GB1585063A

GB1585063A - Display systems

Info

Publication number: GB1585063A
Application number: GB2438076A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1977-06-13
Filing date: 1977-06-13
Publication date: 1981-02-25

Description

(54) IMPROVEMENTS RELATING TO DISPLAY SYSTEMS (71) We FERRANTI LDW1TED, a Company registered under the Laws of Great Britain, of Hollinwood in the County of Lancaster, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to display systems and in particular to circuits for automatically adjusting brightness and contrast of a cathode ray tube display.

Contrast and brightness of a cathode ray tube display are varied according to the ratio between maximum and minimum values of amplitude respectively of a controlling video signal. It is usual to adjust video input amplifiers of the display system so that minimum or black level of the video signal corresponds to cut-off for the cathode ray tube, i.e. the point at which electrons emitted by the gun just fail to reach the tube screen, and so that maximum peak white levels of the video signals corresponds to saturation of the tube phosphor. Generally alteration of the gain of the amplifiers alters the ratio between maximum and minimum values of the video signal, that is, contrast and alteration of the offset of the amplifier alters the position of the video signal with respect to zero light output, that is, brightness.

After this adjustment, contrast and brightness often vary because of thermal variations in the tube and its associated circuits.

It is known to provide automatic adjustment of cutoff with respect to black level of the video signal to compensate for the above variations. Such adjustment affects the brightness mainly and it is assumed that contrast variations will be slight and will be compensated for by the eye of the observer.

However, in some applications the contrast setting is critical and even slight variations cannot be tolerated. For example, if the display is to be photographed, a high speed film is used i.e. one which is very sensitive. The contrast range for correct exposure of the film is much less than that for direct viewing, and small variations can result in substantial loss of detail.

It is an object of the present invention to provide a circuit and method for automatically adjusting brightness and contrast which mitigates the above problem.

According to one aspect of the present invention a circuit for controlling the brightness and contrast of the display presented on a cathode ray tube includes sources of first and second video test signals, variable gain, variable offset means operable to apply the test signals and a display signal to the cathode ray tube, light sensitive means operable to deliver first and second detected signals indicative of the light output of the cathode ray tube display resulting from the application of the first and second test signals respectively, first control means responsive to the first detected signal and to a reference voltage to develop a first control signal for application to vary the offset of the variable gain, variable offset means, second control means responsive to the second detected signal and to a second reference voltage to develop a second control signal for application to vary the gain of the variable gain, variable offset means, and switching means operable to apply the first test signal to the variable gain, variable offset means and the first detected signal to the first control means during a first time interval, and to apply the second test signal to the variable gain, variable offset means and the second detected signal to the second control means during a second, subsequent, time interval.

Each control means may include means for storing the signals from the light sensitive means in the intervals between periodic applications of the respective test signals. In the intervening period the display signal is applied to the variable gain, variable offset means.

According to another aspect of the present invention a method of controlling the brightness and contrast of the display presented on a cathode ray tube comprises feeding video signals to the cathode ray tube by way of a variable gain, variable offset means, detecting light emitted by the cathode ray tube due to the application in a first time interval of a first video test signal having an amplitude being a first fraction of the signal causing maximum light output, producing a first detected signal of magnitude related to the levels of light detected and comparing it with a first reference signal, being said first fraction of the detected signal caused by maximum light output, and varying the offsets of the variable gain variable offset means in accordance with the difference between the signals, detecting light emitted by the cathode ray tube due to the application in a second time interval of a second video test signal having an amplitude being a second fraction of the signal causing maximum light output, producing a second detected signal of magnitude related to the level of light detected and comparing it with a second reference signal being said second fraction the detected signal caused by maximum light out-put, and varying the gain of the variable gain variable offset means in accordance with the time integral of the difference between the signals for the duration of the second time interval.

The second reference voltage may be derived from the signal representative of actual light output of the cathode ray tube when the light output is equal to the level determined by the first control signal.

Each adjustment is preferably carried out immediately prior to the tube being scanned, or at the beginning of each scan. The offset adjustment may be carried out during consecutive scans or frames, and the gain adjustment periodically after a predetermined number of frames.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a diagram representing a cathode ray tube display and a circuit for automatically adjusting the brightness and contrast of the display, and Figure 2 shows a graph of light output of the cathode ray tube against video signal amplitude.

