Video Display System Employing Drive Pulse of Variable Amplitude and Width

1. Field of the Invention

This invention relates generally to a video display system, and more particularly to such a system which includes a flat panel of X-Y matrix type and an improved brightness control circuit for driving the panel.

2. Description of the Prior Art

Recently, video display systems utilizing a flat panel of X-Y matrix type have become the object of considerable interest.

In such systems, different kinds of flat panels, such as, gas discharge panels, liquid crystal panels, electroluminescent panels, and the like have been used, and extensive research has been conducted in respect to the flat panels and their driving circuits.

However, the video display systems of the prior art are still unable to reproduce pictures with good quality on the flat panel.

One of the sources of the difficulties is to provide a brightness control circuit which responds to changes in the level of the video signal with sufficient accuracy. In more detail, the brightness control of the panel used in the prior art is usually achieved by varying either the amplitude or the duration of the current or driving pulse of the display cell of the display panel. These two methods are respectively referred to as "PAM" for pulse amplitude modulation and "PWM" for pulse width modulation. Inherent in the "PAM" method is the difficulty in obtaining driving transistors with sufficiently uniform characteristics to achieve brightness control with good quality for a substantial range of different pulse amplitudes.

On the other hand, the "PWM" method is disadvantageous in that a sufficient number of steps or grades of brightness cannot be obtained because of the time delay of the display panel.

Accordingly, it is an object of this invention to provide an improved video display system of the type referred to above, and in which the inherent disadvantages of the prior art are avoided.

It is a further object of this invention to provide an improved video display system having a flat panel of X-Y matrix type with a brightness control circuit which responds to changes in the level of the video signal with sufficient accuracy to afford a picture of good quality or resolution.

In accordance with an aspect of this invention, the brightness control circuit of a video display system having a flat panel of the X-Y matrix type provides driving pulses for the panel which vary or are modulated both in amplitude and duration in response to variations in the sequentially sampled level of input video signals.

In a preferred embodiment of the invention, variations in the sequentially sampled level of the input video signals are converted into digitally coded signals which selectively energize respective pulse generators to cause the latter to produce pulses of correspondingly weighted durations or widths and amplitudes and which overlap in time, and the pulses produced in response to the sampling of any particular detected or sampled level of the video signals are combined to provide the width or duration- and amplitude-modulated driving pulses for the display panel.

FIG. 1 is a schematic front view of a flat display panel that may be used in the video display system according to the present invention;

FIGS. 2 and 3 are waveform diagrams to which reference will be made in explaining the sampling of an input video signal;

FIG. 4 is a waveform diagram to which reference will be made in explaining how brightness control is effected in one of the video display systems of the prior art;

FIG. 5 is a block diagram showing an embodiment of the video display system according to the present invention; and

FIGS. 6 to 9 are waveform diagrams to which reference will be made in explaining the operation of the video display system shown in FIG. 5.

As shown in FIG. 1, a video display panel or flat panel 1 of the known X-Y matrix type which may be employed in a video display system according to this invention includes a plurality of row lines X.sub.1, X.sub.2, X.sub.3, . . . parallel with one another and a plurality of column lines Y.sub.1, Y.sub.2, Y.sub.3, . . . parallel with one another and perpendicular to the row lines X.sub.1, X.sub.2, X.sub.3, . . . Light emission is made to occur selectively at the intersections of the row and column lines to display a picture or image corresponding to the video signal of a television signal or the like. The display panel 1 may be a gas discharge panel, a liquid display panel, an electroluminescent panel or the like. Where a gas discharge panel is used, by way of example, the row lines X.sub.1, X.sub.2, . . . are formed as its anodes, while the column lines Y.sub.1, Y.sub.2, . . . are formed as its cathodes spaced a suitable distance from the row lines X.sub.1, X.sub.2, . . . . Switching transistors connected to the row lines X.sub.1, X.sub.2, . . . are made conductive sequentially in successive horizontal periods to apply a predetermined voltage to the row lines X.sub.1, X.sub.2, . . . in succession, and a video signal occurring between horizontal sync. signals P.sub.H, as shown in FIG. 2, is sampled sequentially. Then, the sampled video signal, which is held at a suitable sampled value for a suitable time interval, is sequentially supplied to transistors connected to the column lines Y.sub.1, Y.sub.2, . . . to sequentially emit light at the intersections of the column lines Y.sub.1, Y.sub.2 . . . with the row line to which voltage is then applied, with the brightness at the successive intersections varying in accordance with the sequentially sampled levels of the video signal.

The brightness of the gas discharge panel can be controlled by varying either the amplitude or the duration of a current flowing in the display element. These two methods are respectively referred to as PAM for pulse amplitude modulation and PWM for pulse width modulation.

