GB2332582A

GB2332582A - Large screen television display system

Info

Publication number: GB2332582A
Application number: GB9726605A
Authority: GB
Inventors: Richard Murray-Shelley; Philip David Murray-Shelley
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-12-18
Filing date: 1997-12-18
Publication date: 1999-06-23
Also published as: GB9726605D0

Abstract

A large screen video display system consists of a matrix of light emitting diodes (LEDs) with usually one red, one green and one blue making up each pixel. The pixel pitch might typically be 25mm. The current through each LED is controlled in a continuous way rather than the LED's brightness being controlled by pulse width modulation techniques. The control signal necessary to set the current through each LED - and thus its brightness - is derived by sampling the incoming video waveform at a suitable instant. The sampling point is derived with reference to the physical position of the LED within the display and to the line and frame synchronising signals in the video waveform.

Description

2332582 LARGE SCREEN TELEVISION DISPLAY SYSTEM This invention relates to a

large screen television or video display system using light emitting diodes (LEDs) as the display elements.

Conventional television receivers and video monitors, such as those used in the home or office, usually use either a cathode ray tube or a liquid crystal display as the device on which the picture is produced.

In some cases there is a need to show television pictures to large audiences - at sporting events, for example - and then the screens are very large, often having display areas of some tens Of M2. Traditionally such screens are made using small individual cathode ray tubes with three such tubes, one red one green and one blue, making up a single picture element or "pixel". The number of pixels which form the entire display depends on the screen resolution and, for very large arrays the number of lines used is usually substantially less than the 625 employed in domestic television receivers. Nevertheless a large display can contain over 150,000 cathode ray tubes.

Such displays are cumbersome, expensive and consume large amounts of power. Replacing the cathode ray tubes with LEDs offers many advantages, especially since improvements in LED technology mean that they can now rival the light output of the small cathode ray tubes previously used. However the drive circuitry associated with each LED-based pixel remains complex since the brightness of each LED or group of LEDs is usually controlled using pulse-width-modulation (PWM) techniques. Each LED is operated "digitally" being either fully on or fully off at any given time and 1 variations in apparent brightness are produced by controlling the ratio of on to off time. A number of commercial large screen displays are available using this or similar techniques.

According to the present invention there is provided a matrix of LEDs arranged in rows and columns of pixels where each pixel contains one or more red, one or more green and one or more blue LEDs. At any instant the brightness of any LED or group of LEDs is determined by the current passing through it since a small current will produce a low light output which will increase as the current is increased. This current is, in turn, determined by taking a sample of the video voltage waveform relevant to the position of the pLxel within the array and converting it to a corresponding current through the LED(s). Since the uaperture time" (the time which is available during the television scan for each pixel to "see" the video waveform) is very small, each LED or group of LEDs is furnished with a form of "sample and hold" circuit which enables it to remain illuminated at the appropriate level until the pixel is next scanned. Each pixel is suppliedwith timing waveforms which are themselves derived from the normal television vertical and horizontal synchronising (sync) signals and these, when suitably gated together within each pixel's circuitry, control the instant at which the sample-and-hold circuits are triggered. A control unit allows the incoming video signals (which could comprise composite video or red/green/Mue and synchronising signals or any other suitable form) to be shaped as necessary to compensate for the colour characteristics of the LED display. In addition "special effects" such as preferentially enhancing one part of the picture can be produced.

A specific embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows the functions contained within a single pixel.

2 Figure 2 illustrates how several pixels are arranged within a single line.

Figure 3 shows the general arrangement of the entire display.

Referring to figure 1, the pixel has three identical channels for red (R), green (G) and blue (B) colours. Note, however that other forms using fewer or greater numbers of constituent colours are possible. The red video signal 1 is supplied to the input of a sample-and-hold circuit 2. The output of this circuit is fed to a voltage to current conversion circuit 3 and the resulting current drives the red LED 4. Similarly the green video signal 5 passes to its own sample-and hold 6 and thus to the voltage to current converter 7 and the LED 8. The blue channel is represented by the path g10-11-12.

The sample-and-hold circuits can take many forms though the use of integrated circuits for this application is preferred. Similarly the voltage to current converters would normally exist as operational amplifiers but many specific circuits could be employed.

The sampling gates of the sample-and-hold circuits are held open for a very short period (typically less than two microseconds per frame). The sampling pulse required to achieve this operation is derived from a gate associated with the pixel. This need be nothing more than a simple AND gate and combines a "line scan pulse" 15 with a line select pulse 14. The derivation of these pulses is discussed below. The time constant associated with each sample-and hold circuit - which dictates how the sampled voltage decays with time - is selected to produce an aesthetically pleasing picture.

