GB2274535A

GB2274535A - Grey-scale large screen display

Info

Publication number: GB2274535A
Application number: GB9301509A
Authority: GB
Inventors: Peng Seng Toh; Wee Leng Goh
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-01-26
Filing date: 1993-01-26
Publication date: 1994-07-27
Anticipated expiration: 2013-01-26
Also published as: GB9301509D0; GB2274535B

Abstract

A display 21 comprising a plurality of incandescent lamps and having grey scale capability in which the brightness of the lamps is determined by an "ordered table" 16 i.e. a device which stores the location of each lamp in the display in an order sorted in accordance with the desired brightness of the lamp. A sampled video signal is stored in a sample memory 14 which represents the location of a pixel in the display. The pixels are then sorted by a sorter 15 into different brightness bins in the ordered table. The ordered table is sequentially accessed beginning with highest brightness element by means of a counter 18 when the counter is addressed by a zero-crossing signal. The content of the ordered table is then sent to the row and column decoders 19 and 20 to activate the appropriate lamps by turning on an associated TRIAC. <IMAGE>

Description

GREY-SCALE LARGE SCREEN DISPLAY This invention relates to an improved technique of producing grey-scale images on a large screen display consisting of a large number of incandescent lamps. The invention is generally used in conjunction with solid-state power electronics devices that control the power delivered to the incandescent lamps.

Background of the Invention A large screen display is a very effective means of visual communication for a large audience. It finds applications in entertainment, transport, sport, conference trade show, advertising among others. The size of a large screen display is generally in the order of several metres both in width and height.

Large screen displays are commonly configured from array of discrete lamps, projected LCD images and video-walls. Each of these displays is suitable for different occasions and ambient light conditions. The radiant output of a projected LCD image is rather weak and is most suitable for in-door use where the ambient light level is low. The video-wall comprises a large number of Cathode Ray Tubes (CRT) fonning a display array. It has medium contrast and can be used in rather bright ambient conditioned such as a well-lit hall but is not bright enough for displaying outdoor. The array of discrete lamps is most suitable outdoor as it has the highest contrast and will be visible even under strong sunlight.

The discrete lamp array is predominantly used to display binary images or images with only two brightness levels: black and white. It is well known that a grey-scale image will contain much more information and will appear to be more realistic to viewers than a binary image. A grey-scale image is capable of displaying more than just two levels of brightness. A grey-scale image that displays at least 64 levels of brightness will appear to be continuous and realistic to a viewer. It is possible to control the brightness of each of the lamps in the large screen display so that grey-scale images can be shown. Brightness is the output radiant of a lamp and is controlled by the electrical power supplied to it. The electrical power can be adjusted by controlling the conduction angle of the supply AC voltage to the lamp.The means to control the conduction cycle of the supply voltage can be accomplished by solid-state power electronics devices such as a TRIAC. A controlled amount of power can be delivered to the load by turning on or firing the TRIAC at a suitable phase angle (delay angle) relative to the zero-crossing of the supply voltage. Zero-crossing refers to the instance when the AC supply voltage crosses the zero voltage level or whenever the AC voltage changes from positive to negative or from negative to positive.

A direct implementation for controlling the brightness of a incandescent lamp is illustrated in Fig. 1. Generally, such a method of implementation will require a number of digital integrated circuits including registers 1 and counters 2, in conjunction with power electronic devices (SCR, TRIAC 4) for controlling the brightness of a lamp 5.

In the case of a large screen display, there are a large number of lamps involved which will require a large number of digital circuits. The number of wires and connections in the large screen display will also be huge. The cost of such a display will be high and its reliability low.

Summary of the Present Invention According to one aspect of the present invention is that the brightness of lamps in the large screen display is controlled by an ordered table which contains the addresses of lamps in an order sorted according to their brightness. The order of the ordered table determines the delay angle in firing the TRIAC of a particular lamp. The address of the lamp that has the maximum brightness in the display is placed higher up in the said ordered table while the address of the lamp that has the minimum non-zero brightness is placed nearer to the bottom of the said ordered table. The addresses of the remaining lamps that have zero brightness will not be placed on the ordered table. The ordered table will be sequentially accessed to retrieve the addresses of the lamps.The address of a lamp will be decoded to activate a switch so that a firing pulse is sent to turn on the lamp. The scanning of the ordered table is synchronised by the zero-crossing signal detected from the AC supply voltage.

