Abstract:
A method and apparatus for suppressing aesthetic imperfections of a video signal when displayed on a cathode ray tube (CRT) display by compensating for both the picture tube characteristics (e.g. spread, temp drift and ageing of the red, green, blue amplifier and picture tube) and the temperature behavior (e.g. a warm and a cold picture tube start-up behavior) of the picture tube.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains to televisions, and more particularly to a television having a switch-on procedure that suppresses of the aesthetic imperfections of a picture as the result of spread, temp drift and ageing of the red, green and blue (RGB) amplifier and picture tube.  
         BACKGROUND OF THE INVENTION  
         [0002]    After a television is switched on, the viewer has to wait several seconds until the picture becomes visible. As a result, the sound of the television is audible prior to the picture being visible. The viewer may become bored when the wait time (the time before the picture appears) is too long. However, if the picture is released too early, the picture is subject to being laden with aesthetic imperfections (e.g. bad picture quality). Attaining a picture free of imperfections with the shortest possible wait time is challenging because there is a large diversity in the switch-on conditions for conventionally manufactured televisions.  
           [0003]    For example, the start-up phase of a television having a cathode ray tube (CRT) base display requires a warm-up time for the picture tube. Depending on the construction of the picture tube and environmental parameters, such warm-up time can take 5 to 10 seconds. During this time, the emission of electrons around the cathodes is undefined, that may result in a poor quality picture that is unfocused and discolored. In an attempt to avoid the poor picture quality during the start-up phase, the CRT is blanked until it is warmed.  
           [0004]    During warming of a picture tube, the voltage/current (V/I) curve of the tube depends on how long the television has been switched off. For example, the start-up phase of a warm picture tube (such as the result of a quick switch-off and then on again) is different from the start-up behavior of a cold picture tube. A fixed delay for unblanking the picture tube after switching on or detection of a threshold current for unblanking in previous designs did not give the optimal start-up phase. Also the release of the picture (with a fixed delay) in combination with an in-range detector for the black current loops did not solve various problems experienced during the start-up phase.  
           [0005]    Examples of patents that perform blanking of the picture include U.S. Pat. No. 5,194,954, issued to Duffield, entitled “AUTOMATIC CHANNEL SAMPLING PICTURE-IN-PICTURE CIRCUITRY”; U.S. Pat. No. 4,748,497, issued to Sengoku, entitled “TELEVISION RECEIVER AUTOMATIC COLOR TEMPERATURE ADJUSTING SYSTEM WITH START UP CONTROL”; U.S. Pat. No. 4,188,641, issued to Baker et al., entitled “STARTUP CIRCUIT FOR A TELEVISION RECEIVER”; and U.S. Pat. No. 4,129,885, issued to Chovanec, entitled “WARM-UP COMPENSATION SYSTEM FOR PICTURE TUBE” none of which meet the needs of the present invention.  
           [0006]    As will be seen more fully below, the present invention is substantially different in structure, methodology and approach from that of prior switch-on procedures that blank the picture in televisions upon start-up.  
         SUMMARY  
         [0007]    The present invention provides a start-up procedure and circuit that minimizes the time before release of a video signal to the television or picture tube without having significant aesthetic imperfections. The television start-up control circuit compensates for both the picture tube characteristics (e.g. spread, temp drift and ageing of the red, green, blue amplifier and picture tube) and the temperature behavior (e.g. a warm and a cold picture tube start-up behavior) of such picture tube.  
           [0008]    In one embodiment, a last stored gain setting stored when the television was switched “OFF” is used to control the video signal and stabilize the cathodes cutoff and drive level when the television is switched “ON”. Such gain setting is independent of the picture tube characteristics (e.g. spread, temp drift and ageing). In another embodiment, a prediction can be made using start-up curves that are fixed by these characteristics and temperature behaviors to achieve an optimal start-up behavior when the television is switched “ON”. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 illustrates a general schematic diagram of the television start-up control circuit for a television according to the present invention.  