In Figure 1, a cathode ray tube 10 has its display controlled by the output of variable gain, variable offset means 11, which includes a variable offset amplifier 12 and a variable gain amplifier 13. In-put to the means 11 is by way of switching means 14, which comprises switches 15, 16 and 17 connected to corresponding terminals 18, 19 and 20 to which are supplied a normal video signal, a first video test signal and a second video test signal respectively.

Light output from the cathode ray tube is detected by a photocell 21, secured to the screen 22 of the tube.

The photocell output is amplified by amplifier 23 to provide on line 24 a detected signal representative of light output from the cathode ray tube. This signal is applied to either first control means 25 by way of switch 26, or second control means 27 by way of switch 28.

First control means 25 includes a first storage capacitor 29 connected from switch 26 to ground. The junction between capacitor 29 and switch 26 is connected to the inverting input 30 of an amplifier 31 by way of buffer 56 and resistor 32.

The inverting input of the amplifier is connected also by resistor 33, equal in value to resistor 32, to a first reference signal voltage line 34 and by resistor 35 to the output terminal 36 of amplifier 31. The non-inverting input of amplifier 31 is connected to ground. The output terminal 36 of the amplifier is connected by way of resistor 37 and line 38 to the variable offset terminal 39 of amplifier 12.

The second control means 27 includes a second storage capacitor 41 connected between switch 28 and ground. The junction 42 between switch 28 and capacitor 41 is connected to the input terminal 43 of a buffer amplifier 44. Output terminal 45 of the buffer amplifier is connected by way of resistor 46 to one side of a switch 47. The other side of switch 47 connects to input terminal 48 of an integrator 49 comprising an amplifier 50 which has a capacitor 51 connected between its input terminal 48 and its output terminal 52. This output terminal 52 is connected by line 53 to variable gain terminal 54 of variable gain amplifier 13.

Means for deriving a second reference signal voltage includes an inverter 55 comprising amplifier 61, its input terminal 60 connected to the output of buffer 56 by way of resistor 59.

A feedback resistor 62 connects the input terminal 60 to output terminal 63 of the amplifier 61, this output terminal 63 being connected by resistor 64 to that side of switch 47 to which resistor 46 is connected.

Operation of the circuit to adjust brightness and contrast of the cathode ray tube display involves two test signals being applied successively to the display system in the absence of the normal video signal. First and second test signals have different amplitudes which may be proportions of the maximum value of the normal video signal. For example normal video signal maximum may be 2 volts and the first and second test signals square pulses of 0.75 volts and 0.5 volts respectively. The normal video signal modulates the intensity of the cathode ray tube display during the usual scanning of the display, which is a rotary scan in the particular embodiment but could be a raster scan. During dead time between scans, switches 16 and 26 are closed for the duration of a first time interval. The 0.75 volts test signal is applied by way of variable gain, variable offset means 11 to the cathode ray tube and the corresponding light output detected by photocell 21 which receives light from a pattern of radial lines generated by the test signal. A signal representative of the light output produced by the 0.75 volt test signal is then applied to capacitor 29 which is charged to a corresponding voltage.

This voltage is compared with the first reference voltage on line 34, which is predetermined by the amount of light output desired for 0.75 volts of video input signal, that is, the same fraction of the maximum detected signal as the test signal is of the maximum video signal.

If the capacitor voltage is greater than the reference voltage, the feedback signal applied to the variable offset terminal 39 lowers the bias or offset of the amplifier 12 to reduce the light output of the cathode ray tube, and if the capacitor voltage is less than the reference voltage the converse applies and the light output is increased.

Hence at the end of the first time interval, the light output of the cathode ray tube is equal to that desired of a 0.75 volt video signal and the voltage stored on capacity 29 is equal and opposite to the reference voltage on line 34.

During the succeeding time interval, switch 16 and 26 are opened and switches 17, 28 and 47 are closed. The second video test signal, 0.5 volts in amplitude, is then applied to the variable gain, variable offset means 11 and the corresponding light output of the cathode ray tube detected by photocell 21. A signal representative of the light output corresponding to the 0.5 volt test signal is supplied by way of switch 28 to second control means 27. Capacitor 41 charges to a voltage corresponding to the signal representative of the 0.5 volt test. This voltage is applied to resistor 46 by way of buffer amplifier 44.

