defined in claim 2 wherein said control means causes said modified dot displays to be substantially cruciform.

4. In an electronic character generating system as defined in claim 2 wherein said control means includes an output gate having successive outputs of unequal time duration.

5. In an electronic character generating system of the type having signal-conveying means for providing brightening signals of fixed time duration during successive television line periods so as normally to create a pattern of rectangular dot displays to generate the relevant character on an associated television screen, the combination of:

gate means for receiving said brightening signals and having an output producing a pattern of dot displays; and

control means connected to said gate means for causing the output thereof to produce dot displays of a shape differing from said rectangular dot displays said control means also including an output gate having successive outputs of unequal time duration, a pair of gates providing two inputs for said output gate, one gate of said pair of gates providing sustained enabling input to said output gate during alternate frame periods and periodic enabling inputs to said output gate during remaining frame periods, the other gate of said pair providing sustained enabling inputs to said output gate during said remaining frame periods and periodic enabling inputs to said output gate during said alternate frame periods, said periodic enabling inputs being at half line frequency and 180.degree. out of phase as between the two gates of said pair.

6. In an electronic character generating system as defined in claim 5 wherein said control means also includes means for providing pulse input to said output gate which is of a period equal to the period of said brightening signals.