WO1990001848A1

WO1990001848A1 - Cathode ray tube

Info

Publication number: WO1990001848A1
Application number: PCT/GB1989/000896
Authority: WO
Inventors: Michael Grange; Michael Kent; Salim Al-Hasso
Original assignee: Facit Limited
Priority date: 1988-08-05
Filing date: 1989-08-04
Publication date: 1990-02-22

Abstract

This invention relates to a cathode ray tube (1) for use in a display screen of a computer monitor, and to a control circuit for the cathode ray tube. The cathode ray tube (1) with the display screen (2) is adapted to be pivotally mounted on a frame or housing (8, 9) so as to be pivotable between a landscape mode and a portrait mode. A scanning coil (3) located on the tube (1) is secured to the frame (9) in such a way that the cathode ray tube (1) is pivotable relative to the scanning coil (3), the coil remaining stationary during the pivotal movement of the tube (1). The invention includes a control circuit to automatically adjust the screen display to the size appropriate to the mode selected.

Description

Cathode Ray Tube

This invention relate* to a cathode ray tube for use in a display screen of a computor monitor, and to a control circuit for the cathode ray tube.

In modern computor systems, particularly personal computors, there is a requirement for different screen sizes and shapes of screen, depending on the application. The display screen of a personal computor is generally rectangular, with the longest side of the screen being horizontal. Typically, such screens display about 25 lines, each line being 80 characters wide. This is referred to as "landscape" mode. An alternative requirement, which is particularly useful in Desktop Publishing, is to have the longest side of the screen in the vertical direction, so that the screen can display an A4 size page, for example. This is known as "portrait mode" .

One solution to this problem is to use two monitors, one portrait and one landscape and to connect the monitor required for the task in hand. This solution has disadvantages in that two monitors are required, which is expensive, valuable desk space is taken up, and there is also the extra work in continually changing the monitors over.

The present invention seeks to overcome this problem by providing a monitor which can be used in either mode.

According to a first aspect of the present invention there is provided a cathode ray tube with a display screen adapted to be pivotally mounted on a frame or housing, a scanning coil located, on the tube being secured to the frame in such a way that the cathode ray tube is pivotable relative to the scanning coil, the coil remaining stationary during the pivotal movement of the tube.

Preferably, the screen is essentially rectangular and is mounted so as to be pivotable between two mutually perpendicular positions to enable the longer side to be horizontal or vertical, as required. In a preferred embodiment the coil is a sliding fit on the neck of the cathode ray tube .

In the two modes the height and width of the screen display is determined by the vertical and horizontal scan generators which are normally designed to give one fixed line frequency and one fixed field scan frequency. It is not possible from a practical aspect, both as far as software and hardware are concerned, to rotate the coil and for the line scan generator circuit to generate the field scan and for the field scan generator circuit to generate the line scan.

It is possible to provide manually operable switches or controls to enable the required parameters for screen height and width to be set for the selected mode, but some software demands regularly changing screen modes which makes operation of the monitor complicated and tedious. The present invention therefore further seeks to provide an additional control circuit responsive to the mode selected to automatically adjust the screen display to the appropriate settings .

According to a second aspect of the present invention there is provided a control circuit for a cathode ray tube according to the first aspect the circuit being responsive to the position of the screen to adjust the line and field scan frequencies generated by the scanning circuits and to provide the appropriate screen height and width for the selected mode.

A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:-

Figure 1 shows an exploded view of the mechanism for mounting a screen in a computor housing,

Figure 2 shows an assembly of the mechanism,

Figure 3 shows a cross-sectional schematic view of the cathode ray tube and the scanning coil,

Figure 4 shows an exploded view of the mounting means of the coil,

Figure 5 shows a block diagram of a computor system including the present invention,

Figures 6A and 6B shows part of the field scan control arrangement, and

Figure 7 shows a circuit diagram, in block form of a multisync, board.

Referring now to the drawings, there is shown a cathode ray tube 1 which includes a generally rectangular screen 2. A scanning coil 3 is mounted slidably on the neck 4 of the tube 1. The tube and coil assembly is mounted on a mechanism 5 which in turn is mounted in a computor casing (not shown).

The mechanism 5 includes a mounting ring 6 which is secured to the tube 1 by means of screws 7 and which is rotatable in a housing 8. The housing 8 is fastened to a frame 9 which is secured to the computor casing through a vertical slide to enable the height of the screen relative to the computor casing to be varied to suit the operator. The weight of the screen is balanced by a spring 10 which acts on the frame 9 through a pair of links 11.

