GB2094116A - Improvements in visual display devices - Google Patents

Abstract

A control circuit for a monitor unit incorporating a CRT 3 in a VDU has as read/write memory a RAM (4a) connected to a character and control data course (MPU1) via a buffer device (10). A multiplexer (11) supplies the RAM with the current character address which is fed back to it whenever the raster is being modulated to produce a displayed character. The RAM output is supplied to a series of buffers (12) controlling character generator (6) and attribute logic generator (20) whose combined outputs are fed to the CRT as well as vertical and horizontal synchronising signals also controlled by the series of buffers and timed by counters (15 and 14). The circuit allows part of the display on the screen of the c.r.t. to be retained in position while the remainder is moved or changed and also permits all or part of the display to be "soft scrolled" i.e., moved incrementally in vertical or horizontal directions, one dot or raster line at a time, or "hard scrolled" i.e. a character or a row at a time. <IMAGE>

Description

SPECIFICATION Improvements in visual display devices This invention relates to visual display devices and, in particular, to means for controlling a visual display unit (VDU) incorporating a cathode ray tube having a display screen on which an image of lines of character data is created by repeated sequential raster scanning.

According to the invention in its broadcast aspect the control means for such a VDU comprises a store for receiving the characters and character attributes to be displayed together with information for controlling the manner in which the data is to be displayed, which store also receives control information for controlling in a digital manner the way in which the stored character data is displayed.

Before describing an embodiment of the invention, reference will be made to Figure 1 of the accompanying drawings which is in the form of a block diagram of a known control circuit for a monitor unit incorporating a cathode ray tube.

The control circuit shown in Figure 1 comprises a main processor unit (MPU) 1 that processes data from a source such as a keyboard (not shown) and feeds it into a cathode ray tube (CRT) controller (shown in the block 2) which in turn controls a monitor 3.

The CRT controller 2 comprises four basic areas of logic circuitry, namely: a) A Read/Write memory (RAM) 4 in which are stored simple binary codes representing the charac tersigraphics and allocated attributes to be displayed. They are generally stored in the same order as they are to be displayed on the cathode ray tube (not shown).

b) Timing counters (block 5) that divide down the output of the basic high frequency oscillator. The various counts decoded are used for overall logic control, for positioning the finally displayed characters and for creating the necessary synchronising pulsesforthe monitor 3, c) A character generator 6 in the form of a ROM and a parallel to serial convertor 7 for translating the simple codes received from the RAM 4, into serial data that creates the actual complex character shapes seen on the screen, and d) An attribute logic generator 8 that creates the various attributes such as highlight, reverse video (black on white) and blink. The attributes are associated with single character cells.

The raster-scan monitor 3 is fed with horizontal and vertical synchronisng signals by the timing counters 5 and receives the combined outputs of the generator 8 and the convertor 7 via a mixer and synchroniser 9. The monitor 3 comprises a cathode ray tube together with deflection coils and all the circuitry necessary to produce the various voltages that drive the tube. The basic control circuitry in the monitor 3 consists of: a) A video amplifier to convert the small amplitude video signal produced by the CRT controller 2 into a signal fed to the tube cathode for modulation of the electron beam, and b) Synchronisation and deflection circuitry accepting vertical and horizontal synchronisation pulses from the controller in order to build up the CRT raster lines which are modulated to build up the steady character shapes.

An embodiment of the invention will now be described by way of example with reference to Figure 2 of the accompanying drawings which shows diagrammatically the basic address and data flow of a control circuit to replace that of the type shown in Figure 1, like reference numerals being used where appropriate. Control paths have been omitted for clarity.

In the following text "line" will refer to a single raster line and "row" to a group of raster lines that are needed to make up a row of characters. Each raster line in a row contains different parts of the characters that are thus built up for display.

Block 10 is a two-way buffer used by the MPU 1 to write or read data into or from a controller RAM 4 a.

The latter may be physically similar to or even identical to RAM 4 of Figure 1 but it functions to contain not only the character and character attribute data but also information for controlling in a digital manner the way in which the stored character data is to be displayed.