         [0010]    [0010]FIG. 2 illustrates a graphical representation of the different release moments of the picture after switch-on of a warm and cold picture tube with suppression of the aesthetic imperfections.  
         [0011]    [0011]FIG. 3 illustrates a general flowchart of the start-up phase according to the present invention.  
         [0012]    [0012]FIG. 4A illustrates the switching diagram for the offset-loop measurements in accordance with the present invention.  
         [0013]    [0013]FIG. 4B illustrates the switching diagram for the gain-loop measurements in accordance with the present invention.  
         [0014]    [0014]FIG. 4C illustrates the switching diagram for the start-up phase in accordance with the present invention.  
         [0015]    [0015]FIG. 5 illustrates a general flowchart of a second embodiment of the start-up phase according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring now to FIG. 1, the television start-up control circuit  10  includes a gain loop L 1 , an offset loop L 2  and blanking control sub-circuit B 1  to suppress aesthetic imperfections of a picture resulting from spread, temperature drift and ageing of the red, green and blue (RGB) amplifier  28  and picture tube  70 . The television start-up control circuit  10  provides continuous cathode calibration with an offset loop L 2 , during every even field, and a gain loop L 1 , during every odd field. The feedback signal in offset loop L 2  is an analog signal. During every even field, a measurement of a 10 uA point is performed to stabilize the cutoff of the three cathodes. The hold time is short due to the capacitor C 1  in the offset loop L 2 . The drift in cutoff can be very fast due to picture content changes.  
         [0017]    On the other hand, the feedback signal in gain loop L 1  is a digital signal whose output is converted to an analog signal. During every odd field, a measurement of a reference current 150 or 220 uA point is performed to stabilize the drive of the three cathodes. It is not necessary to perform a measurement every 40 msec because the hold time is relatively long due to the digital-to-analog converter (DAC)  48  in the gain loop L 1 . Moreover, the drift in gain is relatively very slow.  
         [0018]    Referring still to FIG. 1 the schematic diagram of the television start-up control circuit  10  will now be described in detail for a television  5 . The television start-up control circuit  10  comprises a video input source (video) on line  12   a  and a voltage reference source (Vref) on line  12   b . The line  12   a  and line  12   b  are coupled to switch SW 1 . Switch SW 1  has an output on line a. Line a has coupled thereto first and second test current reference sources Itest1 and Itest2 via switches SW 2  and switch SW 3 , respectively.  
         [0019]    The input source, on line a, is also coupled to the multiplier  20  (hereinafter referred to as the “gain-loop multiplier  20 ”). The output from the gain-loop multiplier  20 , on line b, is coupled to the input of summer  22  (hereinafter referred to as the “offset-loop summer  22 ”). The output from the offset-loop summer  22 , on line c, is input into the summer  24  (hereinafter referred to as the “blanking summer  24 ”) of the blanking control sub-circuit B 1 . The output from the blanking control sub-circuit B 1 , on line d, is input to the current-to-voltage converter  26 . The output of the current-to-voltage converter  26 , on line e, is input into the RGB amplifier  28 . The output from the RGB amplifier  28  is sent to picture tube  70  in the television  5 .  
         [0020]    The blanking control sub-circuit B 1  will now be described in detail. The blanking control sub-circuit B 1  includes first and second blanking current reference sources  30  and  32 . The first and second blanking current reference sources  30  and  32  are coupled to first and second switches SW 4  and SW 5 , respectively. During the start-up phase, the picture is blanked by blanking control sub-circuit B 1  in accordance with the switch states shown in FIGS. 4A, 4B and  4 C.  
         [0021]    The black current loops (e.g. the gain loop L 1  and the offset loop L 2 ) will now be described in detail. A feedback current I, on line g, from the RGB amplifier  28  feeds the gain loop L 1  and the offset loop L 2 . In an exemplary embodiment of the present invention, a reference current from current source  34  provides a 10 μA current and is switched on via switch SW 6 . The feedback current I, on line g, flows through switch SW 8  and to the offset-loop summer  22  via capacitor C 1  when the gain loop L 1  in inactive and the offset loop L 2  is active. The capacitor C 1  is coupled to ground and between switch SW 8  and the offset-loop summer  22 .  
