IE47294B1 - Video circuit with screen-burn-in protection - Google Patents

Abstract

To prevent display screen burn if a picture having, for example, bright lines e.g. in a T.V. game, remains stationary for a long period of time, a video circuit is examined to detect for how long the information has not been changed. A timer 8, reset whenever a change in any of one or more luminance circuits 3 is detected by detector 5, progresses towards completion of its run. If the run reaches completion a signal 9 causes, via circuit 11, the display 11 brightness to be reduced, e.g. to zero, possibly by switching off the supply to part of or all the apparatus. A restart circuit may be provided.

Description

The invention relates to a video circuit for generating video signals for displaying pictures on a picture screen, the video circuit comprising a picture signal source, at least one luminance circuit, coupled thereto, for generating a luminance signal, and a protection circuit for protecting the picture screen from burning-in due to a prolonged stationary picture.

Video signals of this type are used for generating at least a Y-1uminance and/or R-, G- and B- luminance signals for the pic10 torial display of information from an information processing device, including Teletext and Viewdata systems as well as videogames .

The luminance signals can be applied to corresponding inputs of a display device or can be modulated on a carrier and anplied to an aerial input of a television receiving device.

It frequently happens that a certain picture remains stationary on the picture screen for a long peris·1 of time, so that bright portions of the picture may cause an accelerated local burning-in of the picture screen. Particularly, it often happens that users forget to switch off a television game so that, for example, a line pattern of a playing-field remains on the screen for a whole night. The video circuit is provided with a protection circuit which tries to obviate this harmful effect.

A video circuit of the type defined in the preamble is known from Canadian Application No 257 742. In this video circuit the protection circuit comprises a temperature-dependent element which _ 2 _ effects that the picture to be displayed is slowly moved over a short distance across the picture screen owing to variations in the ambient temperature. However, it appeared tnat in rooms having a thermostatically controlled heating system a temperature is adjusted at the beginning of a night which remains very constant thereafter, which is partly caused by the fact that, for example, doors and windows remain closed for a long period of time so that the picture can yet remain stationary hours on end in the same position on the picture screen for the remainder of the night. it is an object of the invention to provide a circuit of tne type mentioned in the preamble which can prevent burning-in of tne picture screen.

According to the invention, a video circuit is therefore characterized in that the protection circuit comprises an electric protection switch for suppressing in a switched-off state completely or partly the luminance signal generated in the video circuit, a detection circuit for detecting changes in an electric signal of the picdetection output of the ture signal source, ar.d a timer circuit, a/detection circuit being coupled to a starting input of the timer circuit a time signal output whereof is coupled to a control input of the protection switch in such a manner that this switch is adjusted to the switched-off state when the detection circuit has not detected a signal change for a period of time determined by the timer circuit.

This achieves in a surprisingly simple and inexpensive manner that if the display device is not supplied with new information and is not switched off, the picture luminance for all picture elements - 3 4729« of the picture screen after the period of time determined by the timer circuit is reduced to zero or to such a low average value that burning in is completely or substantially completely obviated.

Several embodiments can be used in all sorts of combinations for the protection switch as well as for the detection circuit and the timer circuit. These embodiments wil l be further explained by way of non-limitative examples with reference to the drawings and the description of these drawings.

In the drawings:Figure 1 shows a simplified block diagram of a video circuit having a protection switch for suppressing a luminance; Figure 2 shows a time diagram of the most important electric signals of the protection circuit; Figure 3 shows a simplified block diagram of a video circuit having a protection switch for switching-off a supply voltage; Figure 4 shows a block diagram for suppressing the luminance signal for a black-white display device; Figure 5 shows a time diagram for a video circuit as shown in Figure 4; Figure 5 shows a detailed circuit of a further embodiment with suppression of the luminance signal for a black/white display device; Figure 7 shows, schematically, an example of the use of a touch contact in a video circuit as shown in Figure 4 or Figure 6; Figure 8 shows a further embodiment of a video circuit according to the invention; Figure 9 shows a circuit diagram for a detection circuit having timing circuits; Figures iO to 13 inclusive show four embodiments of video circuits comprising a protection circuit, for use in colour display devices; Figure 14 shows a time diagram associated with the embodiment shown in Figure 13; Figure 15 shows a diagram of a simple detection circuit, and Figure 16 shows a simplified block diagram of a timer circuit implemented with a digital counting circuit.