According to another aspect of the invention, the large screen display can be divided into a number of smaller arrays of reasonable size. Each of these smaller arrays is associated with an independent ordered table. All the ordered tables for each and every sub-array will be concurrently scanned to generate a scale grey image.

According to another aspect of the invention, if a binary image instead of a grey-scale image is to be displayed, the scanning sequence of the ordered table will be alternated in synchronism with the AC supply voltage. In other words, the scanning sequence of the ordered table will be top-down in the first half-cycle and reversed in direction during the next half-cycle. In so doing, the brightness of the lamps that are being turned on will appear to be uniform.

Brief Description of the Drawings The present invention is described with the accompanied drawings: Fig. 1 shows the block diagram of the direct or conventional method of controlling the brightness of a lamp.

Fig. 2 shows the overall block diagram of the present invention.

Fig. 3 shows the voltage waveform across the lamp, the zero-crossing signal and the trigger pulse which determines the delay angle in firing the TRIAC.

Fig. 4 shows an example of a grey-scale image histogram and the ordered table.

Fig. 5 shows the division of a large screen display into a plurality of sub-arrays.

Fig. 6 shows the relationship between the power delivered to a lamp and the conduction angle of the TRIAC in a full-wave configuration.

A Specific Embodiment of the Present Invention The overall block diagram of the present invention is illustrated in Fig. 2(a). The video signal 11 generated by a video source or other graphical display means will be sampled by the re-sampler 12 according to the number of picture elements in the large screen display 21. The sampled video signal is stored in the sampled memory 14 after passing through a Look-Up-Table 13. The sample memory 14 is arranged into a one dimensional array that contains the brightness of each and every picture element in the sample image. The order of the sampled memory 14 represents the location of a picture element 22 in the large screen display array 21. The picture elements in the sampled memory 14 are then sorted by the sorter 15 into different brightness bins in the ordered table 16 according to their brightness. The ordered table 16 contains the addresses of the picture elements 22. The address of each and every picture elements is represented by row and column pointers, or X and Y pointers. Fig. 2(b) is the detailed block diagram of a picture element including a lamp 5, the pulse generator 25, switch 24, trigger 3, TRIAC 4 connected to the AC supply 10. Whenever a picture element is addressed, the switch 24 will be turned on so that the firing pulse generated by the pulse generator 25 will be sent to the trigger 3 to fire the TRIAC 4.

The ordered table 16 will be sequentially accessed beginning from the highest brightness picture element. When a location of the order table 16 is accessed, the content of the ordered table which is the row and column pointers (X and Y) of a particular picture element 22 will be retrieved. The row and column pointers will be decoded by the row and column decoders 19 and 20 respectively to close the switch 24. Retrieval of the content of the ordered table 16 is accomplished by a counter 18 which sequentially addresses the ordered table 16. The counter 18 can be activated to count up or count down. In displaying grey-scale images, the counter 18 will be programmed to count down beginning at a count that points to the top of the ordered table. The total number of elements in the ordered table 16 is much higher than the total number of picture elements 22 in the large screen display 21.The ordered table 16 is a non-compact table where elements that have not been occupied will be pointing to NULL. Ali the elements in the ordered table 16 will always be initialised to NULL before the arrival of a new image. The counter 18 is enabled to count by the ZERO CROSSING signal 23. The ZERO-CROSSING signal 23 is detected by the zerocrossing circuit 17. A ZERO-CROSSING signal 23 is generated when the AC supply voltage 10 changes from negative to positive or from positive to negative cycles as shown in Fig. 3. In other words, a ZERO-CROSSING signal 23 is generated whenever the AC supply voltage 10 passes the zero voltage level.

The function of the Look-Up-Table 13 is to correct for the non-linearity between the TRIAC 4 delay angle and the power delivered to the lamp as depicted in Fig. 6. The brightness of a lamp 5 is directly proportional to the electrical power supplied to it.

