CA1114535A

CA1114535A - Video circuit with screen-burn-in protection

Info

Publication number: CA1114535A
Application number: CA313,084A
Authority: CA
Inventors: Jean-Pierre Asmus; Joseph Soto; Sebastianus J. Op Het Veld; Henri W. Schneider
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-10-11
Filing date: 1978-10-11
Publication date: 1981-12-15
Also published as: DE2844112A1; GB2007471B; GB2007471A; NL7810170A; JPS5946471B2; JPS5488722A; IE782030L; IE47294B1; DE2844112C2

Abstract

ABSTRACT:
To prevent display screen burn if a picture having, for example, bright lines or other picture components remains stationary for a long period of time, a video circuit is examined to detect how lone the information has not been changed. If a predetermined time limit is exceeded the picture bright-ness is limited to a sufficiently low average value, if so desired to zero brightness, by means of one or more protection switches.
A number of embodiments are described.

Description

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Video circuit with screen-burn-in protection.
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The invention relates to a video circuit for generating video signals for dis~laying pictures on a picture screen, -the video circuit comprisi~g ~ a Picture signal source, at least one luminance circuit, ;~ 5 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-luminance signal and/or 10 R-G- and B- luminance signals for the pictorial display of information from an in~ormation processing device, inclu-ding Teletext and Viewdata systems as well as video-games.
The luminance signals can be applied to corresponding inputs of a display device or can 15 be modulated on a carrier and applied 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 period of time, so that bright portions of the 20 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 protec-r.
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. tion circuit which tries to obviate this harmfull effect.
.~ A video circuit of the type defined in the preamble is known from Canadian Patent . Application No. 267,742. In. this video circuit the protec-. . 5 tion circuit comprises a temperature-dependent element which 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 that in rooms having a thermostatically controlled 10 heating system a temperature is adj~ted 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 15 same pogition on the picture screen for the remainder of - .:.r the night.
. It is an object of the inven-tion to pro.vide a video circuit of the type mentioned in the preamble which prevents burning-in of the picture screen in 20. all circumstances.
According to the invention a video circuit is therefore characterized in that the protec-tion circuit comprises an electric protection switch for suppressing in a switched-off state completely or partly 25 the luminance si~nal generated in the video circuit, a de-tection circuit for detecting changes in a electric signal of the picture signal source, and a timer circuit, a detec-tion output of the detection circuit being coupled to a starting input of the timer circuit a time signal output 30 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 surprising~
lr 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 of the . ' . .
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~ 3 ~1$ - -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 com- -pletely obviated. 1 Several embodiments, as des-cribed in the Claims 2 to 13, inclusive, which can also be used in all sorts of combinations are possible for the protection switch as well as for the detection circuit and the timer circuit. These embodiments will be further ex-lO plained by way of non-limitative examples with reference to the Figures and the description of these Figures In thedrawing:
Figure 1 shows a simplified block diagram of a video circuit having a protection switch for suppressing a luminance signal, Figure 2 shows a time diagra~
of the most important electric signals of the protection circuit;
which is on the same sheet as Fig. 1, Figure 3Jshows a simplified 20 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-whi$e display device;
Figure 5 shows a time diagram for a video circuit as shown in Figure 4;
Figure 6 shows a detailed cir-cuit of a further embodiment with suppression of the lumi-nance 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 embo-diment of a video circuit according to the invention;
Figure 9 shows a wiring diagram for a detection circuit having timing circuits;
Figures 10 to 13, inclusive show four embodiments of video circuits comprising a .
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~r ~ ` - -protection circuit, for use in colour display devices;
, which is on the ~me sheet as Figure 1~,/snows a time ~la- Fl 9 . 2, gram associated with the embodiment shown in Figure 13;
~ igure 15 shows a diagram of - 5 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 con-trol devioe 1 comprises one or more control elements which arehnown 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 buildingup the complete picture information for the chosen purpose.
The picture information controls at least one luminance circuit 3, which is coupled to tne picture signal source 2, for applying a luminance signal to the displa~- device 4.
20- For the display of colour pic-tures three luminance circuits 3 are generally used for the so-called ~ G- and B- luminance signals.
The coupling between the control device 1 and the picture signal source 2 may, if so desired~ be effected ~ means of a remote control channel with signal transfer by means of, for e~ample, infrared radiation.
At least one signal of the r picture signal source 2 is applied to a detection circuit 5 having a detection output 6 for an electric detection signal. This detection signal is pulse-shaped and is produced after a change in the signal 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 therefrom by means of processing in the picture signal source 2. It is, of --- course, possi~le to apply two or more signals to the de-.. ....... .. . . . . . . . : . . ..