The mounting ring 6 includes two stop members 12 and 13 which are disposed 90° apart and which cooperate with spring loaded latches fastened on the frame 9 to positively locate the ring and hence the screen in the selected one of the two positions.

The scanning coil 3, as shown particularly in Figure 3 consists of a housing 14 formed of a plastics material, which carries a scanning coil winding of known construction, and is shaped to be a sliding fit on the neck 4 of the tube 1. In order to reduce friction between the housing 14 and the tube 1, the housing has a plurality of ribs 15, typically 3 or 4 equi-distantly disposed around the housing, which space the main body of the housing 14 from the tube 1. A lubricant may also be used. As shown also in Figure 4, a mounting bracket 16 surrounds the coil housing 14 and two screw threaded arms 17 are secured to opposed sides of the bracket 16. The arms each carry a link 18 through which the coil is secured to the frame 9. Each of the links is fastened to the frame through a slot 19 which serves to accommodate the slight dimensional changes of production tolerances to ensure that the coil is always accurately positioned on the tube 1.

In operation, when an operator wishes to change screen mode, he/she merely grasps the screen and twists it manually through 90° . The initial movement frees the stop member 12 or 13 which is engaged with its corresponding latch and the screen can be twisted until the other stop engages in its latch to thereby lock the screen in its second mode. During this movement, the ring 6 carrying the screen and tube pivots in the housing 8, but the coil 3 remains stationary. Thus, whichever screen mode is selected, the coil continues to generate a conventional horizontal scan. The different dimensions of the screen can be accommodated by software or firmware quite easily, whereas it would be extremely difficult to redesign software to accommodate a vertical scanning mode if the coil were also twisted with the screen.

The embodiment illustrated in Figures 5 to 7 is particularly intended for personal computors used for desk top publishing systems using an IBM Enhanced Graphics Adaptor (E.G.A. card) and is intended to support the following screen modes generated by the E.G.A. card:-

(a)All E.G.A. and SUPER E.G.A. modes except 132 column mode.

(b) All C.G.A. modes.

(c) 80 by 72 (Text) Custom Portrait mode.

(d) 600 by 800 SUPER E.G.A. Custom Portrait mode. LINE SYNC

The line sync output from the E.G.A. card is fed via a monostable to a Frequency To Voltage Converter (F.V.C.). The monostable is important since the F.V.C. is sensitive to varying input waveshape as well as frequency and also allows the sync pulse width to be fixed to a suitable value for the monitor board. The F.V.C. timing components are chosen so that its output is typically 2.9V-4.5V for an input of 31Khz-46Khz. The line V.C.O. requires a control voltage from 6.76V-3.1V to produce this output frequency range. This is provided by the next stage, an operational amplifier (op.amp) which has three functions: 1) Inverter 2) Voltage amplifier 3) set V.C.O. control voltage operating point. By making the third function adjustable, it effectively becomes a line hold control. The output of the op.amp is fed to the line V.C.O. LINE PHASE

Each time the operating conditions of the line scan circuit are changed which is achieved by switching the width coil or the tuning capacitor, a different line phase setting is also required, and this is achieved by using the outputs of the dual comparator to control a one pole three way analogue switch I.C. which selects one of three possible line phase reference signals through to the line V.C.O. each of which is preset to suit one of the following possibilities: 1) width coil and tuning capacitor normal: 2) width coil short circuited, tuning capacitor normal: 3) width coil short circuited and tuning capacitor changed. FIELD SYNC

As with line sync, the field sync output from the E.G.A. card is fed via a monostable to a F.V.C. (The monostable is timed to give a positive going output pulse which is 0.2ms wide). THe F.V.C. timing components have been chosen so that its output is typically 2.08V-4.5V for an input of 60Hz-120Hz. The field V.C.O. requires a control voltage from 3.35V-1.95V to produce this output frequency range. As with the line sync control circuit, this is provided by an op.amp which as three functions: 1) Inverter, 2) Voltage attenuator, 3) Set F.V.C. control voltage operating point. By making the third function adjustable it effectively becomes a field hold control. The output of the op.amp is fed to the field V.C.O. FIELD HEIGHT