         [0022]    In general, the function of the offset loop L 2  is stabilization of the cut-off voltage of the cathodes of the CRT. The cut-off measurement is performed during three successive lines in the overscan every even field (40 msec.). The loop is continuously calibrating because the voltage grid  2  (VG 2 ), which is part of the picture tube  70 , depends upon the load of the high tension voltage (EHT). For television applications, the EHT may be approximately 30 kV.  
         [0023]    When the gain loop L 1  is active, the feedback current I, on line g, feeds into first and second op-amps  38  and  40  which receive reference currents Iref1 and Iref2, respectively. The first op-amp  38  feeds into the up/down counter  42 . The output of op-amp  40  feeds both the up/down counter  42  and the picture tube warm (PTW) register  52 . The operation of the first and second op-amps  38  and  40  in the gain loop L 1  is set forth below in TABLE 1.  
         [0024]    The up/down counter  42  receives as input the register contents of the loaded preset gain (LPG) register  58 , preset gain register  56 , the enable gain loop (EGL) register  54  and the PTW register  52 . The output of the up/down counter  42  is sent to the summer  44 . Summer  44  also receives as input the register contents of the white point (WP) RGB adjustment register  64 . The WP register  64  and the cathode drive level register (CL) register  62  store data prestored by the manufacturer, and are used on the production line during the manufacturing process to align the television according to manufacturer specification.  
         [0025]    In operation, when the gain loop L 2  is enabled and the feedback current, on line g, is too low (lower than Iref1), the up/down counter  42  counts up and increases the output to the gain-loop multiplier  20 , via DAC  48 , until the feedback current, on line g, is above Iref1. The PTW register  52  is high if I input is &gt;5 uA (the feedback current exceeds the chosen offset current of 150 or 220 μA) and low if I input is &lt;5 uA. The status of the PTW register  52  is based on the output of op-amp  40 . Switch SW  7  is adapted to switch between two current sources  36   a  and  36   b.  In the exemplary embodiment, the two current sources  36   a  and  36   b  are 220 μA and 150 μA, respectively.  
         [0026]    The gain loop L 1  stabilizes the white point of the picture tube  70 . Therefore, the gain becomes independent upon spread, temp drift and ageing of the red, green and blue (RGB) amplifier  28  and picture tube  70 . The gain loop L 1  is activated with bus bit from the EGL register  54  and adjusts the gain during the odd field in three successive lines. Preferably, the gain loop L 1  will be only active at certain short moments, e.g. during channel switching to prevent interaction between the gain loop L 1  and the offset loop L 2  and the visibility of the test lines.  
         [0027]    In an exemplary embodiment, the reference current Iref1 of the gain loop L 1  is optional between 200 and 150 μA. The result of the gain measurement is stored in the status gain measurement register  60  and will be stored in external memory during the switch-off mode and will be loaded in the preset gain register  56  after the television is switched “ON” again.  
         [0028]    The white point RGB adjustment register  64 , the cathode drive level register  62  and the status gain measurement register  60  are combined via summer  44  and stored in register  46  to drive the gain multiplier  20  via the digital-to-analog converter (DAC)  48  to get optimal signal-to-noise ratio (SIN) for the video signal, on line a, from video source  12   a.    
         [0029]    When the preset gain is loaded, the gain loop L 1  is disabled. When the EGL is enabled the gain loop is enabled. The gain of the loop is controlled by the value in the up-down counter  42 .  
       Gain-Loop  
       [0030]    Referring now to FIG. 4B, the states of switches SW 1 , SW 2 , SW 4 , SW 5 , SW 6  and the up/down counter  42  for the gain-loop L 1  measurements are shown. When the gain loop is active, the EGL register  54  equals “1” (EGL=1) and the LPG register  58  equals “0” (LPG=0). During three successive lines, just before the end of the vertical blanking the gain measurements are performed every odd field or every other field. During the test line the feedback current I is checked in accordance with parameters set forth in TABLE 1.  
