POWER SUPPLY CIRCUIT

Abstract: A power supply circuit for applying a rapidly varying high voltage to a penetron-type cathode ray tube (1) for a colour display system comprises a first voltage generator (2) for supplying a constant high voltage to the anode (3) and a second voltage generator (4) for supplying a voltage, varying in magnitude with the different colours to the cathode (5). The latter voltage, varying in magnitude with the different colours, is modulated with the video signals to be displayed.

Description: Power supply circuit. The invention relates to a power supply circuit for applying a rapidly varying high voltage to a penetron-type cathode ray tube for a colour display system. To enhance the legibility of a display, a multicolour tube is employed. In order that a high positioning accuracy be obtained, while considering the relatively short working distance with such a display system, it is undesirable to use a shadow- mask tube for this purpose; a penetron tube gives considerably better results. The penetron tube has two different phosphor layers. With a certain voltage applied to the tube, the electrons excite mainly the first phosphor layer, while with a certain higher voltage applied to the tube the electrons are sufficiently accelerated to penetrate the barrier layer between the two phosphor layers and excite the second phosphor layer. The two voltages applied to the tube light up the screen in a certain colour; with intermediate voltages mixed colours are obtainable. In practice, four - colour displays are in common use, for instance as described in U.S. patents 3,780,339 and ,099,088. With the four-colour display described in U.S. patent 3,780,339 the anode of the tube is supplied with a voltage switchable to two values and the cathode with a d.c. voltage which is modulated with a voltage switchable to two values. In this way, using two switches, the four voltage levels can be obtained to display the particular colours. With the four-colour display described in U.S. patent 4,099,088, a "split anode" penetron tube is employed, where one anode is supplied with a constant voltage and the other with a voltage switchable to four values. Conventional colour display systems, using a penetron- type cathode ray tube, show the remarkable phenomenon that, with sufficiently rapid switching of the colours, sound waves are propagated from the front of the cathode ray tube. These sound waves are of such an intensity that they cause noise pollution in the usual working area of the display. The present invention has for its object to provide a power supply circuit as set forth in the opening paragraph, whereby such a noise pollution is avoided. It should be borne in mind that, for the application here intended, noise pollution is said to exist already at about 50 dBA. According to the invention, the power supply circuit comprises a first voltage generator for supplying a constant high voltage to the anode and a second voltage generator for producing a voltage varying in magnitude with the different colours, which voltage after being modulated with the video signals to be displayed is applied to the cathode to provide, together with the anode voltage, for the display of the video signals in the particular colours. The fact that the choice of the voltage to be applied to a certain electrode is determinative of the presence or absence of noise pollution is explained as follows. The front of the penetron tube is protected by an implosion screen. This screen is cemented with a silicon rubber on the actual picture screen. At the vacuum side of the picture screen the phosphor layer is provided with a conductive coating, which serves as an electrode (anode). The penetron tube is mounted in a metal frame. This frame and, in so far the implosion screen is also provided with a conductive coating, this coating function as an electrode as well; this electrode is however connected to earth. Consequently, the combination of picture screen and implosion screen may be regarded as a capacitor of which the plates are connected together not by a rigid medium, but by a resilient medium (rubber). On supplying the anode with a high voltage alternating at a certain frequency, the alternating force acting on the implosion screen will cause this screen, as one rigid mass, to perform a harmonic motion. The noise produced is attributed to this isophase motion of the implosion screen. A considerable reduction of the noise can be obtained by keeping the anode voltage constant. Further advantages of the invention are the strongly reduced energy loss on the tube capacity and the greater durabili¬ ty of the cathode ray tube, owing to the absence of the anode currents which are caused by the switching and which flow in the cathode ray tube from and to the particular terminal via the con¬ ductive coating at the inside of the front. The invention will now be described with reference to the accompanying figure which shows a block diagram of the power supply circuit. In this figure the numeral 1 represents a penetron- type cathode ray tube for a colour display system. A first voltage