EP0062702B1 - System for displaying characters on a screen - Google Patents

Description

The invention relates to an arrangement for displaying characters on a screen of a display unit according to the preamble of the claim.

With the new telecommunications services, screen text (interactive videotex) and videotext (broadcast videotex), it is possible to use a television to display characters and graphic patterns in addition to the usual television pictures. In this context, the characters are understood to mean alphanumeric characters and special characters as well as linear graphical characters, while graphical patterns are understood to mean relatively crude graphical representations.

When displaying the characters and graphic patterns, the screen content, which is referred to as the page, is divided, for example, into 960 character fields, which are arranged in 24 lines with 40 character fields each. When alphanumeric characters are displayed, one character is shown in each character field. In this case, the character field consists of 8 x 10 or 8 x 12 pixels, by means of which the individual characters are displayed. The same grid can also be used to display fine graphic characters that can span one or more character fields. When displaying the graphic patterns, each field is divided into 2 x 3 graphic elements and the graphic patterns can be constructed from the possible combinations of light and dark graphic elements.

The characters and graphic patterns can be displayed in the basic colors common to television, red, green and blue and their complementary colors yellow, cyan and magenta as well as in white and black. In the display, these colors can also be selected as background colors.

When displaying characters in relatively dark colors, such as red or blue on a dark background, it was found that the characters are difficult to read and recognize because of the relatively low light intensity and the low spectral sensitivity to red and blue. Coarse graphic patterns that are displayed in red or blue color are, however, clearly recognizable.

An arrangement for displaying characters on a screen of a display unit is known from US Pat. No. 3,829,613, in which the brightness of the different colors is controlled as a function of these themselves when a multicolored photograph is taken of the content of the screen. For this purpose, the time period during which the characters are displayed in the different colors is changed depending on the colors in order to achieve a different exposure time for the different colors. For a viewer of the characters on the screen, only a slight brightening is achieved in this way, since if a viewer wants to look at the characters for a longer period of time, it is not possible to end the display of the lighter characters earlier.

From DE-OS 27 40 329 and EP-OS 0 004 197 it is already known to change the brightness of the colors if, instead of television pictures, teletext pages are displayed on the screen of a television set. The setting of the brightness of the colors in the display of the teletext pages in relation to the brightness of the colors in the display of a television picture takes place with a one-time comparison and applies both to the display of the characters and to the display of the graphic patterns. With such a setting of the brightness of the colors, however, there is a risk that either the relatively bright colors such as white, yellow and cyan will be brightened too much relative to the relatively dark colors such as red and blue when displaying the graphic patterns, or during the display the signs that the relatively dark colors such as red or blue are lightened too little. This is particularly true when both characters and graphic patterns are displayed on a teletext page and relatively dark characters are displayed in the vicinity of large bright areas.

The invention is therefore based on the object of specifying an arrangement by means of which particularly good legibility of characters with different colors is achieved on a screen of a display unit when graphic patterns and / or multicolored television images are also displayed by means of the display unit.

According to the invention the object is achieved in the arrangement of the type mentioned by the features specified in the characterizing part of claim 1.

The arrangement according to the invention has the advantage that it requires little effort and can be easily installed in existing display units. A pleasant picture appears on the screen for the viewer. Compared to an increase in the overall brightness, the arrangement has the advantage that flickering of the relatively bright colors is avoided and that the colors are not changed when the graphic patterns or the television pictures are displayed. The color tone is also retained when the characters are displayed, since the colors are only brightened. The mixed colors are not changed and the color contrast is also retained.

One way of relatively brightening the relatively dark colors is that the control signals represent the brightness of the Increase characters by adding appropriate proportions of the basic colors.

Another possibility of the relative brightening of the relatively dark colors is that the control unit increases the overall brightness of the display on the screen when displaying the characters in the relatively dark colors and generates signals by means of which the brightness of the displayed relatively bright colors is reduced.

If a decoding unit is provided, which outputs ordered color signals to the characters to be displayed and which contains an image memory in which data words associated with the characters and the associated colors are stored, an advantageous embodiment of the arrangement is achieved in that the control unit has a converter contains, which converts the relatively dark colors associated first code words into second code words which are assigned to the corresponding lightened colors. It is particularly advantageous that the converter is connected upstream of the image memory, in which the second code words can be stored instead of the first code words. Another possibility is that the converter is arranged between the image memory and an image repetition memory. The image repetition memory does not have to contain the content of a complete page. For example, it may be sufficient if only the content of one line is stored in the image repetition memory.