The voltage stored on capacitor 29 at the end of the first time interval is applied by way of buffer amplifier 56 to inverting means 55 comprising amplifier 61, and the inverting means output is applied to resistor 64. The ratio of the values of resistors 64 and 46 is the same as the ratio of the test signal amplitudes i.e. 0.75/0.5. When the voltage on capacitor 41 corresponds to the correct light output for a 0.5 video input signal, then the current through resistor 64 is equal and opposite to the current through resistor 46, and the output of the integrator remains unchanged. If the light output is incorrect, and the resistor currents are not equal and opposite, current flows into the integrator and the voltage on line 53 changes.

Change of voltage on line 53 causes a change of gain of variable gain amplifier 13 the sense of the change being such that the light output of the cathode ray tube tends towards the value desired for a 0.5 volt video input signal. At the end of the succeeding time interval the light output of the cathode ray tube corresponds to that desired for a 0.5 volt video signal and capacitor 41 stores a voltage representative of that light output.

The reference voltage on line 34 and the values of resistors 64 and 46 are so chosen that the light output for 0.75/0.5 volt video signals are at points on a curve of desired light output against video input signal amplitude, B and A respectively as shown in Figure 2. Adjustment of the offset of the variable gain, variable offset means alters the position of the line until the line intersects the point B. Subsequent adjustment of the gain of the variable gain, variable offset means alters the slope of the line relative to the X axis until the line intersects the point A.

Over the small range concerned, about 2 volts if fast film is to be used, the response of the cathode ray tube is substantially linear and the line joining A and B passes through the origin and through Z, the maximum output/ maximum signal point as shown. This curve represents the desired setting of brightness and contrast but it will be appreciated that selection of point A & B permits different setting of brightness and contrast to be achieved.

Adjustment of the variable gain amplifier 13 may affect the previously adjusted brightness or 0.75 volt light output level, but each time the 0.75 volt adjustment is performed, the correct level is reestablished. After several periodic adjustments both light output levels will be stabilized at their desired values.

In the preferred embodiment, both the 0.75 and the 0.5 volt test signals. are applied in the same interscan period, immediately prior to, and synchronised with, the opening of a camera shutter. However, application of the first test signal and corresponding adjustment of offset, may be carried out during every interscan period and application of the second signal and gain adjustment carried out more infrequently such as periodically after a predetermined number of scans, say during every tenth interscan period.

In the embodiment described above the second reference voltage is derived from the first reference voltage. An advantage of this arrangement is that the desired brightness and contrast levels may be adjusted simultaneously by altering the first reference voltage or the value of resistor 33. It will be appreciated though that an independent reference voltage, of suitable value, could be selected during the successive time interval.

The means for detecting light output of the cathode ray tube is described above as a photocell secured to the screen of the cathode ray tube. In an alternative arrangement, the cathode ray tube display is reflected off a semi-silvered mirror to a camera and the photocell is positioned behind the semi-silvered mirror. In this arrangement the photocell can view a much greater area of the cathode ray tube screen and so average variations in the light output across the tube face. In a further modified embodiment a lens between the mirror and photocell concentrates the light from the mirror onto the active area of the photocell.

Where the cathode ray tube display is of the raster kind, the brightness and contrast adjustment is conveniently carried out at the start of each frame period. The photocell may be positioned to receive light output from one or more of the first few lines of the frame, the first and second test signals being applied immediately prior to the picture information.

Although the invention is especially applicable where the cathode ray tube is to be photographed and the maximum light output is limited by the dynamic range of the film, it is not limited to such applications. Suitable selection of reference voltage values enable automatic adjustment of brightness and contrast over a wider dynamic range as required for direct viewing.

The choice of 0.75 and 0.5 volts in the described embodiment is purely for convenience.

The only limitation to the values chosen is that they lie on the curve and provide a light output when supplied.

WHAT WE CLAIM IS: 1. A circuit for controlling the brightness and contrast of the display presented on a cathode ray tube including sources of first and second video test signals, variable gain, variable offset means operable to apply the test signals and a display signal to the cathode ray tube, light sensitive means operable to deliver first and second detected signals indicative of the light output of the cathode ray tube display resulting from the application of the first and second test signals respectively, first control means responsive to the first detected signal and to a reference voltage to develop a first control signal for application to vary the offset of the variable gain, variable offset means, second control means responsive to the second detected signal and to a second reference voltage to develop a second control signal for application to vary the gain of the variable gain, variable offset means, and switching means operable to apply the first test signal to the variable gain, variable offset means and the first detected signal to the first control means during a first time interval and to apply the second test signal to the variable gain, variable offset means and the second detected signal to the second control means during a second, subsequent, time interval.

