CIRCUIT FOR CONVERGENCE SETTING IN A PROJECTION TELEVISION SET

Abstract: A projection television set provided with a circuit for automatic convergence setting.For convergence setting, a projection television set contains photo sensors (S) for the primary colours R, G, B which supply an output signal (U1) in the event of impingement of a marker (M) contained in the projected picture.The photo sensors are connected in parallel to a single common evaluation circuit. The evaluation circuit is capable to determine which of several sensors has been illuminated.A method for automatic convergence setting is described taking into account that a plurality of photo sensors is connected in parallel to a single common evaluation circuit. In particular for a projection television set with convergence setting using a plurality of sensors.

Description: Circuit for convergence setting in a projection television set. The invention is based on a circuit for convergence setting in a projection television set according to the preamble of Claim 1. A projection television set contains three monochrome picture tubes for the primary colours red, green, blue, which each project a picture in its colour onto a screen. The three pictures are superimposed on the screen and together produce a colour picture. For satisfactory picture reproduction, the three pictures projected onto the screen must be brought exactly into congruence, i.e. converge. Additional deflection circuits and correction coils are used for convergence setting in horizontal and vertical direction for the colours red, green and blue. The correction currents for the convergence are deduced from correction values stored in a digital memory. When such devices are manufactured, use is made of a large number of photo sensors inside or outside the visible picture area during convergence adjustment. The projected picture of each of the three tubes contains so- called markers in the form of monochrome red, green or blue picture locations. For satisfactory convergence, these markers in the projected picture must impinge exactly on the assigned sensor. This means that a manipulated variable, which indicates the two conceivable states "no light on the sensor" and "light on the sensor" has to be obtained from the output signal of each sensor. From DE 199 39 344 it is known to arrange optical sensors outside the visible area of the picture screen. The sensors allow an automatic convergence adjustment, which can be initiated by the user upon pressing a key. The embodiment described in the reference is provided with eight sensors for this purpose, which are placed in the corners of the screen and in the middle of each border of the screen. Based on the electrical currents flowing in the convergence coils and being necessary to place a marker on a particular sensor corresponding convergence values are deduced and stored in a memory. To achieve a complete set of convergence values it is necessary to evaluate the measurement values of each optical sensor in an evaluation circuit. The required connections of the sensors with the evaluation circuit bring on additional expenses for the manufacturing of such a projection television receiver. Therefore, it is desirable to provide a projection TV having the same performance at reduced production costs . The invention suggests a projection television receiver having monochrome picture tubes, which are provided with deflection means. The pictures of the picture tubes are projected onto a screen. The television receiver further comprises a character generator for displaying symbols and characters on the screen. The position of the symbols and characters can be shifted on the screen under the influence of the deflection means. Finally, a plurality of sensors is arranged in the non- visible area of the picture screen. The sensors are connected in parallel by a common single signal connection with an evaluation circuit. The evaluation circuit comprises means for determining the local position of a displayed symbol generated by the character generator. The parallel connection of the sensors presents a significant simplification in the production of the projection TV. It is advantageous to connect the sensors to a single common amplifier because this is particularly cost effective. In practice, it has been found that is very convenient to use latches in the convergence circuit to determine the coordinates of a marker illuminating a sensor. In one embodiment a counter is used to distinguish between different frames of a television picture. This is necessary when a marker is moved from one position to another. In this way it is avoided that two different marker positions are utilized in a single evaluation sequence. According to a second aspect of the invention, a method for convergence correction in a projection television set is suggested. The television set has monochrome light sources, which are imaged on a screen. A number of sensors are associated with the screen on which a raster can be displayed having horizontal and vertical lines which form crossing points. The inventive method comprises the following steps: a) a monochrome marker is imaged on the screen of the projection television set at a first initial point which is associated with a first sensor; b) the marker is shifted until the sensor responds; c) horizontal and a vertical coordinates representing the local position of the marker are read out; d) the coordinates are compared with a window; e) if the coordinates are inside an allowable window the actual convergence values are accepted as convergence values whereas if the coordinates are outside the allowable window, the actual convergence values are discarded; f) the steps a) to e) are repeated until for all sensors acceptable convergence values have been determined, and g) the steps a) to f) are repeated for every monochrome picture tube. The invention is explained below using an exemplary embodiment with reference to the drawing. The drawing shows : Fig. 1 ' the fundamental structure of the projection television set, Figs. 2, 3 characteristic signals of the sensor and the evaluation Fig. 4 shows a simplified block diagram of the evaluation circuit, Fig. 5 the screen divided in small sections by the convergence cross hatch, and Fig. 6 a part of the internal circuitry of the convergence circuit. Fig. 1 shows the structure of a projection television set in a