GB2151117A - Flat cathode tube deflection waveforms - Google Patents

Abstract

The field scan waveform for an electrostatically deflected cathode ray tube having its electron gun placed to project a beam of electrons towards the screen from one side includes a component at line scan frequency to correct distortion of the raster due to the tube geometry. It is essential that the line scan frequency component is synchronized with the line scan deflection waveform which means that the flybacks must be described in the same time interval. A substantial saving in the power needed to produce the line scan waveform is obtained by arranging the field scan waveform to be such that the flyback portions of the line scan frequency component be performed in such directions as to reduce the field scan deflection. This enables the field scan amplifiers to use higher value collector resistances to achieve the charging and discharging of the circuit capacitances as required during the line flyback interval. The power consumption of the field scan amplifiers is thereby reduced. <IMAGE>

Description

SPECIFICATION Cathode ray tube display apparatus This invention relates to cathode ray tube display apparatus and is particularly concerned with such apparatus as uses flat cathode ray tube of the type described in British Patent Specification No. 1 592571.

The flat cathode ray tube described in the above patent specification is compact and consequently is particularly applicable to use in portable apparatus such as television receivers and computers. Since the powerfor portable apparatus must be provided by batteries, it is clearly important to keep the power consumption of such apparatus as low as possible so as to enable the batteries to provide operation of the apparatusfora useful period oftime.One ofthe major consumers of electric power in television apparatus is the scanning drive circuitry for generating the raster on the cathode ray tube screen since the voltages to be applied to the deflection electrodes ofthe cathode ray tube are relatively high, for example 150 volts, whereasthe battery itself might provide, for example, 6 volts. Itfollows therefore thatfor every unit of current consumption at the high voltage, 25 units at least must be drawn from the battery and such 100% efficiency in voltage conversion cannot be realised in practice.

It is an object ofthe present invention to provide cathode ray tube display apparatus in which the power consumption of one ofthe scanning circuits is reduced.

According to the present invention there is provided cathode ray tube display aparatus having a cathode ray tube with electrostatic deflection in which an electron gun is mounted to project a beam of electrons towards a phosphor screen from one side ofthe screen, the apparatus being such asto generate a raster scan on the screen by line and field scan waveforms, the field scan waveform including a component at line frequency to correct the influence ofthe line scan on the rectangular form ofthe raster due to the location ofthe electron gun relative to the screen, wherein the manner of description of the raster on the screen relative to the location of the electron gun is so chosen that the generation ofthe field scan waveform requires a minimum of electrical power.

In one example of the invention the lines ofthe raster are parallel to the axis ofthe electron gun so that in orderto restore the raster to a rectangularform from the trapezium or keystone shape which would otherwise be generated it is necessary to add sawteeth at line frequency to the field deflection sawtooth waveform. This is described in British Patent Specification No. 1 592571 which relates to one form of cathode ray tube which may be included in apparatus according to the invention. The linefrequencysaw- teeth in the field deflection waveform must be described synchronously with in the line deflection sawteeth, which means in particularthatthe flybacks ofthe line frequency sawteeth in the field deflection waveform must be described in the same time as those of the line deflection sawteeth.It follows therefore that thefield deflection drive circuit must have a suitably short time constant for each field deflection electrode, which imposes a minimum output impedance on the drive circuit once the circuit capacitance have been reduced to a minimum. Using a conventional common emitter transistor amplifier with a resistive collector load to drive each deflection electrode it is clearthatthe collector load has a minimum usable value because ofthe limitation imposed by the flyback time constant and the circuit capacitances since at least some ofthe flybacks ofthe line frequency component must be energised by the current through the load resistor.However, because the basicwaveform ofthefield deflection is also a sawtooth and the polarity ofthe line frequency correction sawtooth in the first half of the field deflection is opposite that in the second half, it is possible to arrange the circuit, by suitable choice of the sense of description of the scanning raster, so that the flybacks always occur in a positive-going sense from a more negative part ofthefield frequency sawtooth flank and in a negative-going sense from a more positive part ofthe field frequency sawtooth flank.With the circuit arranged in this way a larger voltage difference is available to energise the flybacks, than if the line frequency correction sawteeth were of the opposite polarity as would be required if the scanning raster were to be described in the opposite sense. It has proved possibleto increasethe values of the collector load resistors ofthe field drive circuit by a factor of three orfourwith a corresponding reduction in the current drawn by the circuit bythe use of the invention. This is of particular importance in battery powered cathode ray tube apparatus such as portable television receivers and computers.

In orderthatthe invention may be fully understood and readily carried into effect, it will now be described with reference to the accompanying drawings, of which: FIGURE 1 shows part of the circuitofapparatus according to one example ofthe invention; and FIGURES 2,3,4 and Sshowvoltagewaveforms which are to be used to explain the operation of the invention.