In Figure 1 a control device 1 is coupled to a picture signal source 2. Usually the control device 1 comprises one or more control elements which are known per se, such as push-buttons, touch controls, switches or rotary potentiometers, these potentiometers being some times operated by means of a so-called joy-stick. The picture signal source 2 serves for building up the complete picture information controls at least one luminance circuit 3, which is coupled to the picture signal source 2, for applying a luminance signal to the display device 4.

For the display of colour pictures three luminance circuits 3 are generally used for the so-called R-, G- and B- luminance signals.

The coupling between the control device 1 and the picture signal source 2 may, if so desired, be effected by means of a remote control channel with signal transfer by means cf, for example, infrared radiation.

At least one signal of the picture signal source 2 is applied to a detection circuit 5 having a detection output for an electric - 5 detection signal. This detection signal is pulse-shaped and is produced after a change in the signal produced derived from the picture signal source 2, which signal can be derived from a control signal applied to the picture signal source 2 as well as from a new signal produced thereforro by means of processing in the picture source 2. It is, of course, possible to apply two or more signals to the detection circuit and to generate a detection signal as soon as at least one of these signals evidences a change. Examples thereof are given in Figure 4 and subsequent Figures. The output 6 of the detection circuit 5 is coupled to an input 7 or a timer circuit 8 having a time signal output 9.

The timer circuit 8 is a circuit block which is known per se, for example a monovibrator circuit or a digital counting circuit.

The timer circuit 8 is adjusted to a starting position, the 0 -position, by a detection circuit at its input 7. As long as no new detection signal is generated the timer circuit starts operating because, for example, a capacitor voltage increases monotonously as a function of the time and causes the monostable multivibrator to change stage after a period of time determined by the circuit blocks, or the counting position increases under the control of a clock signal up to a predetermined maximum counting position. At the end of the predetermined period of time the timer circuit produces an output signal OTC at its output 9. If, however, a net·/ detection signal appears before the predetermined period of time has elapsed, the timer constant is again adjusted to the 0-position. The timer constant of the timer circuit is chosen considerably onger than the period of time which occurs between information changes during normal use of the system, for example one minute or longer. In this situation that no change - 6 47294 is produced in the picture information for a longer period than the predetermined period of time the signal OTC will be generated.

The output 9 of the timer circuit 8 is coupled in an input 10 of a protection circuit 11, in the example it is directly coupled to the luminance circuit 3. The protection circuit 11 is arranged so that it comnletely or partly suppresses the luminance signal as soon as the timer circuit has attained its maximum position and remains in this position until a restart command is given. In this manner it is prevented that a picture screen in the display device 4 burns-in because a stationary-picture, for example bright reference lines, originating from a calculator, teletext system, television game, etc. is displayed for a long period of time with a high intensity on a fixed portion of the screen. Just because of the fact that other portions of the screen are not burned-in, even rather small differences in intensity can visually be very annoying during subsequent use.

The time diagram of Figure 2 illustrates of the procedure. Herein the line 20 shew the pulses of the detection signal and the line 21 shows the characteristic magnitude of the timer circuit, linearly increasing by way of example from 0 tc a maximum value max. If a detection signal is produced at the instant the timer circuit is adjusted to the 0 position and this is repeated at the instantira + l etc., the max-position not having been attained. Only when nothing has happened for a long period of time the timer circuit arrives in the position max at tn and remains in the position until a new detection signal arrives at, for example, the instants^. The line 22 shows the output signal OTC of the timer circuit, indicated here arbitrarily as ON or 1” in the normal situation and OFF in the position max. The output signal OTC adjusts the protection circuit to the OFF position, as shown by curve 23, so that finally the luminance signal Y for black-white display, indicated by the curve 24, between * and is completely or largely suppressed.

Figure 3 shows a further embodiment. In this Figure and subsequent Figures corresponding elements are always given the same reference numerals.

The elements 1, 2, 3, 4, 5 and 8 have the same function in Figure 1.

The Figure now also includes a power supply 12, for example a battery which applies a supply voltage to the other portions of the video circuit through a normally conducting transistor 13, the base of which constitutes the Input 10 of the protection circuit.