generator 2 supplies anode 3 of this cathode ray tube with a constant high voltage. Furthermore a second voltage generator 4 produces a voltage varying in magnitude with the different colours, which voltage after being modulated with, the video signals to be displayed is applied to cathode 5 of the cathode ray tube. The voltages to be applied to grids 6 and 7 must be defined, as will appear later, with respect to the voltage supplied by the second voltage generator 4_. The figure further shows terminals 8 and 9 (which are beside the point) for the focussing and deflection voltages. The second voltage generator _ comprises a voltage regulator 10 with a negative feedback from its output to its input via control amplifier 11. The input voltage of regulator 10 is supplied by a digital-analogue converter 12. This converter receives a computer-supplied digital code of the colour to be displayed. To modulate the voltage supplied by the second voltage generator with the video signals, this voltage must be made a floating reference for these video signals. The power supply circuit thereto comprises a first transforming unit J_3_. In this unit the video signals, supplied via line 14 and defined with respect to a first reference voltage, preferably "earth", are converted into video signals defined with respect to a second reference voltage, that is the voltage supplied by the second voltage generator 4_, which voltage varies in magnitude with the different colours. The video signals defined with respect to the second reference voltage, i.e. the video signals modulated on the voltage varying in magnitude with the different colours are applied to cathode 5 of cathode ray tube via line 15 and amplifier 16. Like grids 6 and 7 of the cathode ray tube, amplifier l≤ must be fed with supply voltages defined with respect to the second reference voltage. A second transforming unit j_7 is thereto incorporated. In transforming unit J_7 a supply voltage, produced by voltage source 18 and defined with respect to the first reference voltage, is converted into a supply voltage defined with respect to the second reference voltage. The latter voltage is applied to supply unit 19, which produces supply voltages defined with respect to the second reference voltage and required for amplifier 16, grids 6 and 7 and for the supply of various circuits of the first transforming unit; these circuits will be dealt with later. The conversion in the first and second transforming units is in principle effected by inductive couplings (trans¬ formers) 20 and 21, respectively. However, the difficulty arising is that the capacitance between the primary and the secondary windings of the inductive coupling 21 through the lines carrying the second reference voltage, together with the capacitance between the primary and the secondary windings of inductive coupling 20, disturbs the video signals, such that a distorted picture on the screen is obtained. These disturbing effects are however eliminated by two measures. First, a shield 22 having the potential of the first reference voltage is inserted between the primary and secondary windings of inductive coupling 22, so that the capacitance between these windings is reduced substan¬ tially to zero. Second, the first transforming unit J_3_ is provided with an amplitude modulator 23 and an amplitude demodulator 24. The video signals are first modulated on a carrier frequency applied via line 25 and, after conversion by the inductive coupling, are again demodulated. When the signals are converted in a higher frequency range, a smaller transformer can be employed, permitting the conversion to be realised with a lower impedance, which considerably reduces the disturbing effect of the capaci- tance between the primary and the secondary windings of coupling 20. c:.:? With the display of a video signal a voltage jump is impressed on line 14. The rise time of this jump is relatively long to keep the video signal to be converted in the first transforming unit in a narrow band, but for the display on the screen the rise time should be very short. However, it is practically impossible to convert the video signal with both sufficient accuracy and a very short rise time; the bandwidth of the signal to be converted would then be too large. For this reason the first transforming unit J_3 comprises a second inductive coupling 26, a differentiating network 27 incorporating this coupling, a bistable circuit 28 connected to network 27, a switch 29, and an amplifier 30. Line 31 carries a square wave signal fed to the first transforming unit 13. This signal has very steep edges (short duration) which are determinative of the commencing and finishing times of the display in a certain colour; the rising edge appears as soon as the relatively long rise time of the video signal has elapsed, while the falling edge appears before the video signal changes. Since only the commencing and finishing times of the square wave signals are of importance, the signal is differentiated in the RC network 27 after passing through amplifier 30. Network 27 also comprises the inductive coupling to