In Figure 1, the cathode ray tube 1 is ofthe type described in British Patent Specification No. 1 592 571, which is offlat construction and has electrostatic line and field deflection plates 2 and 3 respectively and a phosphor display screen 4. It is not proposed to consider in detail the construction ofthetube 1 except insofar as its geometry determines the nature ofthe field scan waveform which must be applied to the electrodes 3. Since the gun of the tube 1 is placed to one side ofthe screen 4, itfollows that if the field deflection voltages are kept constant whilst the line deflection voltage is varied, the spot will describe on the screen 4 a line which is radial to some point on the axis of the electron gun.Therefore, in order to produce a rectangular raster on the screen 4, it is necessary to add to the conventional field deflection sawtooth waveform a sawtooth component at line frequency the amplitude of which will startfrom a maximum value in one sense at the top of the raster and change progressively to a maximum value in the opposite senseatthebottomoftheraster. For example, the line 5 of the raster must be have a field deflection voltage corresponding to the broken line 6 atthe start ofthe line and a field deflection voltage corresponding to the broken line 7 at its end. The exact nature of this deflection waveform is shown in Figure 5 ofthe above-mentioned British patent specification.

The field scan deflection waveform is applied in push-pull via terminals 11 and 12 to the base electrodes of amplifying transistors 13 and 14 respectively. The transistors 13 and 14 have respective emitter resistors 15 and 16, and the emitters themselves are interconnected by a capacitor 17 in parallel with a fixed resistor 18 and a variable resistor 19. The function of the variable resistor 19 is to adjust the interconnection between the emitters of the transistors 13 and 14 and thereby control the amplitude of the field scan by controlling the gains ofthe transistors.

Collector resistors 20 and 21 are provided for the amplifiers 13 and 14, and the collectors are connected respectively via a resistor 22 and a capacitor 23 and a resistor24and a capacitor 25 in series combinations to the field deflection electrodes 3. The electrodes 3 also receive reference DC voltages on which the deflection voltages are superimposed by virtue of the capacitive connection ofthe collectors ofthe transistors 13 and 14to the electrodes 3. Inevitably there will be stray capacitance to ground from the collectors of the transistors and these strays are represented by the capacitors 26 and 27.

Assuming thatthe raster being scanned on the screen 4 has 625 lines perframe and a field frequency of 50Hz, then the line frequency is 15625 Hz corres ponding to a line period of 64 s. The forward stroke of the line deflection sawtooth occupies 521is and the flyback 12 also Since the field deflection waveform includes a component at line frequency as described above in orderto produce the required rectangular raster, itfollows thatthe output circuits of the transistors 13 and 14 must have a sufficiently short time constant to be able to describe the flyback of the line frequency component whenever it occurs within the 12 Ps period.Otherwise, eitherthe left-hand edge orthe right-hand edge or both edges of the rasterwill display curvature which as a distortion ofthe displayed picture will be unacceptable to the viewer. The time constant of the collector circuit ofthe transistor 13 with regard to the charging ofthe capacitor 26 is determined by the value ofthe collector load resistor 20 and the value ofthe capacitive strays represented bythe capacitor26.The dischargetime constant is shorter since the resistance ofthe transistor 13 when conducting plus the emitter resistor is less than the resistance of the collector load resistor. With good design ofthe circuit layout, the value ofthe capacitive strays can be keptto a minimum which meansthatthe maximum acceptable charging time constant deter mines the value of the collector load resistor 20.Since the transistors 13 and 14 must be operated as linear amplifiers in class A, the collector current can never fall to zero and therefore the power consumption of the amplifiers 13 and 14 depends very considerably on the values ofthe collector load resistors 20 and 21. It is therefore desirable for the values ofthe load resistors 20 and 21 to be as high as possible.

Atelevision raster is conventionally scanned from left to right and from top to bottom. Because ofthis, by mounting the cathode ray tube 1 with the electron gun on the right-hand side of the screen 4 as is shown in Figure 1, then the field deflection waveform applied to the lowerfield deflection electrode would take the form shown in Figure 2, and Figure 3 shows the complementary waveform applied to the upper field deflection electrode. In practice, of course, there would be many more line frequency sawteeth than are shown in Figures 2 to 5, the number being limited to clarify the Figures. The short, almost vertical, sections of the waveforms shown in Figures 2 and 3 represent the flybacks at linefrequency which are described in the directions indicated by the arrows on them.Those on the left-hand side of Figure 2 labelled A, B and C, are described in the positive-going direction, whereas those on the right-hand side of Figure 2, labelled D, E and F, are described in the negative-going direction.

