GB2253978A

GB2253978A - Raster scan system without flyback

Info

Publication number: GB2253978A
Application number: GB9027226A
Authority: GB
Inventors: Beverley Hugh Pardoe; Leonard Unsworth
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-12-15
Filing date: 1990-12-15
Publication date: 1992-09-23
Anticipated expiration: 2010-12-15
Also published as: GB9027226D0; GB2253978B

Abstract

A raster scanning system is described that dispenses with both line and field flyback. The system may further provide interlaced scanning. In the case of television signals further circuits are included to match the signal to the display format. <IMAGE>

Description

RASTER SCAN SYSTEM This invention relates to raster scan systems and particularly to systems used in visual display units and television receivers.

Raster scan systems are well known in the field of television receivers, computer displays and latterly in radar displays. In such displays, an electron beam is deflected across the face of a cathode ray tube. The deflection takes place both horizontally (line) and vertically (field) at the same time. The forward horizontal deflection occurs in, typically fifty-two microseconds with an associated flyback period of twelve microseconds, while the downward vertical sweep takes, typically, 18.4 milliseconds with a flyback period of 1.6 milliseconds. A system of interlacing fields is also used in standard television systems.

One field is transmitted and displayed over the whole screen using a raster scan. This is followed by a second field which is displayed within the interline spaces of the first field.

The third field replaces the first field and so on. This leads to the terms 'even' and 'odd fields.

The physics of raster scan dynamics with flyback requires electrical drive circuitry to operate at high currents and voltages. In order to meet these requirements, special-purpose solid-state devices are needed which are expensive and need to be designed with parameters which are near the limits of existing technology, one fact that has tended to forestall the extension of and development of higher quality displays.

According to one aspect of the present invention, the flyback is eliminated, allowing the parameters of the solid-state devices in the drive circuitry to be relaxed. Specific circuitry is needed to generate the sawtooth current waveform for the horizontal deflection and the stepped current waveform for the vertical deflection (see Figure 2). In the case of television pictures, a frame store and synchronising circuits are included to convert the picture signal to the required format for the display.

A specific arrangement of the invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 shows the sequence of events in the particular case of an interlaced raster scan and Figure 2 shows a block diagram of the required system in the case of standard television signals..

The deflection sequence may be described from the centre of the screen and is as follows: The beam is deflected from the centre to the edge of the screen, during which time the vertical deflection current is zero. At the edge of the screen; the vertical deflection current is increased to deflect the beam by, for example, one line, or spot diameter.

During the first half line the horizontal current was steadily increased from zero to a peak value. During the vertical interline movement the horizontal current's rate of change is reversed. To deflect the beam back across the screen the horizontal current is steadily decreased to zero and on to a negative value until it reaches a negative peak equal in magnitude to the positive peak (but opposite in sign). During this line deflection the vertical current is held constant. The beam path in the forward direction is indicated by the solid line in Figure 1. At the picture edge the vertical deflection current is incremented again and the rate of change of horizontal current is reversed. This cycle is repeated unitl the electron beam reaches the bottom of the screen.At this point the vertical current is decremented and is decremented at each edge of the picture until the beam reaches the top of the screen.

The reverse path is indicated by the dotted line in Figure 1.

The horizontal deflection current merely has its rate of changed reversed at each edge of the picture. The vertical deflection current has its small change reversed in polarity at the top and bottom of the picture, e.g. increment going one way and decrement going the other way.

Appropriate adjustment of the change in vertical deflection current allows one to create either an interlaced or a noninterlaced picture.

The scan sequence of an interlaced picture is illustrated in Figure 1.

For the display of standard television pictures extra circuitry is required because the timing relationship between transmitted and displayed pictures is now different. In computer and radar displays the data can be manipulated to suit the display format.

The demodulated television picture information is sampled and stored in memory devices. when the raster is at the bottom of the screen and moving up, the current field picture information is read in reverse order from the store. At the same time the next field is being stored in different memory locations. When the raster is at the top of the screen and moving down it reads the picture information in forward order while the next field is being stored. A complete frame-store and synchronised read and write system is required to store the incoming picture information and transfer the stored information to the screen at the appropriate time for the new raster scan system. The circuit blocks and the idealised current waveforms required are illustrated in Figure 2.

It is evident that a similar system could be implemented wherever a scanning technique is employed, e.g. television cameras and electron microscopes.

Claims

1. A raster scan system which deflects a beam over the whole of a specified area without flyback.

2. A system according to claim 1 including means to deflect the beam, e.g. currents for magnetic deflection or potential differences for electrostatic deflection.

3. A system according to claims 1 and 2 including means to create an interlaced scan.

4. A system according to claims 1 and 2 including means to synchronise the scan to a separately generated picture signal.

5. A system according to claims 1 to 4 including means to store one or more frames of a separately generated picture signal and means to retrive the stored picture signal in a sequence appropriate to the raster sequence.