Good readability of the characters can also be achieved in that the display unit displays the characters with an increased frame rate and in that the characters with the relatively dark colors are displayed more frequently than the characters with relatively bright colors or that the control unit emits signals to the display unit by means of which the brightness signals are periodically blocked periodically when the characters are displayed in relatively bright colors.

In particular when using a flat screen, in which the display is carried out using a large number of illuminated dots, it is advantageous for the control unit to generate signals which, when the characters with the relatively dark colors are displayed, trigger a display with more illuminated dots on the display unit than when displaying the characters with the relatively bright colors.

• Figur 1 einen Ausschnitt aus einer Videotexseite,
• Figur 2 eine erste Ausführungsform der Anordnung,
• Figur 3 eine zweite Ausführungsform der Anordnung,
• Figur 4 eine dritte Ausführungsform der Anordnung.

In the following, advantageous embodiments of the arrangement according to the invention are described in more detail with reference to drawings. Show it :

• FIG. 1 shows a section of a videotex page,
• FIG. 2 shows a first embodiment of the arrangement,
• FIG. 3 shows a second embodiment of the arrangement,
• Figure 4 shows a third embodiment of the arrangement.

In the section from a teletext page shown in FIG. 1, characters AZ and GZ and a graphic pattern GM are shown. Alphanumeric characters AZ and a graphic character GZ formed from fine lines are shown as characters. The characters AZ and GZ and the graphic pattern GM are arranged in character fields ZF, the limits of which are shown only for reasons of clarity, but are not visible on the screen. Each character field ZF is divided into an invisible grid of 8 x 12 pixels BP when characters are displayed. The characters AZ are shown on the screen due to the different brightnesses of the pixels BP. The graphic characters GZ can be displayed in the same grid as the alphanumeric characters AZ. You can also span several ZF character fields. The characters AZ and GZ can be displayed in the three primary colors red, green, blue, the mixed colors yellow, cyan and magenta and in the colors black and white. However, the color cannot be changed within a ZF drawing field. The basic colors, the mixed colors and the colors black and white can also be used as the background.

When displaying the graphic patterns GM, the character fields ZF are divided into 2 x 3 graphic elements GE. The graphic patterns GM are then represented from character fields ZF with different combinations of the graphic elements GE. The graphic elements GE and the background color can also assume the basic colors, the mixed colors and the colors black and white.

If the characters AZ and GZ are displayed on the screen in relatively dark colors such as red or blue, there is a risk, particularly in the case of a dark background, that the characters AZ, GZ are not clearly legible. This is particularly the case if, as shown in FIG. 1, a graphic pattern GM, for example with a relatively bright color, such as white, yellow or cyan, comes very close to the characters AZ or GZ in blue or red color.

Furthermore, it was found that when characters are displayed purely, they are difficult to read if a part of the teletext page is described with characters of a relatively light color and a further offset part is described in a relatively dark color.

In the first embodiment of the arrangement shown in FIG. 2, a control unit SE is arranged between an image control unit BST and a display unit AE which is provided with the screen. The image control unit BST is designed in a known manner. It contains switching units via which color signals coming from a television receiver or color signals coming from a teletext decoder can be sent to the display unit AE be switched. Switching units of this type are generally known and are described, for example, in the magazine “Wireless World” May 1976, pages 64-68.

The color signals R, G and B assigned to the primary colors red, green and blue are supplied to the display unit AE via delay elements V. In addition, the color signals R, G, B are supplied to the control unit SE. When displaying television pictures on the display unit AE, the color signals are analog signals, the instantaneous values of which determine the colors to be displayed and their brightness. In the representation of the characters and the graphic patterns, the color signals are binary signals which assume the binary values 0 or 1. In the representation of the color red, for example, only the color signal R has the binary value 1. In a corresponding manner, in the representation of the color blue or green only the color signal B or G has the binary value 1. The color black is represented in that all color signals have the binary value 0 and when displaying the color white, hall color signals have the binary value 1. The mixed colors yellow, cyan and magenta are represented by the fact that the color signals R and G, G and B or R and B have the binary value 1.