simplified form. Three monochrome picture tubes la, lb, lc project three pictures for the primary colours red, green and blue (R, G, B) onto the screen 2, where they are superposed in order to display a colour picture. For superposition of this type, the convergence of the three projected pictures must be correct, i.e. mutually corresponding parts of the three pictures must coincide at every point of the picture on the screen 2. In order to set the correct convergence, a plurality of stationary sensors S in the form of photodiodes are assigned to the screen 2 outside the visible picture area. The sensors S are indicated around the screen 2 only by dots. For the sake of simplicity, in the right part of the block diagram of Fig. 1 visualizing the signal flow, only one sensor S is shown. It is a representative for all eight sensors S shown on the left hand side of Fig. 1. It is also possible to place the sensors 2 inside the visible picture area if this is advantageous or desirable for a certain embodiment. The picture projected onto the screen 2 contains a respective marker M in the form of a monochrome, that is to say red, green or blue, picture location within a picture area that is black in the setting region of the marker M. For satisfactory convergence, the marker M must impinge sequentially on the sensors S. This impingement is detected by the fact that when the marker M sweeps over the sensor S, the latter emits an output signal U = "1" = "light" via an evaluation circuit. If the marker M is situated outside the sensor S, the sensor S emits the output signal U = "0" = "dark". The marker M transmits light onto the sensor S, when sweeping over the sensor S. The output signal Ul of the sensor S is AC coupled with the detection circuit 4. The AC coupling eliminates from the output signal of the sensor a DC component generated by ambient light. The presence of a DC component could create a flawed evaluation of the sensor signals. The evaluation circuit 4 derives from the signal Ul, a binary output signal U2. If the marker M does not impinge on a sensor S, the output signal takes the value U2 = "0" = "no light or dark". If the marker M impinges on a sensor S, the output signal takes the value U2 = "1" = "light". The binary output signal U2 from the output of the circuit 4 passes to the digital convergence circuit (DKS) 5. The circuit 5 operates in such a way that digital values for the convergence are stored for the individual frames and are used for convergence setting during reproduction after digital to analogue conversion. Convergence coils 3a 3c attributed to each picture tube influence the electron beams of the picture tubes la lc individually. In Fig.l only one coil is shown for each picture tube. In a practical embodiment there are two coils for each tube for convergence adjustment in horizontal and vertical direction. The control of the convergence circuit 5 and a marker inserter 6 is performed by a microprocessor 7 that is also utilized to control other functions of the projection TV. Details of the light detection by the sensors and the processing of the sensor output signals are described in DE 197 02 452 and are not subject matter of the present invention. Fig. 2a to 2c show signal diagrams to explain the method of operation of the circuit for evaluating the output signal Ul of the sensor S for an interference pulse 8. The interference pulse 8 (Fig. 2a) passes to the input of the evaluation circuit 4 comprising a monostable circuit. On account of the pulse 8 the evaluation circuit 4 generates an output pulse 9 having a duration D, which is somewhat longer than the duration T of a frame as it is shown in Fig. 2b. According to Fig. 2c, the output signal 9 is sampled periodically, as the arrows 11 on the abscissa illustrate it. In this case, the monostable circuit supplies only three samples different from a zero 1 value because the pulse U2 has ended at the fourth sampling. The evaluation circuit 4 is dimensioned in such a way that it responds only in the event of more than three samples having a value different from zero. The evaluation circuit 4 consequently suppresses the interference pulse 8 and generates an output signal U2 = "0", indicating that there is no light of a marker M on a sensor S. Figs. 3a to 3c show similar signal diagrams for the useful pulses 12 and 12' , which are triggered by the impingement of the marker M on the sensor S and is therefore repeated with the period T of a frame. Before the output pulse 9 of the monostable circuit is reset after the duration D as in Fig. 2b, it is set anew by the second pulse 12' . Consequently, the duration of the output pulse 9 is now two times D, i.e. 2D as it is illustrated in Fig. 3b. The equidistant sampling indicated by the arrows 11 of the output pulse 9 according to Fig. 3c now supplies four samples having a value different from zero. In this case the evaluation circuit 4 outputs a signal U2 = "1" indicating that a marker M impinges on a sensor S. In this way the evaluation circuit 4 allows to discriminate between an interference pulse 8 and the useful pulses 12, 12' . Fig. 4 illustrates a more detailed block diagram of the evaluation circuit 4. The DC component of the output signal Ul of a sensor S is removed in block 13 and only the AC component is further processed in an amplifier 14. Block 15 represents a monostable circuit having the toggle duration T + ΔT. The output pulse 9 of the monostable circuit 15 is sampled in the sampling circuit 16 as it is described above in connection with Figs. 2 and 3. A counter 17 counts the number of samples, which are different from a zero value. In block 18 it is decided if the result of counter 17 indicates that a marker impinges on a sensor or not and the output signal U2 is generated accordingly, i.e. U2 = "1" if the marker impinges on a sensor and U2 = "0" if not. Also in Fig. 4 only one sensor S is shown for the sake of simplicity. However, the projection TV according to the invention is provided with eight photodiodes as sensors, which are connected in parallel. This configuration generates difficulties in regard of identifying, which one of the eight sensors is actually illuminated by the marker. The solution for this difficulty is described in the following. Within the digital convergence system the picture is divided in small areas where independent correction values are applied. In order to output the correction values