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Simulation of liquid crystal colour TV displays
Simulatien of liq.._id crystal displays
TV
K G FREEMAN AND P T ROGERS*
Colour liquid crystal displays are now being widely developed for consumer television. To assist this task, Philips is using a computer-based image-processing system to simulate their appearance on the screen of a high-resolution cathode-ray tube. This has enabled comparison of the merits of different cell arrangements and drive techniques without having to construct actual devices. Features that can already be simulated include the intrinsic appearance of various colour cell arrangements in direct view displays, and the consequences of operating such dL,,,playsin both halfand full-resolution modes. It has therefore been possible to examine the spatisl and temporal artefacts which can occur in each case on still and moving pictures and text, not only for uncoded (i.e. direct) red, green and blue signals, but also for the decoded PAL signals which exist in a normal colour receiver. The software also allows comparison of the effects of different transfer characteristics (gamma) and different display primary colours. As a result it has been possible to show for a direct view display not only that a horizontally-staggered arrangement of triads of red, green and blue cells is intrinsically more acceptable than other arrangements (confirming earlier work at the Institute for Perception Research), but also that this arrangement leads to much less disturbing signal/drive artefacts. This work is now being extended to include the simulation of liquid crystal projection displays. Keywords: colour, LCDs, consumer television, simulation
The successful development of liquid crystal colour television displays for the consumer market requires the investment of considerable resources and a number of crucial design decisions. Not only does the manufacturer have to select the most suitable liquid crystal material and the most promising technology for the matrix drive circuits; he also has to choose the size and spatial arrangements for the red, green and blue cells and the method of driving them which will give acceptable picture quality, free from disturbing structure and artefacts, on still and moving pictures and text.
record the consequences of different combinations without having to construct hardware. Using a computer-based television image-processing system already available at PRL, Philips have therefore developed a method of simulating the visual appearance of liquid crystal colour displays on a high-resolution shadowmask cathode-ray tube (CRT). As far as practicable, this emulates the relative size, shape and spatial arrangements of the red, green and blue cells, their colour and transfer characteristics and the way in which the video information is applied to them.
To investigate merely the most promising options by actually constructing different displays and drive circuits would still involve substantial time and effort. Both could be saved if it was possible to predict and
The following list indicates the topics we can already examine by simulation:
Philips Research Laboratories, Cross Oak Lane, Redhill, Surrey RH1 5HA, UK * SGS Services, Camberley, Surrey, UK This material was originally presented in verbal form at the S1D Symposium on European Display Activity held in York, UK, 12-13 September 1989. 16
• • • •
Coiour cell arrangements, 'half-' and 'full-resolution' displays, effects with RGB, PAL & MAC signals*, temporal effects with moving pictures.
*RGB = Red, Green, Blue; PAL is the standard television system used throughout most of Western Europe; M A C is the Muhiplcxcd Analogue Components system for satellite broadcasting in Europe.
0141-9382/91/010016-04 ~) 1991 Butterworth-Heinemann Ltd
DISPLAYS, JANUARY 1991
These can be studied either separately or in combination, which may, of course, reveal artefacts that are not otherwise immediately apparent. Not surprisingly, first on the list is the choice of cell size and shape and the way in which the cells are spatially arranged, since, for a given viewing distance this determines the extent to which the intrinsic structure is visible or disturbing. However, we must also take into account how the display is driven. So far, virtually all small liquid crystal displays (LCDs) for television which have appeared on the market have been half-resolution displays, containing about 280 rows of cells. These are normally driven by successively applying to each row the video information from adjacent pairs of lines in the interlaced signal raster, i.e. the two interlaced fields of the standard TV signal are effectively superimposed. For a full-resolution display with, say, 560 rows of ceils one might expect to use normal interlaced scan, but unfortunately the storage effects which occur with LCDs when driven in this mode give rise to unacceptable effects with moving objects.I It is therefore necessary to examine alternative drive modes capable of full- or nearly fullresolution performance which do not produce such undesirable artefacts. One can also compare the effects which occur with uncoded red, green and blue signals from a laboratory source with those which arise in a real television receiver using P A L (or MAC) coded signals. For example, does the information sampling by a particular cell structure and drive mode improve or further degrade the coarse colour-patterning on fine-luminance detail (cross-colour), which is an intrinsic feature of the P A L system? Finally, one must take care in each case to examine what happens with both still and moving pictures and text, especially since the combined effect of any spatial and temporal artefacts is difficult to predict.