2. A circuit as claimed in claim 1 in which the variable gain, variable offset means comprises a variable gain amplifier the gain of which is variable in response to the first control signal and to which the video signals are applied, and a variable offset amplifier arranged to receive signals from the variable gain amplifier and operable to offset the signals with respect to a datum level in response to the second control signal.

3. A circuit as claimed in claim 1 in which the first and second control means each include means for storing the first and second detected signals in the intervals between periodic applications of the respective test signals.

4. A circuit as claim in claim 1 or claim 2 in which the first control means includes means for storing the first detected signal, comprising a first storage capacitor arranged to be charged by the first detected signal by way of the switching means, a buffer to provide a signal voltage proportional to that stored on the capacitor, amplification means arranged to receive both the stored voltage and the reference voltage and operable to produce the first control signal related to the difference in magnitude between the said voltages.

5. A circuit as claimed in claim 4 in which the second control means includes means for storing the second detected signals, comprising a second storage capacitor arranged to be charged by the second detected signal by way of the switching means, a buffer to provide a signal voltage proportional to that stored on the capacitor, integration means arranged to have applied thereto the signal voltage and the second reference voltage and to provide as the second control signal the integral of the difference between said applied voltages for the duration of the interval that the second test signal is applied to be variable gain, variable offset means.

6. A circuit as claimed in any one of the preceding claims in which the second reference voltage is derived from the first detected signal by an amplifier having a gain equal to the ratio of the second to first test signal voltages.

7. A circuit as claimed in any one of the preceding claims in which the light sensitive means comprises a photodetector attached to the screen of the cathode ray tube to receive light from a portion of the scanned display area.

8. A circuit for controlling the brightness and contrast of the display presented on a cathode ray tube substantially as herein described with reference to, and as shown by, the accompanying drawings.

9. A method of controlling the brightness and contrast of the display presented on a cathode ray tube comprising feeding video signals to the cathode ray tube by way of variable gain, variable offset means, detecting light emitted by the cathode ray tube due to the application in a first time interval of a first video test signal having an amplitude being a first fraction of the signal causing maximum light output, producing a first detected signal of magnitude related to the level of light detected and comparing it with a first reference signal, being said first fraction of the detected signal caused by maximum light output, and varying the offset of the variable gain variable offset means in accordance with the difference between the signals, detecting light emitted by the cathode ray tube due to the application in a second time interval of a second video test signal having an amplitude being a second fraction of the signal causing maximum light output, producing a second detected signal of magnitude related to the level of light detected and comparing it with a second reference signal, being said second fraction of the detected signal caused by maximum light output, and varying the gain of the variable gain variable offset means in accordance with the time integral of the difference between the signals for the duration of the second time interval.

10. A method as claimed in claim 9 in which the first and second detected signals are stored in the intervals between periodic applications of the respective test signals.

11. A method as claimed in claim 9 or claim

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. adjustment of brightness and contrast over a wider dynamic range as required for direct viewing. The choice of 0.75 and 0.5 volts in the described embodiment is purely for convenience. The only limitation to the values chosen is that they lie on the curve and provide a light output when supplied. WHAT WE CLAIM IS:

1. A circuit for controlling the brightness and contrast of the display presented on a cathode ray tube including sources of first and second video test signals, variable gain, variable offset means operable to apply the test signals and a display signal to the cathode ray tube, light sensitive means operable to deliver first and second detected signals indicative of the light output of the cathode ray tube display resulting from the application of the first and second test signals respectively, first control means responsive to the first detected signal and to a reference voltage to develop a first control signal for application to vary the offset of the variable gain, variable offset means, second control means responsive to the second detected signal and to a second reference voltage to develop a second control signal for application to vary the gain of the variable gain, variable offset means, and switching means operable to apply the first test signal to the variable gain, variable offset means and the first detected signal to the first control means during a first time interval and to apply the second test signal to the variable gain, variable offset means and the second detected signal to the second control means during a second, subsequent, time interval.

11. A method as claimed in claim 9 or claim

10 in which the second reference signal is derived from the first detected signal stored during the first time interval as a fraction of the first stored signal equal to the ratio of the amplitude of the second and first video test signals.

12. A method as claimed in any one of claims 9 to 11 in which the first video test signal is applied at the beginning of each frame of a scanned display and the second video test signal is applied periodically after a pre-determined number of frames.

13. A method of controlling the brightness and contrast of the display presented on a cathode ray tube substantially as herein described with reference to the accompanying drawings.