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tection circuit and to generate a detection signal as soon as at least one of these signals evidences a change.
Examples thereof are given in Fig. 4 and subsequent ~igures. The output 6 of the detection circuit 5 is coup-5 led to an input 7 of a timer circuit 8 having a timesignal output 9. The timer circuit 8 is a circuit bloc~
which is known ~_ se, for example a monovibrator circuit or a digital counting circuit. The timer circuit 8 is adjusted to a starbng position, the "O"-position, by a detection circuit at its input 7. As long as no new de-tection signal is generated the timer circuit starts operating because~ for example, a capacitor voltage in-creases monotonously as a function of the time and causes the monostable multivibrator to change state a~ter a period of time determined by the circuit blocl~s, 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 20 however, a new detection signal appears before the prede-termin~d period of time has elapsed, the timer circuit is again adjusted to the "O"-position. The time constant of the timer circuit i9 chosen considerably longer than the period of time which occurs between information changes during normal use of the system, for example one minute or longer. In the situation that no change is produced in the picture information for a longer period than the predetermined period of time the signa] OTC will bs generated.
The output 9 of the timer circuit 8 is coupled to an input 10 of a protection circuit 11, in this example it is directly coupled to the lum~nance circuit 3. The protection circuit 11 is arrangGd so that it completely or partly suppresses theluminance signal as soon as the timer circ;ait 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 .-,.

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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 inten- -5 sity on a fixed portion ofthe screen. Just because of the -*
fact that other portions ofthe 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 i~ustrates the procedure. Herein the line 20 shows the pulses of the detection signal and the line 21 shows a characteristic magnitude of the timer circuit, linearly increasing, by way of example, from "O" to a maximum value "max". If a detection signal is produced at the instant tm the timer circuit is adjusted to the "O" position and this is repeated at the instant tm+1 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 this position until a new detection signal arrives at, for ex-~ample, the instant ti. The line 22 shows the output signalOTC 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 pro-¦25 tection 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 tn and ti is completely or largly suppressed.
Figure 3 shows a further embodiment. In this Figure and subsequent Figures corres-ponding elements are always given the same reference numerals.
The elements 1, 2, 3, 4, 5 and 8 have the same function as 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 .. ... . . , , . . ~

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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 transis-tor 13 conducts and the video circuit is supplied with the supply voltage. If the signal OTC becomes "OFF" at, for example, the instant ti, 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. -In the case of a battery supply this is useful to prolong the life of the battery. A further advantage occurs when a 15 television receiver is used as the display device ll, if the luminance signal, modulated on a carrier, is applied to an aerial input 14 of the receiver. When the supply voltage disappears the carrier also disappears in response whereto 'he majority of receivers start hissing and thus 20 furnish an auditive warning for the user. A further possi-bility is to adjust in receivers having a so-called "stand-by" switch, this switch to the "stand-by" position as soon as the supply voltage dis~ppears.
Figure 3 shows the circuit of 25 the transistor 13 schematically only. The design of a sui-table circuit is outside the scope of the invention and must be evident to the average person skilled in the art.
If in addition to the timer r circuit 8 also further components, such as the picture 30 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 b~ means of a switch 15, which is here assumed to be included in the control device 1. The ~5 switch 15 may, for example, be implemented as a push-button on as a second contact provided on a control ele--- ment which is already present for other reasons.
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:~ ~ Figure 4 shows schematically a video circuit 20 comprising a picture signal source and a . luminance circuit in the form of a block 21, a synchroni-zation signal circuit 22 an.d a mixing circuit 23 having an 5 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 t~ corres-10 ponding inputs 34, 35 of an OR-gate 36, an output 37 of which is coupled to an input 38 of a timer circuit 8 having .. a time signal output 9. The luminance signal at the output 4O of the luminance circuit 21 is applied to the mixer . circuit 23 through an AND-gate 42 for the time the signal 15 OTC at the output.9 `of the timer circuit is "ON". In the case a detection signal does not immediate~ result in a ! luminance signal ~ter a re-start owing to the flyback t.ime : of a monostable multivibrator having a long time constant, the detection signal is applied, if so desired, to the AND-20 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
25 becomes "OFF"-Figures Sa to 5e inclusiveshow a simplified time diagram for the synchronizationsignal (5a), the picture information signal (5b~ which can be modulated together as the video signal (5e) on a carrier

