Note: Descriptions are shown in the official language in which they were submitted.
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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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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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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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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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; 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,
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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
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The same happens for the other
colours. The patto~n obtained last remains visible on the
picture screen but with a markedly reduced average bright-
ness.
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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can be dispensed with or be used for, for example, to
connect an other control element t such as a touch contact,
to an additional input 158.
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
35 OTC
,
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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