at the right time and synchronism to the horizontal and vertical deflection the convergence circuit is synchronized to the deflection. Internal counters for the horizontal and vertical direction are representing always the current place of the electron beam and the projected position on the screen 2, respectively. This general concept is described e.g. in EP-A 1 061 751. To implement an easy use of photo sensors for an optical loop system, which are simply connected in parallel to a common amplifier the digital convergence circuit S is provided with special circuitry. The general idea for position finding with a photo sensor is to move a monochrome marker towards a sensor and note the number of steps needed until the marker is lighted. Fig. 5 shows the division of the screen 2 into small sections by the convergence cross hatch with the numbering of the grid lines indicated along the top and right border of the screen 2. Fig. 6 illustrates in a schematic way details of the convergence circuit 5, which is realized e.g. as an integrated circuit. An example of such an integrated convergence circuit is the STV2050D supplied by ST electronics. Inside the convergence circuit there is a register 21 where the μP 7 can read if a marker has been detected by a sensor and if so, where. Four bit of the register 21 represent the status of a four bit rollover counter 22 (counting 0-15) which is clocked with the vertical sync signal Vsync applied to the convergence circuit 5. The rollover counter 22 is effective as a field counter. Furthermore, the register 21 contains the read out of two latches with four bit. The first four bit are associated with the horizontal grid counter. The second four bit are associated with the vertical grid counter. The output of the evaluation circuit 4 is connected to the latch input of these latches . When the μP 7 reads the register 21 then the latches 23, 24 are reset to 1111 and the latch input is enabled. With a trigger at the latch input 26 caused by illumination of a sensor the present status of the grid counters is latched and the latch input 26 is disabled until the next read out of the register 21 has been performed by the μP 7. When no trigger event has occurred, the readout of the register will be "1111" for the vertical grid counter state. Since the 1111 = 15 is a value, which is never reached by the vertical grid counter, the μP 7 decides that a marker M did not illuminate a sensor. If a trigger event was present then the two latches will contain the counter values of the trigger moment. If the counter values are within a window around the position of the sensor where the marker is, then the - in correct sensor was illuminated. If the counter values are outside the window then the trigger was caused by any disturbance, e.g. a sequence of interference pulses. The rollover counter enables the μP 7 to read out the register 21 without hardware synchronization to the vertical deflection frequency. If the μP 7 changes the position of the marker it waits at least one frame before the next result from the register 21 is read. In this way it is secured that the result is not originating from an illumination of a sensor before the position of the marker has been changed. By tracking, the value of the rollover counter 22 the μP 7 checks when one frame has passed. A second advantage of this configuration is that the processing power has to be provided only when reading the register 21 containing the relevant data also using the latch register saves processing power because it is not needed to act immediately when a sensor output signal is detected.

Claims: Claims 1. Projection television receiver having monochromatic picture tubes (la, lb, lc) , the pictures of which are projected onto a screen 2 and which are provided with deflection means, the television receiver further comprising a character generator (6) for displaying symbols and characters on the screen (2) , their position can be shifted on the screen under the influence of the deflection means, and a plurality of sensors (S) being arranged in the non-visible area of the picture screen, characterized in that, the sensors are connected by a common single signal connection with a processing unit (4, 5) and that processing unit (4, 5) circuit is provided comprising means (21, 22, 23, 24) for determining the local position of a displayed symbol generated by the character generator (6) . 2. Projection television receiver according to claim 1, characterized in that, a single amplifier (14) is used to amplify the output signals of all sensors (S) . 3. Projection television receiver according to claim 1, characterized in that, the processing unit (4, 5) comprises latches (23, 24) for counting coordinate values corresponding to the local position of the displayed symbol on the screen. 4. Projection television receiver according to claim 3, characterized in that, the processing unit (4, 5) is provided with a counter (22) to distinguish between different frames of a television picture. 5. Method for convergence correction in a projection television set having monochromatic light sources which are imaged on a screen, with which a number (L) of sensors (Sι..S L ) are associated and on which a raster can be displayed having horizontal and vertical lines which form crossing points, in which case the method has the following steps: a) a single-colour marker is imaged on the screen of the projection television set at a first initial point which is associated with a first sensor (S k ) ; b) the marker is shifted until the sensor (S k ) responds; c) horizontal and vertical coordinates representing the local position of the marker are read out, d) the coordinates are compared with a window, e) if the coordinates are inside an allowable window they are accepted as convergence values whereas the coordinates are discarded if they are outside the allowable window, f) the steps a) to e) are repeated until for all sensors acceptable convergence values have been determined, and g) the steps a) to f) are repeated for every monochromatic picture tube.

Inventors: GLEIM GUENTER, HEIZMANN FRIEDRICH, RUNTZE ALBERT, GLEIM, GUENTER, HEIZMANN, FRIEDRICH, RUNTZE, ALBERT
Applicants: GLEIM GUENTER, HEIZMANN FRIEDRICH, RUNTZE ALBERT, THOMSON LICENSING SA, GLEIM, GUENTER, HEIZMANN, FRIEDRICH, RUNTZE, ALBERT, THOMSON LICENSING S.A.
Application year: 2001
Main classification: H04N9
IPC codes: H04N9