3~by the modulation circuit 23.
Figure 5c shows the end of a detection signal at an instant t1 and Figure 5d shows the : variation of the signal OTC whi.ch becomes "OFF" after a long period of time has elapsed. This is shown in a com-35pressed form, actually there are many synchronization pulses between t1 and t2~ The information signal is applied to the -- modulator 23 until the instant t2, from this instant t2 ,~, ,.
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1~ 53 -~'--'- --- --- -- ' outwards the AND-gate 42 blocks the information signal so that only the synchronization signal is applied to the mo-dulator 23 and the picture on the display screen goes da-rk.
It may be desirable that the p~ture is not completely suppressed butthat it is limited to a picture having a harmless luminance level. It appeared that no noticeable burn-in phenomena occur when the lumi-nence level is limited to approximately 50 % of the maximum luminance level.
Figure 6 shows an embodiment in which the luminance is limited. An e~ample of a circuit 23 having resistors 50, 51, 52 is symbo~cally shown, whereas the carrier modulation is not shown for the sake of clarity.
The modulator 23 functions in a manner which is known se.
~ In this embodiment the AND~
- ~ gate 42 is replaced by a protection circuit ~ consisting of a series arrangement of a resistor 54, an adjustable resis-tor 55 and a diode 56, an anode of which is colmected 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 ~upply 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 resis-tor 54.
As soon as OTC becomes "OFF", the diode 56 is conductive and the luminance signal is re-duced by the additional load formed by the series arrange-ment 55, 56. Alternatively, it is possible to combine the resistors- 54, 55 to one adjustable resistance potentiometer.
In Figure !~ and Figure 6 an output 60 of the video circuit is connected to the output of the OR-gate 43. In ~he case the video circuit can be in-- - cluded entirely in a television receiver the output signal ,, ' ~
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. r, 10 ; of the OR-gate 43 can be used to switch the receiver over to television reception. If the OR-gate 43 is omitte,d the output 60 can be coupled to the output 9 of the timer circuit 8.
: J 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 two 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, 15 whereas their outputs 78 and 79, respectively, are connec-ted to the inputs 35 and 34, respectively, of the OR-gate 37.
Figure 9 shows an embodiment of the detection circuits 74, 75. Each of these detection cir-20 cuits is provided at the input side (76 and 77, respective-ly), with an impedance matching circuit 121 a, b, followed by two pairs of comparator circuit6 122a 123a and 122b,123b the four outputs of ~ch are connected to four corresponding inputs of OR-gate 36.
Of each pair of two comparator circuits the inputs of opposite polarities are mutually interconnected, one of these connections 128a, being coup-led to the output of the impedance matching circuit 121a, b through a RC circuit 124a, b and 125a, b and the other 30 connection 129a, b through an additional RC circuit 125a, b and 127a, b9 the total time delay between 121 and 129 consequentl~ being greater than the total time dela~
between 121 and 129 consequently being greater th~n the total time delay between 121 and 128.
As soon as the input si~nal at, for exa~ple, the input 76 of detection circuit 74 ~Taries, ----- either the comparator 122a or 123a will effect an output ,:
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signal which induces the "OFF"-condition and the output 37 of the R-gate 36 will temporarily become "ON".
In the absence of an operation ¦ all inputs of the comparators will assume an equal voltage 5 level and the output voltage of the OR--gate becomes equal to . lo~
If the time constant of the RC circuits 126, 127 is chosen sufficiently high, that is to say in the order of magnitude of one minute or longer 10 they perform also the function of the timer circuit 8 so that ihe output 37 of the OR-gate 36 can be connected directly to the OR-gate 43 or to the AND-gate 42.
The Figures 10, 11 and 12 show implementations for the display of colour pictures but 15 otherwise they fully correspond to circuits shown in Figure 4, Figure 8 and Figure 6, respectively.
In these Figures 1 0 , 11 and 12 three protection switches 42a, b, c or circuits 53a, b,c are included between the picture signal source and 20 luminance circuit 200, having three outputs for R, G and B B, and a modulation circuit ~e4 in accordance with, for example, the PAL system or the SECAM system, this modula-tion circuit furthermore being coupled-to a synchroniza-tion signal circuit 300. An output 100 of the modulation circuit 400 can be coupled to an aerial input of a televi-sion 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 o~
yideo recorders.
A very simple solution in the case of colour display is shown ln Figure 13, in which the protection circuit is implemented with three EXCLUSIVE-OR gates 110, 1 l1 and 112 having outputs 113, 35 114 and 115, respectively, which produce the R, & and B
signals and a NOR-gate 116. For a simple video circuit the colour information is given with three one-bit signals SR, .......