A second output feed is taken from the field F.V.C. and fed to the inverting input of an op.amp in the field height control circuit. Referring now to Figures 6A and 6B, the basic field oscillator on the monitor board uses a capacitor, charging via a fixed value resistor. At the end of each field scan the field oscillator circuit discharges the ramp capacitor. The time from which it begins to recharge to the time when it is discharged again corresponds to one field scan. Since the difference between these two times will become smaller as field frequency increases, at higher field frequencies the charge voltage achieved by the ramp capacitor will become progressively smaller. This has to be corrected, otherwise picture height will vary considerably from mode to mode. The op.amp acts as a variable gain inverting amplifier. The output of this amplifer sets the base bias on the next stage, an adjustable constant current source. The gain of the op.amp is adjusted so that if the field scan frequency increases, the current available from the constant current charging source increases by an appropriate amount to maintain the correct charge voltage across the ramp capacitor. This ensures that at all possible field frequencies the picture height is correct. A small d.c. feedback circuit from the collector of the constant current source to the non inverting input of the o .am . and a diode in the base circuit of the constant current source prevent any picture height drift due to temperature changes. A secondary adjustable biasing circuit is introduced into the base circuit of the constant current source so that when the display is twisted into portrait mode, picture height is accordingly adjusted.

WIDTH AND E.H.T. CONTROL

The line V.C.O. control voltage is used to sense when a critical frequency has been reached where a change is required in the line scan circuit on the monitor board. It is fed to the inverting inputs of two voltage comparators. The voltage on the non-inverting inputs is set so that at 36Khz, relay 1 is switched which shorts out a width coil in series with the line scan coils on the monitor board to correct horizontal scan width above 36Khz. At 40Khz, relay 2 is switched which connects a 5.6nF capacitor in series with the line flyback tuning capacitor on the monitor board, thus increasing the resonant frequency of the scan circuit.

The E.H.T. supply to the C.R.T. display is indirectly controlled by regulating the monitor board video amplifier supply. This voltage is dropped by an adjustable potential divider and fed to the inverting input of an op.amp this is compared with a 10V reference voltage on the non-inverting input. The op.amp output is fed via an emitter follower to one input of a differential amplifer. The other input is connected via a fixed potential divider to the collector of a series regulator transistor. The differential amplifier compares the voltage reference with the voltage on the collector of the series regulator transistor and corrects any errors by varying the regulator transistor's base bias. If only one screen mode was being used, the output of the op.amp would be constant and the circuit will behave as a conventional regulated power supply, if, however a screen mode change occured such that, for example the line frequency changed from 34.93Khz to 31.63Khz, the E.H.T. supply and consequently the video amplifier supply would try to increase. This change when compared with the 10V reference, lowers the output voltage level from the op.amp which causes the differential amplifier to adjust the bias to the series regulator transistor until an output level is reached where the video amplifier supply returns to 49V.

Claims

1. A cathode ray tube with a display screen adapted to be pivotally mounted on a frame or housing, a scanning coil located on the tube being secured to the frame in such a way that the cathode ray tube is pivotable relative to the scanning coil, the coil remaining stationary during the pivotal movement of the tube.

2. A cathode ray tube according to claim 1 wherein the screen is essentially rectangular and is mounted so as to be pivotable between two mutually perpendicular positions to enable the longer side to be horizontal or vertical, as required.

3. A cathode ray tube according to claim 1 or 2 wherein the coil is a sliding fit on the neck of the cathode ray tube.

4. A cathode ray tube according to any one of claims 1 to 3 wherein a mounting ring is secured to the tube, the mounting ring being rotatable in a housing and Ha ing at least one engaging element cooperable with latch means fixed relative to the housing and spaced 90 degrees apart, the latch means being engageable with the engaging element to lock the tube in a selected one of two positions.

5. A cathode ray tube according to any one of claims 1 to 4 wherein the scanning coil includes a housing formed of a plastics material which carries a scanning coil winding and which is a sliding fit on the neck of the tube.

6. A cathode ray tube according to claim 5 wherein the said housing of the coil has a plurality of internal ribs disposed about its inner periphery to space the main body of the housing from the neck of the tube.

7. A cathode ray tube according to any one of claims 4 to 6 wherein the housing carrying the mounting ring is mounted in a frame so as to be vertically adjustable to enable adjustment of the height of the screen, the weight of the tube and screen being balanced by counter balancing springs .

8. A control circuit for a cathode ray tube according to any one of claims 1 to 7 wherein the circuit is responsive to the position of the screen to adjust the line and field scan frequencies generated by the scanning circuits and to provide the appropriate screen height and width for the selected mode.

9. A cathode ray tube with a display scren substantially as described herein with reference to, and as illustrated in, the accompanying drawings.

10. A control circuit for a cathode ray tube substantially as described herein with reference to and as illustrated in Figures 5 to 7 of the accompanying drawings .