If the teletext decoder determines that a teletext page is to be displayed, the corresponding color signals R, G and B are fed to the display unit AE. When displaying the graphic patterns GM and when displaying the background colors, the color signals are supplied to the display unit AE in a known manner. When representing alphanumeric characters AZ or graphic characters GZ, the image control unit BST generates corresponding signals SAZ or SGZ. If one of these signals occurs, an OR gate G1 in the control unit SE outputs a signal S1 to two AND gates U1 and U2. If the characters are to be displayed in blue color, the color signal B has the binary value 1, while the color signals R and G have the binary values 0. In this case, the AND gate U1 outputs a signal S2 via a switching stage SS1 and an analog OR gate G2 as signal S3 to the display unit AE. Using this signal S3, which serves as a brightness signal, the blue characters are lightened relative to the relatively bright colors, such as white, yellow or cyan. The brightening can take place in a known manner in that small portions of the primary colors red and green are generated in the display unit AE by means of corresponding additional color signals. It is also possible to supply the signal S3 to the grid of a picture tube designed as a cathode ray tube in order to increase the brightness. In the same way as when the characters are displayed in blue color, a signal S4 is generated by means of the AND gate U2, which brightens them when the characters are displayed in red color. Using the switching stages SS1 and SS2, the amplitude of the signals S2 or the signals S4 and thus the signals S3 can be changed so that the basic colors red and blue can be brightened to different extents. It is particularly advantageous here if the blue color is lightened more than the red color. The delay elements V are provided to compensate for additional transit times which arise from the AND elements U1 and U2, from the switching stages SS1 and SS2 and the OR element G2, so that the signals S3 occur simultaneously with the corresponding color signals.

With the help of the control unit SE, it is achieved in a simple manner that when characters are displayed in the relatively continuous colors red and blue, these are brightened relative to the relatively bright colors.

The relative brightening of the relatively dark colors can also take place in that the display unit AE is designed for a higher frame rate and that the characters AZ and GZ are displayed with relatively bright colors with a lower frame rate than the characters AZ and GZ with relatively dark colors. In this case, a higher frame rate is set with signal S3, so that the relatively dark colors then appear brightened.

If a screen formed from a multiplicity of luminous dots, for example a flat screen, is provided as the screen, the relative brightening of the relatively dark colors can take place in that the relatively bright colors are displayed with a smaller number of luminous dots, while the signal S3 is caused that correspondingly more luminous dots are used in the representation of the characters in the relatively dark colors.

In the arrangement shown in FIG. 3, the relative brightening of the relatively dark colors is achieved by increasing the overall brightness when displaying teletext pages and by selectively darkening the relatively bright colors when displaying graphic patterns GM or when displaying background colors . In addition to the color signals R, G and B, the videotex decoder in the image control unit BST again outputs the signals SAZ and SGZ. When the graphic patterns GM are displayed or the background is shown, the signals SAZ and SGZ each have the binary value 0, so that three AND gates U3 to U5 are enabled. If the graphic pattern GM or the background is to be displayed in the colors white, yellow or cyan, the AND gates U3, U4 or U5 emit a signal S5, S6 or S7 which is transmitted via a switching stage SS3, SS4 or SS5 and an analog OR gate G3 is supplied as signal S8 to the display unit AE and darkens these colors, so that the relatively dark colors also appear brightened in this way relative to the relatively bright colors. The delay elements V are in turn provided for the delay times through to balance the AND gates U3 to U5, the switching stages SS3 to SS5 and the OR gate G3. Switching levels SS3 to SS5 in turn can be used to set the level of darkening for each color individually. The signal S8 is expediently fed to the grid of the cathode ray tube in order to reduce the brightness of the corresponding colors.

If the display on the display unit is carried out with an increased frame rate, the signal S8 can be used to reduce the frame rate when the relatively bright colors are displayed, so that the relatively dark colors appear brightened here too. When displaying a large number of illuminated dots on a flat screen, the signal S8 can be used to reduce the number of activated illuminated dots when displaying the relatively bright colors.

Another possibility for selectively darkening the relatively bright colors is that the corresponding brightness signal is chopped using the signal S8, i. H. is periodically interrupted briefly. This can be done, for example, by an AND operation with a clock signal output by an oscillator.

In the block diagram shown in FIG. 4, the control unit SE is part of the image control unit BST. From the teletext decoder, only an image memory BSP is shown, in which data words associated with the characters to be displayed and graphic patterns are stored.

These data words contain, for example, 16 binary characters, of which 8 are assigned to the alphanumeric characters AZ to be displayed or the graphic elements GE and of which 8 binary characters represent control characters which identify the color or the type of representation or which indicate that they are data words which are assigned to the alphanumeric characters AZ or the graphic elements GE. For the representation of the colors, first code words consisting of 3 binary characters are provided in accordance with the color signals R, G and B.

The control unit SE contains an image repetition memory BWS which, for example, temporarily stores the content of a line on the screen. With the exception of the first code words assigned to the color signals, all other binary characters are fed directly to the image repetition memory BWS as signals S9. The color signals are fed to the frame buffer BWS via a converter UM1.