In the case of the PAM method, it is difficult to provide the column line driving transistors with sufficiently uniform characteristics to achieve uniform brightness control over a substantial range of variation of the sampled video signals so that good quality brightness control of the display panel by means of the PAM method cannot be realized.

In the PWM method, pulse signals with a constant amplitude and with the pulse widths being varied in response to the sampled values of the video signal are applied to the transistors connected to the column lines Y.sub.1, Y.sub.2 . . . to make them conductive in sequence for periods corresponding to the respective pulse widths. Thus, the duration of light emission is varied at the successive intersections to control the brightness. In practice, the level of the video signal is divided into, for example, sixteen steps or grades of 0 to 15, as shown in FIG. 3, and the pulse width is changed in a step-wise manner in accordance with the step or grade of the sampled value, as shown in FIG. 4.

Since the transistors associated with the column lines Y.sub.1, Y.sub.2 . . . are used as switching elements in the PWM method, uniform brightness control can be achieved at the respective column lines.

However, where gas discharge elements, liquid crystal elements electroluminescent elements and so on are employed as display cells of the display panel, there is a certain delay time between the application of driving voltage to the display cells and the onset of light emission from the display cells. By way of example, in the case of glow discharge of gas discharge elements, the delay time is about 10 micro seconds.

Accordingly, when the driving voltage with its pulse width being varied as described above is applied to the display cells, the time intervals during which the light emission is actually taken place for the several steps or levels 0-15 of the sampled video signal are indicated by cross-hatching on FIG. 4. When the step [1] is detected or sampled, the pulse width is 63.5/15 micro seconds, that is, approximately 4 micro seconds, where 63.5 micro seconds correspond to one horizontal or scanning period T.sub.H. Similarly, when the step [2] is detected or sampled, the pulse width is about 8 micro seconds, so that, in each of the above cases, the time delay exceeds the pulse width and no light emission takes place. When the sampled level of the video signal is step [8], the pulse width is about 32 micro seconds; whereas, when the sampled levels are [9] and [10], the corresponding pulses are constituted by a short initial pulse of about 4 micro seconds and 8 micro seconds duration, respectively, followed by a pulse of about 32 micro second duration. By reason of the above mentioned time delay, no light emission would actually occur during the initial short pulse for the sampled step [9] or [10], so that the durations of light emission would be the same for all of steps [8], [9] and [10]. Accordingly, if the level of the sampled video signal is divided by as many as sixteen steps, the PWM method cannot achieve accurate variation of the brightness level in response to variations in the level of the video signal. Therefore, in existing video display systems employing the PWM method for brightness control, it is the practice to divide the range of variation of the level of the video signal into a relatively low number of steps, for example, no more than eight steps, so that the shortest pulses for driving the column lines will have a width or duration exceeding the time delay of the display cells. However, in that case, the resulting displayed picture or image is of rather poor resolution or quality as the variation of brightness in the displayed picture only roughly correspond to the variations in the video signal.

In order to avoid the foregoing problems associated with the PAM and PWM methods of brightness control, the video display system according to the present invention generally comprises a row line driving circuit which drives the row lines X.sub.1, X.sub.2, X.sub.3 . . . sequentially in synchronism with the horizontal sync signals, a column line driving circuit which drives the column lines Y.sub.1, Y.sub.2, Y.sub.3 . . . sequentially during each horizontal period, and a pulse generating circuit providing pulses for the column line driving circuit which vary or are modulated both in amplitude and duration in response to variations in the sequentially sampled level of the input video signals. Since such pulses are varied both as to amplitude and duration or width, a relatively large number of steps in the sampled level of the video signals can be characterized by corresponding pulses which differ from each other by a relatively small number of substantially different amplitudes and by a relatively small number of substantially different durations or widths, whereby to avoid the above described problems associated with the PAM and PWM methods of brightness control.

An embodiment of the video display system according to this invention will be now described with reference to FIG. 5.

In FIG. 5, only a portion of the display flat panel 1 is shown to have a plurality of row lines X.sub.1, X.sub.2, . . . and a plurality of column lines Y.sub.1, Y.sub.2, . . . in the same manner as described in connection with FIG. 1. In the embodiment of FIG. 5, the light emission or radiation by gas discharge elements is employed for display, so that the row lines X.sub.1, X.sub.2, . . . are used as anodes and the column lines Y.sub.1, Y.sub.2, . . . are used as cathodes. A row line driving circuit 2 is shown to include transistors T.sub.1, T.sub.2, . . . having their collector electrodes connected to the row lines X.sub.1, X.sub.2, . . . respectively, while their emitter electrodes are connected together to a voltage source terminal 3 to which a positive voltage E.sub.A is applied. A column line driving circuit 4 is shown to include transistors Q.sub.1, Q.sub.2, . . . having their collector electrodes connected to the column lines Y.sub.1, Y.sub.2, . . . respectively, while their emitter electrodes are grounded through resistors R.sub.1, R.sub.2, . . . , respectively.