SIMULATION PROCEDURE Figure 1 outlines the stimulation system. First, one or more frames of a 625-1ine television sequence are captured and stored as digital information in a hard disk memory associated with a VAX computer. This is then processed off-line under the control of the simulation program to give an output signal suitable for a 1200-line high-resolution shadowmask display. The following list shows the procedure in more detail.
• Store 625-1ine signal as 720 × 576 bytes of Y and U/V. (See text below for definitions of Y, U and V.) • Create LCD row/column drive-mode RGB file. • Sample RGB file according to cell configuration. • Modify display coiour and 'gamma'. • Specify representation of individual cells by CRT pixels. • Create and read 1440 × 1200byte YUV output file. Those operations shown in italic take place in real time, whilst the others are performed off-line as software computations; for a 25-frame, 1 second, short moving sequence these can take as much as five hours to complete. First, the simulator takes the input 625-1ine video information and samples it 720 times along each of the 576 active lines to provide a file in which every TV frame is represented by 720 × 576 bytes each of full-bandwidth luminance (Y), and time-multiplexed reduced bandwidth U and V (B-Y and R-Y) colourdifference signal components. This form of signal coding was adopted for the simulator architecture to give more economical use of memory space and faster processing than would be possible with coding in terms of full-bandwidth red, green and blue signals. However, although more than sufficient to represent all the perceivable information content of a standard 625-1ine signal, this representation limits the minimum number of simulator picture elements (pixels) which may be used to represent the width of a single LCD cell. It also
VAX computer 12OO-line CRT 6 2 5 -line RGB video
t LCD simulation program Figure 1. Outline of LCD simulator DISPLAYS, JANUARY 1991
17
involves some additional signal conversions in the software. This digital information, which may represent a single frame of a still R G B picture or text, at least four frames of a decoded P A L signal, or several tens of frames of a moving sequence, is then stored, either on disk or a backup tape, until it is required for creating a new simulation. The first part of the off-line simulation program processes this information to create a file representing the R G B information for all the cell locations of the LCD being simulated (irrespective of their actual colour) and, equally important, the way in which the display is driven (i.e. whether it is a full or half-resolution display). Given the spatial colour cell arrangement, this information can then be sampled as appropriate to represent a particular design of directview LCD. Next, suitable corrections are incorporated which, as far as practicable, compensate for the differences between the transfer characteristic and primary colour points of the LCD and the shadowmask tube on which it is being simulated. Then the height and width of the rectangular C R T pixel array are specified, e.g. 5 × 4, used to represent each LCD cell. This is inevitably a compromise. On the one hand, we would like to represent each cell by as many C R T pixeis as possible so that, despite the finite size of the C R T spot, we obtain a good simulation of the comparatively sharp boundaries of the real LCD cells. On the other hand, we obviously wish to simulate as much of the LCD picture as practicable in order to make realistic assessments of the various artefacts which occur. For the time being, therefore, we have restricted our simulations to simple rectangles, although more complex shapes are in principle possible. An additional constraint when simulating multi-frame sequences is the maximum rate (54 Mbits/ s) at which the final output data needed to drive the C R T can be read from the hard-disk memories. Because of these two practical constraints the software therefore includes a 'window' facility, so that those areas of the original picture which are likely to exhibit unusual effects can be selected and examined. Finally, using the above cell specification, an output file is created in which each frame is represented by up to 1440 × 1200 bytes of Y and multiplexed UV information. This is stored until it is required to view the simulation on the sequentially-scanned 20 inch diagonal 1200-line C R T display, at which point it can be transferred to the display simulator disks. Because of the limited space on both the computer and simulator disk memories, which must be shared with other users, it is possible to store continuously in either location only a few tens of frames of simulations (though they can, of course, be backed up on tape). To facilitate subsequent immediate comparisons to be made, colour slide photographs are therefore taken of each simulation for record purposes. For sequences, photographs of single consecutive display fields are also taken to indicate any temporal changes which occur. However, it will be apparent that any colour and brightness flicker effects will only be perceived by viewing the actual simulation. 18