' ~5 ~45~i 12 ~ ~-SG and SB so that eight colours, including black and white, are formed with the various "O" and "1" combina-tions. These one-bit signals are applied to first inputs 120, 121 and 122, respectively, of the EXCLUSIVE-OR gates 110, 1i1 and 112 respectively, second inputs 130, 131 and 132, respectively, of which are jointly connected to the output 133 of the NOR-gate 1 16 . For the period of time this output is "O", 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 "OF~", 116 follows the clock signal CLI~ and in response thereto be-comes alternatirly "ON" and "OFF" in the rhythm of CLK:
Each time the output 133 is "ON", the combination R, G and B becomss precisely the inverso of the combination SR, SG
and SB, respectively, 000 is replaced by 111, 011 by 100 ` etc., so that now ~ternately the colours determined by SR, SG and SB or a colour which is complementary thereto is 20 passed on through R, G and B. If one or more of the colour signals is "1", the result becomes alternately "O" and "1"
so that each colour signal has an average luminance equal to 50 % of the maximum luminance.
For, for exainple, R this is illu-strated i~ the time diagram of Fig. 14 having the same timeaxis as that of Fig. 2.
Until the instant tn, remains identical to a signal SR, which, for clarity, is shown as a multi-bit signal, of which all bits of all three colour luminance signals can be switched-over when OTC = "O".
After tn, or already before that instant, SR does not change anymore and is maintained at a, in this example, high arnplitude a at the appearance of OTC = "O". All bits are periodically and simultaneously inverted in the rhytllm of CL~ so that R periodically assumes the value "rnax" -a), average ~"ma~", which follows from , ! a + ('Imæx~ - a) ~ 2 .... . .