As long as the signal OTC at the output 9 of the timer circuit 8 is ON the transistor 13 conducts and the video circuit is supplied with the supply voltage. If the signal OTC becomes OFF at, for example, the instant transistor 13 cuts off the supply voltage so that, obviously OTC will now remain in the OFF position. Now the timer circuit 8 need not have a holding function. Although it is sufficient for this purpose to block only the supply voltage of the timer circuit 8 it may be desirable to block the entire supply of the video circuit, e.g. some or all of the elements 1, Z, 3 and 5.

In the case of a battery supply this is useful to prolong the life of the battery. A further advantage occurs when a television receiver is used as the display device 4, if the luminance signal, modu30 lated on a carrier, is applied to an aerial input 14 of the receiver. - 8 47294 When the supply voltage disappears from the video circuit the carrier also disappears in response whereto the majority of receivers start hissing and thus furnish an auditive warning for the user. A further possibility is to adjust in receivers having a so-called stand-by switch, this switch to the standby position as soon as the supply voltage disappears.

Figure 3 shows the circuit of the transistor (3 schematically only. The design of a suitable circuit is outside the scope of the invention ano must be evident to the average person skilled in the art.

If in addition to the timer circuit 3 also further components, such as the picture signal source 2 or the detection circuit 5 are switched-off, a new control signal will not automatically effect a restart. The transistor can be briefly adjusted to the conducting state by means of a switch 15, which is here assumed to be included in the control device 1. The switch 15 may, for example, be implemented as a push-button or as a second contact provided on a control element which is already present for other reasons.

Figure 4 shows schematically a Yideo circuit in the form of a block 21, a synchronization signal circuit 22 and a mixing circuit 23 having an output 24 for a video signal modulated on a carrier. In addition to the video circuit two control dividers 30, 31 are coupled to control inputs of the picture signal source 2 and also to two detection circuits 32, 33 for detecting a control operation. Outputs thereof are coupled to corresponding inputs 34, 35 or an OR-gate 35, an output 37 which is coupled to - 9 gS 4 an input 38 of a timer circuit 8 having a time signal output 9.

The luminance signal at the output 40 of the luminance circuit 21 is applied to the mixer circuit through an AND-gate 42 for the time signal OTC at the output 9 of the timer circuit is ON. In the case a detection signal does not immediately result in a luninance signal after a restart owing to the flyback time of a monostable multivibrator having a long time constant, the detection signal is applied, if so desired, to the AND-gate 42 through an OR-gate 43. However, the OR-gate 43 is not essential, it is alternatively possible to connect the output 9 of the timer circuit 8 directly to an input of an AND-gate 42. It will be obvious that the AND-gate 42 cuts off the luminance signal as soon as the time signal OTC becomes “OFF.

Figures 5a to 5e inclusive show a simplified time diagram for the synchronization signal (5a), the picture information signal (5b) which can be modulated together as the video signal (5e) on a carrier by the modulation circuit 23.

Figure 5c shows the end of a detect'o^ signal at an instant ¢, and Figure 5d shows the variation of the signal OTC which becomes OFF” after a long period of time has elapsed. This is shown in a compressed form, actually there are many synchronization pulses between f, and ta. The information signal is applied to the modulator 23 until the instant from this instant onwards the AND-gate 42 blocks the information signal so that only the synchronization signal is applied to the modulator 23 and the picture on the display screen goes dark.

It may be desirable that the picture is not completely suppressed but that it is limited to a picture having a harmless luminance - 10 47294 level. It appeared that no noticeable burn-in phenomena occur when the luminance level is limited to approximately 50% of the maximum luminance level.

. Figure 6 shows an embodiment in which the luminance is limited.

An example of a circuit 23 having resistors 50, 51, 52 is symbollically shown, whereas the carrier modulation is not shown for the sake of clarity. The modulator 23 functions in a manner which is known per se.

In this embodiment the AND-gate 42 is replaced by a protection circuit 53 consisting of a series arrangement of a resistor 54, an adjustable resistor 55 and a diode 56, an anode of which is connected to the adjustable resistor 55 and a cathode to the output of the OR-gate 43 or directly to the output 9 of the timer circuit 8.

Taking the use of a positive supply voltage in the video circuit as the starting point, the cathode of the diode is normally at a positive voltage if OTC is ON, diode 56 then being cut-off. The luminance signal is passed on to the modulator 23 through the resistor 54.

As soon as OTC becomes “OFF, the diode 56 is conductive and the luminance signal is reduced by the additional load formed by the series arrangement 55,56. Alternatively, it is possible to combine the resistors 54,55 as one adjustable resistance potentiometer.