Block 11 is a multiple 3 to 1 multiplexer. The MPU address is fed to the RAM on any MPU read or write operation. A character address is fed to the RAM at all times that the raster is modulated to produce characters and a row address is fed to the RAM in order to obtain row, frame and controller set-up information. The row addresses are fed to the RAM during horizontal and vertical retrace times when no video data is being sent to the monitor 3.

Block 12 is a series of buffers that are clocked (staticised) for either a character clock period, a row period or a frame or sub-frame period. A sub-frame period is a multiple row period, for instance an attribute could be set up on the second displayed row and reset on the eighth displayed row. The letter 'C' denotes character rate information, 'R' denotes row rate information and 'F' denotes frame or sub-frame information.

Controller set-up information such as the number of displayed characters per row, the number of raster lines that make up a character row, the number of rows perframe and the positioning and lengths of the horizontal and vertical synchs are all programmed by the MPU on system initialization.

The controller can therefore be fully software programmable to suit user and various monitor requirements.

Character Generator 6 is the read only memory that stores the character matrix shapes. For all raster lines on any particular row the character decodes are presented to the ROM. The parts of character shapes for any particular raster line in a row are selected by the line address in the row. On incrementing the line address from zero to the number of selected lines per row, the character shapes for the row are established in the form of dot matrices.

The parallel to serial convertor 7 converts the parallel output of the character generator 6 into a serial form which (when mixed and synchronised with any selected attributes in the mixer and syn chroniser 9) is the CRT controller video output to the monitor 3.

Block 13 is a programmable counter clocked by a crystal oscillator 21. The counter counts the number of dots that make up the width of any particular character cell. For equally spaced characters, the dot counter is programmed to a count value on initialization. For a proportionally spaced character set, the dot counter can be programmed on a character by character basis either from character width information stored in the RAM 4 as shown in the diagram, or from a separate ROM (not shown) containing the character widths for each character and having, as its address inputs, the character codes.

Block 14 is a series of programmable counters and associated decode logic also clocked by crystal oscillator 21 which are set up according to the number of characters per row and the requisite horizontal blanking and synch timings required by the monitor 3. The counters also provide control signals for clocking the character row and frame buffers and other parts of the controller.

Block 15 consists of two programmable counters, both being set up on controller initialization. The line counter is incremented at the beginning of each horizontal blanking period and counts from zero to the pre-programmed number of lines per row for each row. The row counter counts from zero to the number of pre-programmed rows per frame plus an extra count necessary for monitor vertical retrace blanking period.

One of the most valuable facilities available from a VDU provided with control means according to the invention is that of "Soft Scrolling". For a digitally generated raster data display, soft scrolling is the ability to move data at different speeds and directions through a visual display field, without the observer perceiving any discrete steps of movement. This is achieved by changing the data field at a rate faster than the combined persistence of the eye and the screen phosphor in use. For an observed flicker-free screen, this rate will be at the frame refresh rate or faster.

Character counter block 16 and its row attribute programming allows easy horizontal and vertical soft scroll manipulation of sections of the display field. The counter can be programmed to point to any single character address in the RAM and then to be incremented by a value equal to the number of characters per row, before being reset by block 14.

This process enables any section of the RAM representing a row of characters to be displayed in any row position on the screen. During each horizontal blank time, an actual sequential row address originating in block 15 is fed to the RAM and the value read is used to point the character counter to any section of the RAM 4 in order to obtain the displayed row information. The procedure used to accomplish a row by row or 'hard' scroll of any area of the display field, is to alter the sequence of the row start addresses stored the RAM locations which are pointed to by the actual sequential row addressed.

Block 17 enables soft horizontal (or lateral) scrolling. Soft horizontal scroll can be achieved in either direction for any single row or a combination of rows. The adder adds the row attribute to the dot count 13 and the carry decode increments the character counter 16. In the case of a steady state display, the row attribute is zero. The procedure for scrolling left on a display with a selected 9 dots wide character cell is as follows: 1. At the end of a frame (beginning of vertical synch), the row attribute is made equal to 1. This moves the characters left by one dot with respect to the steady state display window on text created by block 14.

2. At the end of the next 7 frames, the row attribute is incremented to a final value of 8. This moves the row left until only one dot of the first 9 dot wide character cell is displayed.