The video display system embodying this invention is further shown to include an antenna 5 for receiving broadcast television signals and a tuner 6 connected to the antenna 5 for selecting the channel to be viewed. The output of the tuner 6 is applied to an i.f. amplifier 7, which supplies amplified signals to a video detector 8.

The output terminal of the video detector 8 is connected to a video amplifier 9, and also to a sync. separator 10 which separates vertical and horizontal synchronizing signals from the video signals detected by the video detector 8. The output signal from the sync. separator 10 is applied to a start pulse generator 11 which, as shown on FIG. 6, produces a vertical start pulse S.sub.V for each vertical sync signal, with each pulse S.sub.V being delayed by a predetermined time interval from the respective vertical sync signal, and during which time interval the display is taken place for each field. Further, generator 11 produces a horizontal start pulse S.sub.H for each horizontal sync signal, with each pulse S.sub.H being delayed by a predetermined time interval from the horizontal sync signal, and during which time interval the display is taken place for the respective horizontal period. The vertical start pulse S.sub.V is applied to a shift register 12 as an input signal and the horizontal start pulse S.sub.H is also applied to the shift register 12 as a clock pulse train. At output terminals a.sub.1, a.sub.2, . . . of the shift register 12 there are obtained driving pulses which are sequentially delayed by one horizontal or scanning period in respect to each other (FIG. 6), and which have a pulse width corresponding to one horizontal period (1H) from the generation of the horizontal start pulse S.sub.H, as shown in FIG. 6. The driving pulses obtained at the output terminals a.sub.1, a.sub.2, . . . are applied to the base electrodes of the transistors T.sub.1, T.sub.2, . . . , respectively, of the row line driving circuit 2 in such a manner that the transistors T.sub.1, T.sub.2, . . . are made conductive one after another for one horizontal period (1H) and delayed by one horizontal period (1H) from the generation of the horizontal start pulse S.sub.H. Thus, the row lines X.sub.1, X.sub.2, . . . are successively supplied with the positive voltage E.sub.A from the terminal 3 through the transistors T.sub.1, T.sub.2, . . . , respectively. In other words, the shift register 12 serves as a pulse generator for driving the row lines X.sub.1, X.sub.2, . . . sequentially.

At the same time, the video signal from the video amplifier 9 is applied to a level detector 13 in which the level of the video signal is divided into a number, for example, sixteen (0 tp 15), of steps or grades, as shown in FIG. 3. The level detector 13 produces an output signal at a selected one of its output terminals t.sub.0, t.sub.1, . . . , t.sub.15 in accordance with the instantaneously detected level of the video signal. Each of the output signals delivered to the terminal t.sub.0, t.sub.1, . . . , t.sub.15 is applied to a 4-bit coder 14 which produces a 4-bit binary coded signal corresponding to the video signal level represented by the signal appearing at the respective one of the 16 terminals t.sub.o -- t.sub.15.

The 4-bit binary coded signals appear as various combinations of the potential levels [ ] and [1] at the output terminals t.sub.A,t.sub. B,t.sub. C and t.sub.D which represent the decimal values 1, 2, 4 and 8, respectively. The potential appearing at the output terminal t.sub.A is applied to a first group of pulse generators A.sub.1, A.sub.2, . . . ; the potential appearing at the terminal t.sub.B is applied to a second group of pulse generators B.sub.1, B.sub.2, . . . ; the potential appearing at the terminal t.sub.C is applied to a third group of pulse generator C.sub.1, C.sub.2, . . . ; and the potential appearing at the terminal t.sub.D is applied to a fourth group of pulse generators D.sub.1, D.sub.2, . . .

The horizontal start pulse S.sub.H from the start pulse generator 11 is also applied to a timing pulse generator 15 which produces a pulse S.sub.S with a predetermined period which is the same as the sampling time interval, as shown in FIG. 7.

The horizontal start pulse S.sub.H in FIG. 7 corresponds to that in FIG. 6, but is shown on an enlarged scale. The horizontal start pulse S.sub.H and the pulse S.sub.S from the timing pulse generator 15 are applied to a shift register 16 as an input signal and a clock pulse train, respectively. Thus, the shift register 16 produces at its output terminals b.sub.1, b.sub.2, . . . sampling gate signals which are delayed one after the other by the period of the pulse S.sub.S as shown in FIG. 7. Accordingly, shift register 16 can be considered a sampling gate signal generator.