change the reference voltage for the differentiated signal; the differentiated square wave signal defined with respect to the first reference voltage is to be converted into a signal defined with respect to the second reference voltage. In bistable circuit 28 the differentiated square wave is again converted to a pure square wave signal now defined with respect to the second reference voltage. This square wave signal controls switch 29. With the presence of the rising edge of the square wave signal, i.e. the instant of commencing the display of video signals in a certain colour, switch 29 is set to the position as indicated in the figure. At this instant the amplifier 16 is fed with the video signals defined with respect to the second reference voltage. With the presence of the falling edge of the square wave, i.e. at the instant of finishing the display of video signals in a certain colour, switch 29 is set to the other position, to supply the amplifier 16 with the second reference voltage. It is preferred to use a difference amplifier for amplifier 16, the reason being as follows. The acceleration of the electrons in the cathode ray tube is determined by the anode-cathode voltage, and not by the deflection voltage. Variations in the cathode voltage through the video signals dis- played in the different colours result in positional errors in the display on the screen. These positional errors may be limited by allowing the voltage across grid 6 to change with the aforementioned variations in the cathode voltage. Further corrections of the positional errors are to be made In the deflection voltage. It should finally be noted that inductive couplings are employed in the first and the second transforming units, whereas in general electrically isolated couplings could have been used; as substitute for couplings 20 and 26, reference is made to the use of optoelectronic couplers.

Claims: Claims ; 1. Power supply circuit for applying a rapidly varying high voltage to a penetron-type cathode ray tube for a colour display system, characterised in that the power supply circuit comprises a first voltage generator for supplying a constant high voltage to the anode and a second voltage generator for producing a voltage varying in magnitude with the different colours, which voltage after being modulated with the video signals to be displayed is applied to the cathode to provide, together with the anode voltage, for the display of the video signals in the particular colours. 2. Power supply circuit as claimed in claim 1, characterised in that the power supply circuit comprises: a first transforming unit for converting the video signals defined with respect to a first reference voltage into video signals defined with respect to the voltage varying in magnitude with the different colours and functioning as second reference voltage; a second transforming unit for converting a supply voltage defined with respect to the first reference voltage into a supply voltage defined with respect to the second reference voltage; and a supply unit for deriving from the supply voltage defined with respect to the second reference voltage the supply voltages needed to process the video signals defined with respect to the second reference voltage and to display said video signals on the cathode ray tube. 3. Power supply circuit as claimed In claim 2, characterised in that an electrically isolated coupling is employed for the conversion process in each of the transforming units. 4. Power supply circuit as claimed in claim 3, characterised in that the first transforming unit as electrically isolated coupling comprises a (first) inductive coupling, an amplitude modulator, and an amplitude demodulator, whereby the video signals are modulated on a carrier frequency and the modulated signal so obtained is demodulated after the conversion. 5. Power supply circuit as claimed in claim 4, characterised in that the first transforming unit comprises a second inductive coupling, a differentiating network, incorporating said second inductive coupling, and connected to said network a bistable circuit for converting the square-wave voltage, being determinative of the start and the end of displays in a certain colour and being defined with respect to the first reference voltage, into a square-wave voltage defined with respect to the second reference voltage, and further comprises a switch for passing to the cathode either the second reference voltage or, depending on said latter square-wave voltage, the video signals defined with respect to the second reference voltage. 6. Power supply circuit as claimed in claim 3, 4 or 5, characterised in that the electrically isolated coupling in the second transforming unit consists of an inductive coupling, whereby the capacitance between the primary and the secondary windings is reduced substantially to zero through the insertion of a shield between said windings, said shield having the potential of the first reference voltage.

Inventors: PLOUMEN H
Applicants: HOLLANDSE SIGNAALAPPARATEN BV, PLOUMEN H
Application year: 1980
Main classification: H04N9
IPC codes: H04N9