TEST M A T E R I A L For the validation tests described below and [k)r subsequent assessment and comparison of different simulations a variety of test material has been used. Still pictures have included the well-known electronically-generated Philips Test-Pattern, the 'zone plate' (a signal containing a continuous spectrum of horizontal and vertical spatial frequencies) and some electronically-generated text of different sizes. For real pictures (derived from a telecine) we have used four well-known slides used for high-definition television tests, namely Manhattan, Boats on a Shore, a seated couple (Richard and Jean), and Wheels. Moving material of up to 40 TV frames (one and a half seconds) duration has included a vertically-scrolled version of the electronic text, and horizontally-panned sequences of a moving girl and a moving car taken from a broadcast-quality video tape recorder (VTR). VALIDATION The validity of the simulation technique was assessed by comparing the appearance of a real LCD with its simulation. This proved a difficult task. Direct visual comparison of two displays having a number of subtle and not-so-subtle differences, especially size, was far from easy, even when the displays were positioned so as to subtend the same angle at the observer's eyes. To eliminate some of these difficulties, the two displays were therefore photographed and prints made which were balanced as well as possible in respect of grey-scale and colour and were identical in size to the picture on the actual LCD. Even this was not entirely successful since the multiple processes involved resulted in significant degradation of the image quality. Comparisons were therefore also made using colour slide film. None of these separate comparisons was entirely satisfactory on its own and time did not allow thorough subjective viewing tests to be carried out. Nevertheless, when taken together, we felt that the results showed the simulation technique to be a viable basis for comparing different cell arrangements and drive methods. Colour Plate 1 shows a photograph of the 6-inch half-resolution LCD used for these validation tests when displaying the Philips test-pattern. This has the diagonal cell arrangement shown in Colour Plate 2(a). Colour Plate 3(a) shows the corresponding simulation when each LCD cell is represented by a 5 high by 4 wide array of pixels on the 1200-line high resolution CRT. Despite the minor differences described above, the real and simulated displays are very similar, with the diagonal structure being the most noticeable feature. Note also that there are no obvious coloured aliasing artefacts. ALTERNATIVE
CELL ARRANGEMENTS
An alternative cell arrangement, again with equal numbers of red, green and blue cells, but with alternate rows now displaced horizontally by half a cell width, is DISPLAYS, J A N U A R Y 1991
shown in Colour Plate 2(b). The simulation of this is shown in Colour Plate 3(b). Since the cells of each coiour are now equally spaced in several directions, there is no obvious diagonal structure in plain areas of colour and the cell structure is therefore much less noticeable. However, since the red, green and blue cells are no longer arranged in straight columns, some aliasing now occurs in the horizontal direction. The higher-frequency resolution bars consequently give rise to coarse colour striations. Fortunately, the effect is not serious and can be ameliorated by limiting the bandwidth of the video information. Other cell arrangements are, of course, possible. For datagraphic displays, for example, it is often argued that twice as many green ceils as red or blue should be used because around 60% of the luminance information is carried by this colour in white areas. Our simulations suggest that this has doubtful advantage for television. Colour Plate 2(c) shows the most obvious (and probably best) arrangement of this type and Colour Plate 3(c) its simulated appearance for a half-resolution display. As is to be expected, the improved appearance of plain green areas is accompanied by a noticeable degradation in red and blue ones. Probably even more disturbing is that single horizontal white lines are reproduced as either yellow or cyan, depending on their vertical position in the raster, while
DISPLAYS,JANUARY 1991
horizontal high-frequency information produces more severe colour aliasing. More complex 'double-green' arrangements (e.g. that of Colour Plate 2(d), not surprisingly, just make things worse (Colour Plate 3(d))~
CONCLUSIONS Using the PRL TV Image Processor, a method has been developed for simulating the appearance of direct view colour liquid crystal television displays on the screen of a high resolution CRT. This has enabled investigation of the relative merits of different spatial arrangements of the red, green and blue cells from the points of view both of their intrinsic appearance and of the artefacts which arise when driven in half- and full-resolution modes with uncoded (direct RGB) and decoded (PAL), still and moving, pictures and text. As a result a valuable advice and support service is now provided to the Concern LCTV Development Group. This is expected to continue as the technique is developed.