In Figure 4 and Figure 6 an output 60 of the video circuit is connected to the output of the OR-gate 43. In the case the video - 11 4729 £ circuit can be included entirely in a television receiver the output signal of the OR-gate 43 can be used to switch the receiver over to television reception. If the OR-gate 43 is omitted the output 60 can be coupled to the output 9 of the timer circuit 8.

. Figure 7 shows schematically that a detection circuit 71 for a control device 30 may be in the form of a touch contact whose change in voltage, produced when the control device is touched, can be applied to the timer circuit.

Figure 8 shows a slightly different arrangement of the elements. In the Figure too detection circuits 74,75 are now included in the video circuit, their inputs 76 and 77, respectively, being coupled to the outputs of the corresponding control device 30,31, whereas their outputs 78 and 79, respectively, are connected to the inputs 35 and 43, respectively, of the OR-gate 37.

Figure 9 shows an embodiment of the detection circuits 74,75. Each of these detection circuits is provided at the input side (76 and 77, respectively), with an impedance matching circuit 121a,b, followed by too pairs of comparator circuits 122a, 123a and 122 b the four outputs of which are connected to four corresponding inputs of ORgate 36.

Of each pair of too comparator circuits the inputs of opposite polarities are mutually interconnected, one of these connections 128a,b being coupled to the output of the impedance matching circuit 121a,b through a RC circuit 124a,b and 125a,b and the other connection 129a,b through an additional RC circuit 126a,b and 127a,b, the total time delay between 121 and 129 consequently being greater than - 12 47294 the total time delay between 121 and 129 consequently being greater than the total time delay between 121 and 128.

As soon as the input signal at, for example, the input of 76 of detection circuit /4 varies, either the comparator 122a or 123a will effect an output signal which induces the 0FF-condition and the output 3/ of the OR-gate 36 will temporarily become 'ON”.

In the absence of an operation all inputs of the comparators will assume an equal voltage level and the output voltage of the OR-gate becomes equal to 0.

If the time constant of the RC circuits 126,127 is chosen sufficiently nigh, that is to say in the order of magnitude of one minute or longer they perform also the function of the timer circuits 8 so that the output 37 of the OR-gate 36 can be directly to the OR-gate or to the AND-gate 42.

The Figures 10,11 and 12 show implementations for the display of colour pictures but otherwise they fully correspond to circuits shown in Figure 4, Figure 8 and Figure 6, respecti vely.

In these Figures 10, 11 and 12 three switches 42a, b, c or circuits 53a, b, or c are included between the picture signal source and luminance circuit 200, having three outputs for R, G and B, and a modulation circuit 400 in accordance with, for example, the PAL system or the SECAM system, this modulation circuit furthermore being coupled to a synchronization signal - 13 4 7 S 8 4 circuit 300, An output 100 of the modulation circuit 400 can be coupled to an aerial input of a television receiver, it is obvious that the modulation circuit 400 can be omitted if the television receiver comprises separate video inputs R, G, B and SYNC, as is usually the case at present, inter alia to enable the connection of video recorders.

A very simple solution in the case of colour display is shown in Figure 13, in which the protection circuit is implemented with three EXCLUSIVE-OR gates 110,111 and 112 having outputs 113, 114 and 115, respectively, which produce the R, G and B signals and a NOR-gate 116. For a simple video circuit the colour information is given with three one-bit signals SR, SG and SB so that eight colours, including black and white, are formed with the various 0 and 1 combinations. These onebit signals are applied to first inputs 120. 121 and 122, respectively, of the EXCLUSIVE-OR gates 110, 111 and 112 respectively, second inputs 130, 131 and 132, respectively, of which are jointly connected to the output 133 of the NOR-gate 116.

For the period of time this output is “G, R, G and B correspond to SR, SG and SB, respectively. The signal OTC and a clock signal are applied to the inputs of the NOR-gate 116.

For the time OTC is ON during normal use the output 133 of the NOR-gate 116 is “OFF. If, however, OTC becomes “OFF, 116 follows the clock signal CLK and in response thereto becomes alternately ON and OFF in the rhythm of CLK: Each time the output 133 is ON, the combination R, G and B becomes precisely the inverse of the combination SR, SG and SB, respectively, 000 is replaced by 111,011 by 100 etc, so that now alternately the colours determined by SR, SG and SB or a colour which is comple- 14 47294 mentary thereto is passed on through R, G and B, if one or more of the colour signals is 1, the result becomes alternately 0“ and 1 so that each colour signal has an average luminance equal to 50% of the maximum luminance.