However, since by virtue of the overall positive-going ramp atfield frequency on which the linefrequency component is superimposed,the positive-going flyback sections A, B and C which are described by current th rough the collector load resistor 21, have a larger voltage difference across the resistor 21,so that the necessary current can be obtained for a value of the resistor 21 which is higherthan could be used if the field frequency ramp were negative-going and the voltage difference across the resistor 21 were smaller.

The broken line A' represents the charging curve of the capacitance 27 to the voltage v of the conductor 28, Figure 1, which is represented bythe dashed line Pin Figure 2. On the right-hand side of Figure 2, however, where theflyback sections D, E and Fare described in a negative-going direction, they do so from more positive voltages as a result ofthe overall rising ramp at field frequency. The sections D, E and F are described by discharging of the capacitance 27 through the transistor 14.

Figure 3 shows the complementaryvoltagewave- forms for the upperfield deflection electrode from which it is clearthatthe same advantage ofthe greater voltage differences due to the overall ramp of the field frequency sawtooth is obtained.

Figures 4 and 5 showthe waveforms which would have been needed to describe the same raster with the electron gun on the left-hand side of the screen 4, and consideration of these Figures will show that in all cases the positive-going flybacks are described at more positive voltages and the negative-going flybacks are described at more negative voltages, so that lower values ofthe collector load resistors would be necessaryto perform the charging and discharging of the stray capacitances within the lineflyback time of 12sus.

As explained above, a consequence of the use of lower collector load resistors would bean overall increase in the power consumption by the field amplifier circuit. It has been found in practice thatthe increase in power consumption bythefield amplifier circuit resulting from the arrangement ofthe cathode raytubel with itsgun ontheleftofthescreen 4would be aboutfourtimes that needed when the gun is on the right side of the screen 4. Since the supply voltage forthe field amplifierwould be a few hundred volts and the higher power consumption would be ofthe order of a tenth of a watt, the invention could result in a substantial saving in the current consumption from the battery.

Itwill be understood thatthe arrangement ofthe tube f with the gun to the right ofthe screen 4 prnducesa lower power consumption only for a conventional scan raster and forfield amplifying transistors of the NPN type. If the raster were to be scanned in some other way, this could result in the more economical operation being obtained with the gun on the left of the screen ofthetube 1. Had the collector load resistors been connected to the more negative supply conductor, then this would also reverse the favourable orientation of the cathode ray tubes.

Although the invention has been described with reference to particular kind of cathode ray tube, it would be applicable to othertubes using similar geometry.

Claims (9)

1. Cathode raytube display apparatus having a cathode ray tube with electrostatic deflection in which an electron gun is mounted to project a beam of electrons towards a phosphorscreenfrom one side of the screen, the apparatus being such as to generate a raster scan on the screen by line and field scan waveforms, the field scan waveform including a component at linefrequencyto correctthe influence ofthe line scan on the rectangularform ofthe raster due to the location ofthe electron gun relative to the screen, wherein the manner of description ofthe raster on the screen relative to the location ofthe electron gun is so chosen that the generation ofthe field scan wave-form requires a minimum of electrical power.

2. Apparatus according to claim 1 wherein the lines ofthe raster are described in directions towards and away from the electron gun, and the field scan is described from side to side relative to the electron gun.

3. Apparatus according to claim 2 in which the forward stroke of each line is described from left to right and the forward stroke of each field is described from top to bottom ofthe raster when viewed facing the display on the screen ofthe cathode ray tube, wherein the electron gun is located on the right hand side ofthe screen towards which each line is described.

4. Apparatus according to claim 2 wherein the component at line frequency is of sawtooth waveform and the flyback portions of the sawteeth are always described so asto reduce the deflection in the field scan direction.

5. Apparatus according to claim 2 wherein the field scan waveform includes sawtooth waveform at field frequency, the component at line frequency is of sawtooth waveform andtheflyback portions ofthat waveform are arranged to be described in positivegoing sensewhn.the ramp of the field frequency sawtooth is more negative and in negative-going sense whenthe ramp ofthefield frequency sawtooth is morapositive.

6 Apparatus according to claim 3,4 or 5 wherein thefield scan waveform is applied in push-pull to the field deflection electrodes ofthe cathode ray tube.

7. Apparatus according to claim 6, including two transistors connected in common emitter mode with their collector electrodes connected to applythe respective polarities of the field scan waveform to the respective field deflection electrodes ofthe cathode ray tube, wherein the collector load resistors of the transistors have substantially the largest value permitting the flybacks of the line frequency sawteeth to be described within a period substantially equal to the flyback period of the line scan deflection waveform of the tube.

8. Apparatus according to claim 7 including an adjustable resistor connected between the emitter electrodes ofthetransistorsto provide a deflection height control.

9. Cathode raytube apparatus substantially as described herein with reference to the accompanying drawings.