REFERENCES Knapp A G and Powell M J 'The performance of a-Si active matrix LCTV displays' P R L Ann. Rev. (1989)
19
TV
K G FREEMAN AND P T ROGERS*
Colour liquid crystal displays are now being widely developed for consumer television. To assist this task, Philips is using a computer-based image-processing system to simulate their appearance on the screen of a high-resolution cathode-ray tube. This has enabled comparison of the merits of different cell arrangements and drive techniques without having to construct actual devices. Features that can already be simulated include the intrinsic appearance of various colour cell arrangements in direct view displays, and the consequences of operating such dL,,,playsin both halfand full-resolution modes. It has therefore been possible to examine the spatisl and temporal artefacts which can occur in each case on still and moving pictures and text, not only for uncoded (i.e. direct) red, green and blue signals, but also for the decoded PAL signals which exist in a normal colour receiver. The software also allows comparison of the effects of different transfer characteristics (gamma) and different display primary colours. As a result it has been possible to show for a direct view display not only that a horizontally-staggered arrangement of triads of red, green and blue cells is intrinsically more acceptable than other arrangements (confirming earlier work at the Institute for Perception Research), but also that this arrangement leads to much less disturbing signal/drive artefacts. This work is now being extended to include the simulation of liquid crystal projection displays. Keywords: colour, LCDs, consumer television, simulation
The successful development of liquid crystal colour television displays for the consumer market requires the investment of considerable resources and a number of crucial design decisions. Not only does the manufacturer have to select the most suitable liquid crystal material and the most promising technology for the matrix drive circuits; he also has to choose the size and spatial arrangements for the red, green and blue cells and the method of driving them which will give acceptable picture quality, free from disturbing structure and artefacts, on still and moving pictures and text.
record the consequences of different combinations without having to construct hardware. Using a computer-based television image-processing system already available at PRL, Philips have therefore developed a method of simulating the visual appearance of liquid crystal colour displays on a high-resolution shadowmask cathode-ray tube (CRT). As far as practicable, this emulates the relative size, shape and spatial arrangements of the red, green and blue cells, their colour and transfer characteristics and the way in which the video information is applied to them.
To investigate merely the most promising options by actually constructing different displays and drive circuits would still involve substantial time and effort. Both could be saved if it was possible to predict and
The following list indicates the topics we can already examine by simulation:
Philips Research Laboratories, Cross Oak Lane, Redhill, Surrey RH1 5HA, UK * SGS Services, Camberley, Surrey, UK This material was originally presented in verbal form at the S1D Symposium on European Display Activity held in York, UK, 12-13 September 1989. 16
• • • •
Coiour cell arrangements, 'half-' and 'full-resolution' displays, effects with RGB, PAL & MAC signals*, temporal effects with moving pictures.
*RGB = Red, Green, Blue; PAL is the standard television system used throughout most of Western Europe; M A C is the Muhiplcxcd Analogue Components system for satellite broadcasting in Europe.
0141-9382/91/010016-04 ~) 1991 Butterworth-Heinemann Ltd
DISPLAYS, JANUARY 1991
These can be studied either separately or in combination, which may, of course, reveal artefacts that are not otherwise immediately apparent. Not surprisingly, first on the list is the choice of cell size and shape and the way in which the cells are spatially arranged, since, for a given viewing distance this determines the extent to which the intrinsic structure is visible or disturbing. However, we must also take into account how the display is driven. So far, virtually all small liquid crystal displays (LCDs) for television which have appeared on the market have been half-resolution displays, containing about 280 rows of cells. These are normally driven by successively applying to each row the video information from adjacent pairs of lines in the interlaced signal raster, i.e. the two interlaced fields of the standard TV signal are effectively superimposed. For a full-resolution display with, say, 560 rows of ceils one might expect to use normal interlaced scan, but unfortunately the storage effects which occur with LCDs when driven in this mode give rise to unacceptable effects with moving objects.I It is therefore necessary to examine alternative drive modes capable of full- or nearly fullresolution performance which do not produce such undesirable artefacts. One can also compare the effects which occur with uncoded red, green and blue signals from a laboratory source with those which arise in a real television receiver using P A L (or MAC) coded signals. For example, does the information sampling by a particular cell structure and drive mode improve or further degrade the coarse colour-patterning on fine-luminance detail (cross-colour), which is an intrinsic feature of the P A L system? Finally, one must take care in each case to examine what happens with both still and moving pictures and text, especially since the combined effect of any spatial and temporal artefacts is difficult to predict.