The amplitude of the video signal 11 which represents the brightness of the lamp 5 will be modified by the Look-Up-Table 13 so that its relationship with the delay angle of the TRIAC 4 will be linear. For those skilled in the art, the construction of a Look-Up Table 13 can be implemented with memory using the video signal 11 as address and the data output as the linearity-corrected video signal.

The function of the ordered table 16 can be explained by means of an example illustrated in Fig. 4. An image can be characterised by a histogram which indicates the frequency of occurrence or the number of picture elements at different brightness levels. The vertical axis of the example histogram is the frequency of occurrence and the horizontal axis is the discrete brightness levels from 0 to L. A binary image will only occupy two brightness levels without any intermediate levels. The example histogram is sorted and arranged into the example ordered table shown in Fig. 4(b).

The example ordered table has a total number of elements equal to the product of L and b where b is the number of locations allocated to a particular brightness level. In this example, the number of picture elements at brightness level L is 7, it is then ordered into 7 consecutive memory locations in the example ordered table beginning at location Lb, followed by Lb-l, Lb-2 Lb-6. Each of the memory location in the ordered table contains the row and column pointers of the corresponding lamps in the large screen display array 21. The next brightness level, L-1, has 5 picture elements and addresses of which will be stored in the ordered table at 5 consecutive locations beginning at (L-l)b. The memory locations between Lb-6 and (Ll)b will be pointing to NULL.Similarly, the next lower brightness level will be stored in the next lower memory locations in the ordered table. The process of arranging the ordered table will be completed upon reaching lowest brightness level which is 1.

The ordered table 16 will be sequentially addressed by the ordered table counter 18 which will begin to retrieve the content of the ordered table 16 when triggered by a zero-erossing signal 23. The content of the ordered table 16 which is the address of the lamp 5 will be sent to the row and column decoders 19 and 20 to activate the lamp 5 by turning on the TRIAC 4. When a lamp 5 is turned on, it will continue to be on until the AC supply voltage 10 reaches zero voltage. The addressing of the ordered table 16 will continue until the entire ordered table 16 is scanned and will be repeated at the next zero-crossing signal 23.

It is further provided by the present invention that a large screen display array 21 can be divided into a plurality of smaller arrays called the sub-array 26. This is provided to take care of displays that contain a huge number of picture elements. An illustration of dividing a large screen display a plurality of smaller arrays is shown in Fig. 5. Each sub-array 26 is associated with a ordered table 29 which contains addresses of lamps residing within the sub-array 26. The image splitter unit 30 is responsible for dividing the linearized video signal into separate paths that will be stored in separate sub-array memory 27. Each sub-array memory 27 will be sorted by the sorter 28 before being stored in the corresponding ordered table 29. The sub-array ordered table 29 will be similarly scanned to retrieve the addresses for activating the lamps within the sub-array 26.

Claims

1. This invention is a technique of displaying grey-scale images on a large screen display consisting of a large number of incandescent lamps powered by the AC supply through TRIACs where the brightness of lamps are controlled by an ordered table which contains the addresses of lamps in an order sorted according to their brightness; the order of the said ordered table determines the delay angle in firing the TRIAC of a particular lamp; the said ordered table is sequentially accessed to retrieve the addresses of the lamps to activate a switch so that a firing pulse is sent to turn on the TRIAC and hence the lamp; the scanning of the ordered table is synchronized by the zero-crossing signal detected from the AC supply voltage.

2. The grey-scale large screen display as claimed in claim 1 where the large screen display is divided into a number of smaller arrays of reasonable size and each of the smaller arrays is associated with an independent ordered table with all the ordered tables concurrently scanned to generate a compiete scale grey image.

3. The grey-scale large screen display as claimed in claim 1 where the address of the lamp that has the maximum brightness in the display is placed higher up in the said ordered table while the address of the lamp that has the minimum non-zero brightness is placed nearer to the bottom of the said ordered table; the addresses of the remaining lamps that have zero brightness will not be placed on the ordered table.

4. The grey-scale large screen display as claimed in claim 3 where in the case of displaying a binary image instead of a grey-scale image, the scanning sequence of the ordered table will be alternated in synchronism with the supply AC voltage such that the scanning sequence of the ordered table will be top-down in the first half-cycle and reversed in direction during the next half-cycle.