More storage spaces for the data words are provided in the image repetition memory BWS than in the image memory BST. Instead of three binary characters for the first code words, four binary characters for second code words are provided in the data words, so that in addition to the basic colors, their complementary colors and the colors black and white, further colors can be stored. The colors "lightened blue and" lightened red are intended as further colors. If the videotex decoder emits the signals SAZ or SGZ and the characters are to be displayed in the colors red or blue, the corresponding combinations of the binary characters which are assigned to these brightened colors are stored in the second code words in the image repetition memory BWS. The image repetition memory BWS is followed by a second converter UM2, which again assigns three color signals R1, G1 and B1 to these four binary characters. These color signals differ from the color signals R, G and B in that when the characters are shown in blue color, the color signals R1 and G1 also have predetermined instantaneous values and in that when the characters are shown in red color, the color signals G1 and B1 also have predetermined instantaneous values to lighten the blue or red color due to the additive mixture of colors.

Another possibility of designing the image control unit is that the image memory BSP is already being expanded in such a way that second code words formed from 4 binary characters are provided for the colors. In this case, the image repetition memory BWS is not required and the converter UM1 is connected upstream of the image memory BSP.

The converter UM2 can also be designed in such a way that, as shown in FIGS. 2 and 3, it generates additional brightness signals in accordance with the signals S3 and S8 in order to control the grid of the picture tube, the frame rate or the number of illuminated dots of a flat picture tube to change.

The converter UM1 can also be designed such that it passes the color signals R, G and B unchanged to the image repetition memory BWS even when characters are displayed. This can be the case, for example, if the characters are displayed on a light background. For this purpose, the converter UM1 can contain a memory, in which the background color valid for the respective character field is temporarily stored and, depending on its content, the relatively dark colors can be brightened. Such an intermediate store can also be provided in the arrangements according to FIGS. 2 or 3, so that even in this case the relative brightening of the relatively dark colors only takes place if these are displayed on a dark background.

Claims (9)

1. A system for displaying characters on a screen of a display unit on which the characters can be displayed in a plurality of different colours and on which the characters, which are displayed in relatively dark colours, are displayed so as to be made brighter relative to the characters which are displayed in relatively bright colours, characterised in that there is provided a control unit (SE) which when displaying the characters (AZ, GZ) emits control signals (S3), which trigger a brightening of the relatively dark colours (R, B), to the display unit (AE), and which control unit when displaying graphic patterns (GM) and/or multicoloured television pictures, blocks the transmission of the control signals (S3).

2. A system as claimed in claim 1, characterised in that the control signals (S3) increase the brightness of the displayed characters (AZ, GZ) by adding appropriate components of the basic colours (R, G, B).

3. A system as claimed in claim 1, characterised in that when displaying the characters (AZ, GZ) in the relatively dark colours (R, G) the control unit (SE) increases the overall brightness of the display on the screen and produces signals (S8), by means of which the brightness of the displayed relatively bright colours is reduced.

4. A system as claimed in one of claims 1 to 3, wherein the colours which are to be displayed are binary coded and wherein a picture store is arranged in a picture control unit which is connected preceding the display unit, characterised in that the control unit (SE) comprises a converter (UM1) which converts first code words, which are allocated to the relatively dark colours (R, B), into second code words which are assigned to the corresponding brightened colours.

5. A system as claimed in claim 4, characterised in that the converter (UM1) precedes the picture store (BSP) in which the second code words can be stored instead of the first code words.

6. A system as claimed in claim 4, characterised in that the converter (UM1) is arranged between the picture store (BSP) and a picture repetition store (BWS).

7. A system as claimed in one of claims 1 to 6, characterised in that the display unit (AE) displays the characters (AZ, GZ) with increased alternating picture frequency, and that the characters (AZ, GZ) with the relatively dark colours (R, G) are displayed more frequently than the characters (AZ, GZ) with relatively bright colours.

8. A system as claimed in one of claims 1 to 6, characterised in that the control unit (SE) transmits signals (S8) to the display unit (AE), by means of which signals the brightness signals are periodically temporarily blocked when displaying the characters (AZ, GZ) in relatively bright colours.

9. A system as claimed in one of claims 1 to 8, characterised in that the control unit (SE) produces signals (S3, S8) which when displaying the characters (AZ, GZ) with the relatively dark colours (R, B) trigger a display with more luminous dots on the display unit (AE) than when displaying the characters (AZ, GZ) with the relatively bright colours.

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