3. To complete the left soft scroll of one character width, the row attribute (at the end of the next frame) is reset to zero and the row start address fed to block 16 is incremented by 1 for that row.

To soft scroll right, the process is reversed. The row start address is first decremented by 1 at the same time as setting the row attribute fed to block 17 to a value of 8. The row attribute is then decremented to zero on the following 8 end of frame blanking periods.

Blocks 18 and 19 provide soft vertical scrolling of a frame of sub-frame in either direction. A frame or sub-frame attribute is fed to the adder block 18 and added to the line address. The carry decode from block 18 is fed to block 19 to increment the row address. Let the frame or sub-frame attribute be called the raster addition count. The procedure for upward soft scroll of rows pre-selected to having 14 lines per row is as follows: 1. At the end of each frame, increment the raster addition count by 1 until a value of 13 is reached.

2. On the next end of frame blank period, reset the raster addition count and perform a hard scroll up by altering the sequence of the row start addresses fed to block 16.

For soft scroll down, first effect a hard scroll down and simultaneously set the raster addition count to 13. This will drop the area enabled for soft scroll by one raster line. The raster addition counts are then decremented to zero.

Faster speeds can be obtained by increasing the raster addition count to more than 1.

Block 20 provides the attribute logic. Row attributes can consist of row blank, row highlight etc.

Some frame attributes could be frame brightness (allowing an operator to alter the monitor brightness from say a keyboard) and frame formats.

It is to be noted that both vertical and horizontal scrolling can be performed selectively so that part of the displayed text (for example, a heading) can be left in position while scrolling of another part of the display is performed.

The word "character" as used in this specification and claims is not to be regarded as limited solely to alphanumeric characters.

Claims (6)

CLAIMS 1. Control means for a visual display unit of the kind incorporating a cathode ray tube having a display screen on which an image of rows of character data is created by repeated sequential raster scanning, comprising a store for receiving the characters and character attributes to be displayed, which store also receives control information in digital form that permits part of the stored character data to be retained in position on the screen while the position of the remainder of the character data on display is caused to be modified. 2. Control means as claimed in claim 1, in which the control information is effective to cause one or more rows of stored character information to be moved perpendicularly with reference to the raster by increments of one raster line at a time. 3. Control means as claimed in either of the preceding claims in which the character data is stored in the form of dot matrices and in which the control information is effective to cause one or more rows of stored character information to be moved parallel to the raster by increments of distance equal to the distance between successive dots. 4. Control means as claimed in any preceding claim which is interposed between a main processor unit for processing data from source such as a keyboard, and a raster-scan monitor for the cathode ray tube, the control means comprising a read/write memory (RAM) for storing characters and control information supplied by the main processor unit, together with other logic circuitry to provide full digital control ofthe monitor. 5. Control means for a visual display unit substantially as described with reference to Figure 2 of the accompanying drawings. New claims or amendments to claims filed on 27 October 1981 Superseded claims 1 to 5 New or amended claims 6: CLAIMS

1. Control means for a visual display unit of the kind incorporating a cathode ray tube having a display screen on which an image of rows of character data is created by repeated sequential raster scanning, comprising a store for receiving the characters and character attributes to be displayed, which store also receives control information for controlling in a digital mannerthe way in which the stored character data is displayed

2. Control means as claimed in claim 1, in which the control information is effective to cause part of the stored character data to be retained in position on the screen while the remainder of the character data on display is modified.

3. Control means as claimed in claim 1 or 2, in which the control information is effective to cause one or more rows of stored character information to be moved perpendicularly with reference to the raster by increments of one raster line at a time.

4. Control means as claimed in any one of the preceding claims in which the character data is stored in the form of dot matrices and in which the control information is effective to cause one or more rows of stored character information to be moved parallel to the raster by increments of distance equal to the distance between successive dots.

5. Control means as claimed in any preceding claim which is interposed between a main processor unit for processing data from a source such as a keyboard, and a raster-scan monitor for the cathode ray tube, the control means conprising a read/write memory (RAM) for storing characters and control information supplied by the main processor unit, together with other logic circuitry to provide full digital control of the monitor.

6. Control means for a visual display unit substantially as described with reference to Figure 2 of the accompanying drawings.