SIMULATION PROCEDURE Figure 1 outlines the stimulation system. First, one or more frames of a 625-1ine television sequence are captured and stored as digital information in a hard disk memory associated with a VAX computer. This is then processed off-line under the control of the simulation program to give an output signal suitable for a 1200-line high-resolution shadowmask display. The following list shows the procedure in more detail.
• Store 625-1ine signal as 720 × 576 bytes of Y and U/V. (See text below for definitions of Y, U and V.) • Create LCD row/column drive-mode RGB file. • Sample RGB file according to cell configuration. • Modify display coiour and 'gamma'. • Specify representation of individual cells by CRT pixels. • Create and read 1440 × 1200byte YUV output file. Those operations shown in italic take place in real time, whilst the others are performed off-line as software computations; for a 25-frame, 1 second, short moving sequence these can take as much as five hours to complete. First, the simulator takes the input 625-1ine video information and samples it 720 times along each of the 576 active lines to provide a file in which every TV frame is represented by 720 × 576 bytes each of full-bandwidth luminance (Y), and time-multiplexed reduced bandwidth U and V (B-Y and R-Y) colourdifference signal components. This form of signal coding was adopted for the simulator architecture to give more economical use of memory space and faster processing than would be possible with coding in terms of full-bandwidth red, green and blue signals. However, although more than sufficient to represent all the perceivable information content of a standard 625-1ine signal, this representation limits the minimum number of simulator picture elements (pixels) which may be used to represent the width of a single LCD cell. It also
VAX computer 12OO-line CRT 6 2 5 -line RGB video
t LCD simulation program Figure 1. Outline of LCD simulator DISPLAYS, JANUARY 1991
17
involves some additional signal conversions in the software. This digital information, which may represent a single frame of a still R G B picture or text, at least four frames of a decoded P A L signal, or several tens of frames of a moving sequence, is then stored, either on disk or a backup tape, until it is required for creating a new simulation. The first part of the off-line simulation program processes this information to create a file representing the R G B information for all the cell locations of the LCD being simulated (irrespective of their actual colour) and, equally important, the way in which the display is driven (i.e. whether it is a full or half-resolution display). Given the spatial colour cell arrangement, this information can then be sampled as appropriate to represent a particular design of directview LCD. Next, suitable corrections are incorporated which, as far as practicable, compensate for the differences between the transfer characteristic and primary colour points of the LCD and the shadowmask tube on which it is being simulated. Then the height and width of the rectangular C R T pixel array are specified, e.g. 5 × 4, used to represent each LCD cell. This is inevitably a compromise. On the one hand, we would like to represent each cell by as many C R T pixeis as possible so that, despite the finite size of the C R T spot, we obtain a good simulation of the comparatively sharp boundaries of the real LCD cells. On the other hand, we obviously wish to simulate as much of the LCD picture as practicable in order to make realistic assessments of the various artefacts which occur. For the time being, therefore, we have restricted our simulations to simple rectangles, although more complex shapes are in principle possible. An additional constraint when simulating multi-frame sequences is the maximum rate (54 Mbits/ s) at which the final output data needed to drive the C R T can be read from the hard-disk memories. Because of these two practical constraints the software therefore includes a 'window' facility, so that those areas of the original picture which are likely to exhibit unusual effects can be selected and examined. Finally, using the above cell specification, an output file is created in which each frame is represented by up to 1440 × 1200 bytes of Y and multiplexed UV information. This is stored until it is required to view the simulation on the sequentially-scanned 20 inch diagonal 1200-line C R T display, at which point it can be transferred to the display simulator disks. Because of the limited space on both the computer and simulator disk memories, which must be shared with other users, it is possible to store continuously in either location only a few tens of frames of simulations (though they can, of course, be backed up on tape). To facilitate subsequent immediate comparisons to be made, colour slide photographs are therefore taken of each simulation for record purposes. For sequences, photographs of single consecutive display fields are also taken to indicate any temporal changes which occur. However, it will be apparent that any colour and brightness flicker effects will only be perceived by viewing the actual simulation. 18