For, for example, R this is illustrated in the diagram of Figure 14 having the same time axis as that of Figure 2.

Until the instant t , the signal k remains identical to a signal SR of Figure 13 whien, for clarity, is shown as a multi-level signal, of which all bits of all three colour luminance signals can be switched-over when OTC - 0. After tn, SR does not change anymore and is maintained at some amplitude a when OTC = 0, for each position on the display screen. When OTC = 0, however, the value of R will alternate between a and max-a), and the average brightness for this position will be a + (max a) = max The same happens for the other colours. Although a may be different for different positions on the display screen, this does not affect the a'erage brightness, which will be max R + “max C + max B for all positions.

In the simple case of Figure 13, where R, G and B have only levels 1 (max) or 0, R will alternate between SR and SR', i.e. between 1 and 0“ irrespective of SR being I or o during OTC - 0. Again the average will be 'max + 0 = max' -Z~ ~~T~ - 15 47 2S 4 The clock frequency can be chosen at will v/i thin very wide limits, hxamples are the use of a seconds signals of a clock present in the device, a 50 Hz clocking voltage derived from the mains frequency etc.

Figure 15 shows a very simple and inexpensive but nevertheless effective detection circuit for use with a television game. To detect the change in the position of an adjustable resistance potentiometer whose wiper 151 is coupled to an input 125 of the picture signal source 2 the wiper is also connected to a RC differentiating network 153,154. The differentiated signal is applied to a pulse-shaper 155, for example an OR-gate, an ANDgate vrith interconnected ANU-inputs or an inverting circuit.

When the wiper moves this pulse-shaper produces in the two first-mentioned pulse-shaper examples a 1 pulse if the voltage taken from the resistance potentiometer increases, and, in the example of an inverting pulse-shaper, if this voltage decreases. If the video circuit is intended for a television game for too players this detection puise can be combined with the detection pulse derived from a resistance potentiometer intended for the opponent, by combining the too signals with an OR-gate 156 to form the detection signal DET which is applied to the timer circuit 8 via the output 157 of this gate.

As some games are played by one player only, as, for example, in a cardgame, the potentiometer then used must in any case be connected. As also in the case of too players one player will operate his own potentiometer several times per minute in alternating directions the second potentiometer generally need not of necessity be provided with a detection circuit. - 16 47284 In that case, the OR-gate 156 can be dispensed with or be used for, for example, to connect another control element, such as a touch contact, to an additional input 158.

Finally, Figure 16 shows a simple timer circuit formed by means of a digital counting circuit 160 having a counting input 161, a count enable input 162, a resetting input 163 and a counting output 164.

The counting input 161 can be connected to a similar clock signal as mentioned for Figure 13, for example 1Hz.

The resetting input 163 is connected to the detection circuit 5 so that the counting circuit is adjusted to the 0 position by each detection signal, whereafter it starts adding in the clock rhythm. If no detection signals are produced, the counting position finally arrives at its maximum position, given by a decoding of the counter bits. This may, for example, be the highest counter position given by an OFF-going carry signal at the counter output 164. For a 1Hz clock and a 6-bit counter this position is obtained after something more than 1 minute, if no subsequent detection signals GET appear.

To prevent the counter from continuing to counter after 111111 at CLK to 000000, the output 164 is connected to the count enable input 162 so that the counter stops as soon as CA, and consequently CE, becomes OFF. As soon as signal DET is produced again the counter is, however, reset. Several known digital counting circuits can be used as the counting circuit such as, for example, a Signetics type 54161, or two or more cascade counting - 17 A72S 4 circuits of this type if more than 4 bits are required. The output signal CA at the counter output 164 is at the same time the time signal OTC.

If the transistor 13 of Figure 3 is coupled by means of its emitter side to the power supply 12, the base of this transistor can be driven by an inverter circuit consisting of a switching transistor 165 having collector resistor 166 an end 167 whereof is connected to the base of the transistor 13. As soon as OTC is OFF, the transistor 13 does not receive any base current and is cut-off. Normally, however, OTC is ON, transistor 165 conducts and transistor 13 receives a base current which is largely determined by the resistance value of the resistor. in this example a restart is possible by shunting the transistor 165 by a re-starting contact 168.