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The clock frequency can be chosen at will within very widè limits. Examples are the use of a secondssignal of a clock present in the device, a 5O Hz clocking voltage derived from the mains frequency etc.
Figure 15 shows a very simple and inexpensive but neverthel~;s effective detection circuit for use with a television game. To detect the change in the position of an adjustable resistance potentiometer whose whiper 151 is coupled to an input 152 of the picture signal source the whiper is also connected to a RC diffe-rentiating network 153, 154. The differentiated signal is applied to a pulse-shaper 155, for example an OR-gate, an AND-gate with interconnected AND-inputs or an inverting circult. When the whiper moves this pulse-shaper produces in the two first-mentioned pulse-shaper examples an "1"-pulse if the voltage taken from the resistance potantiome-ter increases and, in the example of an inverting pulse-shaper, if this voltage deoreases. If` the video circuit is intended for a tela~ision game for two players this detec-tion pulse can be combined with the detection pulse derived from a resistance potentiometer intended for the opponent, by combining the two 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 two players one player will operate his own potentiometer several times per minute in alterna-ting directions the second potentiometer generally need not of necessity be provided with a detection circuit. -In that case the OP~-gate 156 .
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Finally, Figure 16 shows a S simple timer circuit formed by means of a digital coun-ting 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 10 connected to a similar cloc~ signal as mentioned for Figure 13, for example 1 Hz.
The resetting input 163 is connected to the detection output 6 of the detection circuit 5 so that the counting circuit is adjusted to the / 15 "O"-position by each detection signal, whereafter it v starts adding in the clock rhythm. If no new 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 hlghest 20 position,given by an "OFF"-going carry signal at the counter output 164. For a 1 Hz clock and a 6-bit counter this position is obtained after something more than 1 min~ute, if no subsequent detection signals DET appear.
To prevent the counter from 25 continuing to count after "111111" at CLK to "OQOOOO", the output 164 is connected to the count enable input 162 so that the counter stops as soon as CA, and conse-~uently CE, becomes "OFF". As soon as signal DET is produced again the counter is, however, reset. Several 30 known cligital counting circuits can be used as the coun-ting circuit such as, for example, a Signetics type 54161, or two or more cascade counting circuits of this type if more than 4 bits are required. l`he output signal CA at the counter output 164 is at the same time the time signal , If the transistor 13 of Figure 3 is coupled by means of its emi tter side to the power :
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supply 12, the basc of this transistor can be driven by an i~erter circuit consisting of a switching transistor 165 having collector resistor 166 an 0nd 167 whereof is con-nected to the base of the transistor 13. As soon as OTC

5 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 inventive idea and that they are not limited to the form chosen, numerous variation~ are possible whilst maintaining the inventive idea. Several combinations of various detec~
tinn circuits, timer circuits and protection swltches are also possible.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PRO-PERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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 lumi-nance signal, and a protection circuit for protecting the picture screen from burning-in due to a prolonged stationary picture, characterized in that the protection circuit com-prises an electric protection switch, which in its switched-off state, at least partly, suppresses the luminance signal generated in the video circuit, a detection circuit for detecting changes in an electric control signal applied to the picture signal source or in a second electric signal derived from said electric control signal and a timer cir-cuit, a detection output of the detection circuit being coupled to a starting input of the timer circuit a time signal output of which 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 control signal change for a period of time deter-mined by the timer circuit.

2. A video circuit as claimed in Claim 1, charac-terzied 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, charac-terized in that a supply input of the timer circuit is con-nected to the supply input of the luminance circuit and in that the video circuit comprises a restarting circuit for switching the protection switch on again.

4. A video circuit as claimed in Claim 1, charac-terized in that the protection switch is arranged in series with the luminance circuit for partly or completely suppres-sing the luminance signal in the switched-off state.

5. A video circuit as claimed in Claim 1, for dis-playing 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 switched-on state of the protection switch pass the corresponding luminance signals on essentially unchanged and in a switched-off state of the protection switch period-ically switch the corresponding luminance signal from ampli-tudes 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 Claim 5, comprising digital R-, G- and B-luminance circuits for generating lumin-ance signals having at least one one-bit output per colour, characterized 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 switched-off state.

8. A video circuit as claimed in Claim 1, 5 or 7, 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, charac-terized in that 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 the aerial change-over switch to the second switching state.

9. A video circuit as claimed in Claim 1, charac-terized in that 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 cir-cuit 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, characterized 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 Claim 1, 5 or 9, com-prising at least one adjustable resistance potentiometer for generating an adjustable analog control signal at a potentio-meter output, characterized in that the potentiometer is coupled capacitively to a signal input of the detection cir-cuit.

12. A video circuit as claimed in Claim 1, 5 or 9, characterized in that the timer circuit is in the form of a restartable monostable multivibrator circuit.

13. A video circuit as claimed in Claim 1, 5 or 9, characterized in that the timer circuit is in the form of a digital counting circuit provided with a resetting input which constitutes the start input of the timer circuit and with a final position output which constitutes the time signal output of the timer circuit.