The output signal obtained at the output terminal b.sub.1 is applied to the pulse generators A.sub.1, B.sub.1, C.sub.1 and D.sub.1 ; the output signal at the terminal b.sub.2 is applied to the pulse generators A.sub.2, B.sub.2, C.sub.2 and D.sub.2, and so on. In other words, the sampling gate signals obtained at each of the output terminals b.sub.1, b.sub.2, . . . sequentially are applied to the four pulse generators identified by the letters A,B,C and D to which the same numerical subscripts are appended.

Each of the first group of pulse generators A.sub.1,A.sub. 2, . . . , each of which may represent the decimal value 1, may be formed of, for example, a monostable multivibrator producing a current pulse P.sub.A with the pulse width 1/4 T.sub.H (T.sub.H =horizontal period) and a predetermined amplitude, as shown in FIG. 8, when the output of the 4-bit coder 14 at the terminal t.sub.A is the potential [1] and the sampling gate pulse of the shift register 16 is applied to such pulse generator A.sub.1, or A.sub.2, or A.sub.3 -- . Similarly, each of the second group of pulse generators B.sub.1, B.sub.2, . . . may be formed of, for example, a monostable multivibrator which produces a current pulse P.sub.B with the pulse width 1/2 T.sub.H and the same amplitude as the pulse P.sub.A, as shown in FIG. 8, when the output signal of the 4-bit coder 14 provides the potential [1] at the terminal t.sub.B and the sampling gate pulse of the shift register 16 is applied to the pulse generator B.sub.1 or B.sub.2 or B.sub.3 -- . Each of the third group of pulse generators C.sub.1, C.sub.2, . . . may be formed of, for example, a bistable multivibrator which produces a current pulse P.sub.C with the pulse width T.sub.H and the same amplitude as the pulse P.sub.A as shown in FIG. 8, when the output signal of the 4-bit coder 14 provides the potential [1] at the terminal t.sub.C and the sampling gate pulse of the shift register 16 is applied to the pulse generator C.sub.1 or C.sub.2 or C.sub.3 -- . Finally, each of the fourth group of pulse generators D.sub.1, D.sub.2, . . . may be formed of, for example, a bistable multivibrator which produces a current pulse P.sub.D with the pulse width of T.sub.H and an amplitude twice that of any of the pulses P.sub.A, P.sub.B and P.sub.C, as shown in FIG. 8, when the output signal of the 4-bit coder 14 provides the potential [1] at the terminal t.sub.D and the sampling gate pulse of the shift register 16 is applied to the pulse generator D.sub.1, D.sub.2 or D.sub. -- . It will be apparent that the current pulses P.sub.A, P.sub.B, P.sub.C and P.sub.D are basic pulses and are weighted by their respective widths and amplitudes, to represent the decimal values of 1, 2, 4 and 8, respectively.

As shown on FIG. 5, the output terminals of pulse generators A.sub.1,B.sub. 1,C.sub.1 and D.sub.1 are connected together through suitable resistors to the base electrode of transistor Q.sub.1 ; the output terminals of pulse generators A.sub.2,B.sub. 2,C.sub. 2 and D.sub.2 are connected together through suitable resistors to the base electrode of transistor Q.sub.1 ; and so on. Therefore, in response to the sampling gate pulse appearing at any one of the output terminals b.sub.1,b.sub. 2 -- of shift register 16, for example, at the terminal b.sub.1, one or more of the basic current pulses P.sub.A,P.sub. B,P.sub.C and P.sub.D may be produced by the respective pulse generators A.sub.1, B.sub.1, C.sub.1 and D.sub.1 in dependence on the 4-bit binary coded signal which issues from coder 14 in response to the level of the video signal then being detected in level detector 13. Since any two or more of such basic current pulses P.sub. A, P.sub.B, P.sub.C and P.sub.D overlap in time, as shown on FIG. 8, any basic current pulses produced by generators A.sub.1, B.sub.1, C.sub.1 and D.sub.1 upon the appearance of a sampling gate pulse at terminal b.sub.1 are simply combined into a respective driving current pulse of characteristic width or duration and amplitude which is supplied to the base electrode of the respective transistor Q.sub.1.

As shown on FIG. 9, the simple combination of the four basic current pulses P.sub.A -P.sub.D makes it possible to obtain sixteen different types of driving current pulses which respectively represent the detected levels 0 to 15, inclusive, of the input video signals, and any one of those sixteen different driving current pulses, which are distinguished from each other by their widths and/or by their amplitudes, is supplied to the base electrodes of the transistors Q1, Q2-- in synchronism with the sampling gate pulses sequentially provided at the outut terminals b.sub.1, b.sub.2,-- of shift register 16.