TEST M A T E R I A L For the validation tests described below and [k)r subsequent assessment and comparison of different simulations a variety of test material has been used. Still pictures have included the well-known electronically-generated Philips Test-Pattern, the 'zone plate' (a signal containing a continuous spectrum of horizontal and vertical spatial frequencies) and some electronically-generated text of different sizes. For real pictures (derived from a telecine) we have used four well-known slides used for high-definition television tests, namely Manhattan, Boats on a Shore, a seated couple (Richard and Jean), and Wheels. Moving material of up to 40 TV frames (one and a half seconds) duration has included a vertically-scrolled version of the electronic text, and horizontally-panned sequences of a moving girl and a moving car taken from a broadcast-quality video tape recorder (VTR). VALIDATION The validity of the simulation technique was assessed by comparing the appearance of a real LCD with its simulation. This proved a difficult task. Direct visual comparison of two displays having a number of subtle and not-so-subtle differences, especially size, was far from easy, even when the displays were positioned so as to subtend the same angle at the observer's eyes. To eliminate some of these difficulties, the two displays were therefore photographed and prints made which were balanced as well as possible in respect of grey-scale and colour and were identical in size to the picture on the actual LCD. Even this was not entirely successful since the multiple processes involved resulted in significant degradation of the image quality. Comparisons were therefore also made using colour slide film. None of these separate comparisons was entirely satisfactory on its own and time did not allow thorough subjective viewing tests to be carried out. Nevertheless, when taken together, we felt that the results showed the simulation technique to be a viable basis for comparing different cell arrangements and drive methods. Colour Plate 1 shows a photograph of the 6-inch half-resolution LCD used for these validation tests when displaying the Philips test-pattern. This has the diagonal cell arrangement shown in Colour Plate 2(a). Colour Plate 3(a) shows the corresponding simulation when each LCD cell is represented by a 5 high by 4 wide array of pixels on the 1200-line high resolution CRT. Despite the minor differences described above, the real and simulated displays are very similar, with the diagonal structure being the most noticeable feature. Note also that there are no obvious coloured aliasing artefacts. ALTERNATIVE
CELL ARRANGEMENTS
An alternative cell arrangement, again with equal numbers of red, green and blue cells, but with alternate rows now displaced horizontally by half a cell width, is DISPLAYS, J A N U A R Y 1991
shown in Colour Plate 2(b). The simulation of this is shown in Colour Plate 3(b). Since the cells of each coiour are now equally spaced in several directions, there is no obvious diagonal structure in plain areas of colour and the cell structure is therefore much less noticeable. However, since the red, green and blue cells are no longer arranged in straight columns, some aliasing now occurs in the horizontal direction. The higher-frequency resolution bars consequently give rise to coarse colour striations. Fortunately, the effect is not serious and can be ameliorated by limiting the bandwidth of the video information. Other cell arrangements are, of course, possible. For datagraphic displays, for example, it is often argued that twice as many green ceils as red or blue should be used because around 60% of the luminance information is carried by this colour in white areas. Our simulations suggest that this has doubtful advantage for television. Colour Plate 2(c) shows the most obvious (and probably best) arrangement of this type and Colour Plate 3(c) its simulated appearance for a half-resolution display. As is to be expected, the improved appearance of plain green areas is accompanied by a noticeable degradation in red and blue ones. Probably even more disturbing is that single horizontal white lines are reproduced as either yellow or cyan, depending on their vertical position in the raster, while
DISPLAYS,JANUARY 1991
horizontal high-frequency information produces more severe colour aliasing. More complex 'double-green' arrangements (e.g. that of Colour Plate 2(d), not surprisingly, just make things worse (Colour Plate 3(d))~
CONCLUSIONS Using the PRL TV Image Processor, a method has been developed for simulating the appearance of direct view colour liquid crystal television displays on the screen of a high resolution CRT. This has enabled investigation of the relative merits of different spatial arrangements of the red, green and blue cells from the points of view both of their intrinsic appearance and of the artefacts which arise when driven in half- and full-resolution modes with uncoded (direct RGB) and decoded (PAL), still and moving, pictures and text. As a result a valuable advice and support service is now provided to the Concern LCTV Development Group. This is expected to continue as the technique is developed.
REFERENCES Knapp A G and Powell M J 'The performance of a-Si active matrix LCTV displays' P R L Ann. Rev. (1989)
19