3 In one embodiment of the invention, the pixels are manufactured on one or more printed circuit boards with a spacing which depends on the overall display size and the resolution required. A typical but not essential spacing might be about 25mm. The spatial relationships of the red, green and blue LEDs (or groups of LEDs) can be important depending on the actual characteristics of the devices used. Arrangements where the LEDs are stacked vertically, horizontally or in any other appropriate pattern are possible. It is not necessary to ensure that the same number of LEDs are used to represent each colour within the pixel. Thus it might be appropriate to use one green, one blue and two red LEDs for example.

Figure 2 shows the arrangement of three pixels, 1, 2 and 3, within a single row. Each is driven by the common red 7, green 8 and blue 9 video signals. 10 is the line select pulse which is used to enable all of the pixels within the particular row of interest at any time. This will remain it its "seleef state for the whole of the line scan time. The line scan pulses 11, 12 and 13 are produced from outputs from a shift register 6. At the start of the line scan this shift register will be cleared and a single bit 4 will be inserted into one end. As the scan progresses a shift clock 5 operates and causes this one bit to travel from one shift register location to the next thus activating the pixel sample-and-hold circuits one by one. The shift register - which could take the form of coupled integrated circuits or any other form - has one output for every column of pixels within the display. In the preferred embodiment of this invention there is only a single shift register but other forms are possible whereby a shift register could be used on every line or group of lines. Though the diagrams indicate stpositive logic" for the control pulses, this is not essential and any logic format could be used. Buffering of logic signals and amplification of the video signals Will be necessary as appropriate in accordance with normal electronic design practice.

4 The line select pulse is produced either using a second shift register or by other means and is synchronised to the frame and line synchronising pulses derived from the video source. The exact manner by which this pulse is generated will depend on whether the display is to show interlaced or non-interlaced signals. In the interlaced case, line 17 (say) will be scanned immediately after line 15 and the even numbered lines will be filled in later. This invention is applicable to either format. With most displays built using the principles of this invention, the number of display lines will be less than the broadcast standard. The preferred method of generating the line select pluses will thus involve using every second, third or other multiple of the lines available. Thus if, for example, a display had a resolution of 150 lines and the incoming video was at 625 lines per frame, the line select pulse system would take every fourth line, discarding the other three. The preferred method of choosing lines to be used to provide signal data would involve the use of a microprocessor but other techniques are possible.

Figure 3 describes the overall arrangement of the system. The pixel array 1 is supplied with red, green and blue video signal (6,7 and 8) and power 9. The shift register 2 (or similar arrangement) provides the line scan pulses whilst shift register 3 (or similar) provides the line select pulses. In the event of interlaced operation being required, the preferred embodiment would be to use two shift registers for 3, one driving even lines and one driving odd lines. The control unit 4 contains circuitry and systems to generate the required control pulses from the video synchronising information 5. The video processing unit, 10, may, K required, produce the separate red, green and blue signals and synchronising information from the composite video input 11. It may also modify the video signals to compensate for the colour and brightness characteristics of the LEDs in use in a particular display,

Claims

We claim:

A video display system comprising a matrix of light emitting diodes (LEDs) arranged into groups of three LEDs - one red, one green and one blue each group making up a single pixel within the display in which the light output from each LED is determined by controlling the current through it in a continuous manner. The control voltage for each LED is derived by periodically sampling the video waveform being displayed at a time synchronised to the line and frame synchronising signals provided by the video source having regard to the spatial posilon of the particular LED within the complete display.

2 A video display system as claimed in Claim I but which uses more than one LED of the same colour in some or all of the groups making up a pixel.

3 A video display system as claimed in Claim 1 and Claim 2 which uses fewer or more than three basic colours - for example a system using only two colours per pixel point such as red and cyan.

A video display system as claimed in any preceding Claim where one or more shift registers are used in either or both axes of the display to provide the necessary synchronising pulses to operate the pixel sampling circuits.

6 A video display as claimed in Claim 4 where alternative means other than shift registers are used in either or both axes of the display to provide the necessary synchronising pulses to operate the pLxel sampling circuits.

6 A video display system as claimed in any preceding claim which includes facilities to select sections of the incoming video signal and thus display only portions of the complete video picture.

A video display system as claimed in any preceding claim which is able to operate with both interlaced and non-interlaced video signals and which can be adapted to a variety of line standards and video formats.

8 A video display system as claimed in any preceding claim which includes provision for audio presentations.

9 A video display system as claimed in any preceding claim whicP is manufactured in such a way as to be easily transportable using a modular structure.

A video display system which is substantially as described herein and whose principles are illustrated in Figures 1-3 of the accompanying drawings.

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