The operation of the system shown in FIG. 5 will be now described in detail. When the transistor T.sub.1 is made conductive during the initial horizontal period of each field of the video signal and the positive voltage E.sub.A is applied to the row line X.sub.1. With the sampling gate pulse being first obtained at the output terminal b.sub.1 and assuming that, at this time, the level of the video signal is, for example, [1] at the black level side as shown in FIG. 3, the output at the output terminal t.sub.1 of the level detector 13 becomes [1] and only the output terminal t.sub.A of the 4-bit coder 14 exhibits the logic [1]. Thus, only the pulse generator A.sub.1 is operated to produce the pulse P.sub.A which is shown in FIG. 9 to represent the decimal value [1] and which is applied to the base electrode of the transistor Q.sub.1 as the driving pulse. Accordingly, the transistor Q.sub.1 is made conductive in such a manner that the impedance between the collector-emitter of the transistor Q.sub.1 is controlled in accordance with the width and amplitude of the pulse P.sub.A, and discharge occurs between the row line X.sub.1 and the column line Y.sub.1 to emit light at the intersection of the lines X.sub.1 and Y.sub.1. In this case, the brightness of the emitted light, as determined by the amplitude and width of the pulse P.sub.A, corresponds to step [1] of the video signal level. Thereafter, when the sampling gate pulse is obtained at the next output terminal b.sub.2 and assuming that the detected level of the video signal is then, for example, at the step [3] shown in FIG. 3, the output at the terminal t.sub.3 of the level detector 13 becomes [1] and, accordingly the outputs at the terminals t.sub.A and t.sub.B of the 4-bit coder 14 become [1]. As a result, the pulses P.sub.A and P.sub.B are obtained from the pulse generators A.sub.2 and B.sub.2, respectively, and hence the pulse which results from combining the pulses P.sub.A and P.sub.B with each other as, shown in FIG. 9 at step [3], is applied to the transistor Q.sub.2 as the driving pulse. Thus, a discharge occurs between the row X.sub.1 and the column line Y.sub.2 to emit light at the intersection thereof. In this case, since the duration of the driving pulse is 1/2 T.sub.H, that is, twice the duration of the driving pulse applied at the intersection of the lines X.sub.1 and Y.sub.1, and since the amplitude of the driving pulse in the first half of its duration is twice that in the second half of the pulse duration, which latter amplitude is equal to the amplitude of the driving pulse applied at the intersection of lines X.sub.1 and Y.sub.1, it will be apparent that the brightness of the light emitted at the intersection of lines X.sub.1 and Y.sub.2 will be three times the brightness of the light emitted at the intersection of lines X.sub.1 and Y.sub.1, that is, the light emitted at the intersection of lines X.sub.1 and Y.sub.2 will have a brightness corresponding to the [3] level of the detected video signal. Similarly, light emission is made to occur sequentially at the intersections of the row lines X.sub.1, X.sub.2, . . . and the column lines Y.sub.1, Y.sub.2, . . . and the brightness of the emitted lights is varied in sixteen steps in accordance with the detected levels of the video signal, whereby to display an accurate image of the video signal.

With the video display system according to the present invention as described above, by changing the combination of the width and amplitude of the driving pulse, the suitable step of the brightness is selected, so that even if the video signal level is divided into sixteen steps or grades as shown in the figures, it is sufficient to change the amplitude of the driving pulse over only five steps or grades. Accordingly, even if the transistors Q.sub.1, Q.sub.2, . . . are ununiform in h.sub.FE, the brightness control can be performed uniformly at the respective column lines due to small number of steps. Further, even when the video signal level is divided into, for example, 16 steps, it is sufficient to change the driving pulse width in only three steps. Therefore, the smallest pulse width can be made to be 1/4 T.sub.H, that is, about 16 micro seconds, as shown in FIG. 9, so that, when the start of light emission is delayed by about 10 micro seconds, as shown by the dotted lines in FIG. 9, the variation of brightness can be obtained accurately in correspondence with the change of the driving pulse.

The above described video display system has employed a flat panel in which discharge light emission is used, but it will be apparent that the video display system of the present invention can be applied to flat display panels in which light emission is achieved by liquid crystals, electroluminescence and the like.

Having described an illustrative embodiment of the invention, it will be apparent that many modifications and variations could be effected therein by one skilled in that art without departing from the spirit and scope of the present invention as defined in the appended claims.