It will be obvious that the above examples are only given as an illustration of the scope of the invention as defined in the claims and that they are not limited to the form chosen, numerous variations are possible whilst still falling within the scope of the invention. Several combinations of various detection circuits, timer circuits and protection switches are also possible.

Claims (10)

1. A video circuit for generating video signals for displaying pictures on a picture screen, the video circuit comprising a picture signal source, at least one luminance circuit coupled thereto for generating a luminance signal, and a protection circuit for protecting the picture screen from burningin due to a prolonged stationary picture, characterized in that the protection circuit comprises an electric protection switch for suppressing in a switched-off state completely or partly the luminance signal generated in the video circuit, a detection circuit for detecting changes in an electric signal of the picture signal source, and a timer circuit, a detection output of the detection circuit being coupled to a starting input of the timer circuit a time signal output whereof is coupled to a control input of the protection switch, in such manner that this switch is adjusted to the switched-off state when the detection circuit has not detected a signal change for a period of time determined by the timer circuit.

2. A video circuit as claimed in Claim 1, characterized in that the protection switch is included in a supply line between an electric power supply section and a supply input of the luminance circuit.

3. A video circuit as claimed in Claim 2, characterized in that a supply of the timer circuit is connected to the supply input of the luminance circuit and in that the video circuit - 19 comprises a restarting circuit for switching the protection switch on again.

4. A video circuit as claimed in Class 1, wherein the protection switch is arranged in series with the luminance circuit for partly or completely suppressing the luminance signal in the switched-off state. 5. Input which constitutes the start input of the timer circuit and with a final position output which constitutes the time output signal/of the timer circuit. 14. A video circuit substantially as herein described with reference to the accompanying drawings.

5. A video circuit as claimed in Class 1, for displaying colour pictures with R-, G-and B- luminance signals of corresponding luminance circuits, characterized in that the protection switch comprises R-, G- and B- sub-switches which, in a switchedon state of the protection switch pass the corresponding luminance signals on essentially unchanged and in a switched-off state of the protection switch periodically switch the corresponding luminance signal from amplitudes given by the luminance circuits to amplitudes which are complementary thereto, a complementary amplitude being equal to the difference between the maximum and the given amplitude.

6. A video circuit as claimed in Class 5, comprising digital R-, G- and B- luminance circuits for generating luminance signals having at least one one-bit output per colour, characterised in that the protection switch periodically inverts the bit outputs in the switched off state.

7. A video circuit as claimed in Claim 1, comprising a carrier oscillator and a modulation circuit for applying a video signal, modulated on a carrier to an aerial input of a television receiver, characterized in that the protection switch is arranged in series with the carrier oscillator for suppressing the carrier in the - 20 47294 switehed-off state.

8. A video circuit as claimed in any of the preceding Claims, comprising a carrier oscillator, a modulation circuit, an auxiliary aerial input and an aerial change-over switch, for applying a video signal, modulated on a carrier to an aerial input of a television receiver, the aerial change-over switch coupling in a first switching state the aerial input to an output of the modulation circuit and in a second switching state to the auxiliary aerial input, wherein the aerial change-over switch is coupled to the protection switch which in a switched-on state adjusts the aerial change-over switch to the first switching state and in a switched-off state adjusts tne aerial change-over switch to the second switching state. y.

9. A video circuit as claimed in any of the preceding Claims, wherein the picture signal source has at least one control input for an electric control signal, this control input being coupled to a signal input of the detection circuit for detecting changes in the control signal. 10. A video circuit as claimed in Claim 9, comprising at least one touch contact for generating an electric control signal, characterised in that a control signal output of the touch contact is coupled to the signal input of the detection circuit. 11. A video circuit as claimed in any of the preceding Claims, comprising at least one adjustable resistance potentiometer for generating an adjustable analog control signal at a potentiometer output, characterised in that the notentiometer is coupled capacitively to a signal input of the detection circuit. 12. A video circuit as claimed in any of the preceding Claims, wherein the timer circuit is in the form of a - 21 4728« restartable rnonostabtemultivibrator circuit. 13. A video circuit as claimed in any of the Claims 1 to Tl inclusive, wherein the timer circuit is in the form of a digital counting circuit provided with a resetting

10. 15. A television game comprising a video circuit as claimed in any of the preceding Claims.