                           TABLE 1                                   Up/Down Counter       I input   Upper Level   Lower Level   Action                   &lt; −5 μA   0   0   Count Up       &gt; −5 μA, &lt;5 μA   0   1   Hold       &gt;5 μA   1   0   Count Down                  
 
         [0031]    The preset gain registers  56  are inputs (read) into the gain loop L 1  and the status registers  60  are outputs (write) from the gain loop L 1 . The status of the gain can be loaded in external memory (values can be held during switch off condition) and be reloaded in the preset gain registers during switch on.  
       Offset-Loop  
       [0032]    Referring now to FIG. 4A, the states of switches SW 1 , SW 3 , SW 4 , SW 5 , SW 7  and SW 8  for the offset loop L 1  measurements are shown. When the offset-loop L 2  is active, the gain-loop L 1  is not active. The offset loop is active in one field and the gain loop can be active in the other field. The offset loop controls the cutoff levels of the three channels. During three successive lines, just before the end of the vertical blanking the offsets measurements are performed. When the offset-loop L 2  is active, the EGL register  54  equals “0” (EGL=0) and the LPG register  58  equals “1” (LPG=1). The up/down counter  42  is not active. The feedback current, on line g, is not checked for gain settling. However, the values present in the preset gain registers  56  are loaded in the up/down counter  42  to control the gain of the gain-loop multiplier  20 .  
       Start-Up Phase  
       [0033]    Referring now to FIG. 4C, the states of switches SW 1 , SW 2 , SW 4 , SW 5 , SW 6  and the read status of the PTW register  52  for the start-up phase are shown. During the start-up phase, the EGL register  54  equals “1” (EGL=1) and the LPG register  58  equals “1” (LPG=1). The test lines are available, but the gain loop L 1  is not active. Instead, the gain of the gain loop L 1  is fixed by the preset gain values in the preset gain register  56 . The status bit in the PTW register  52  becomes high (“1”) when I input&gt;5 μA.  
         [0034]    The present gain register  56  is loaded with the information in the status gain register  60  when the television is switched “ON” during the start-up phase. For simplicity of design of circuit  10 , it is assumed the status registers  60  are part of external memory. When the television is switched “OFF”, the values of the status registers  60  are stored in the external memory. After the television is switched “ON”, the values stored in the external memory are loaded in the preset gain registers  56  and can be used to define the software start-up algorithm before the picture is released.  
         [0035]    In operation during the start-up phase, switch SW 1  disables the video during the vertical blanking period and blanking is inserted by switch SW 5  except during the test lines. During three successive lines, just before the end of the vertical blanking the gain measurements are performed. In one field the offset loop controls the cut-off of the cathodes of the CRT, and in the other field, the gain (drive level) of the cathodes is stabilized. Test pulses are generated internally by with switches SW 2  and SW 3 . During the start-up phase, the EGL equals “1”, the LPG equals “1”, and the value of the preset gain registers  56  equals the previous value of the status register+“x”, wherein the value of x is fixed by the customers&#39; software. The software start-up algorithm determines the value of “x”. When the input current exceeds 5 μA during the RED gain measurement the PTW status bit toggles from “0” to “1”.  
         [0036]    In the start-up procedure according to the present invention, the time before release of the picture tube can be minimized without having aesthetic imperfections. In conventional start-up systems, such solutions did not generally distinguish between the picture tube characteristics (e.g. spread, temp drift and ageing of the RGB amplifier  28  and picture tube  70 ) and temperature behavior (e.g. a warm and a cold picture tube start-up behavior). By predicting the start-up curve which is fixed by these characteristics and temperature behavior it is possible to achieve an optimal start-up behavior for the varying characteristics and temperature behavior.  
         [0037]    During the start-up phase, the picture is blanked by blanking control sub-circuit B 1  and only in the vertical interval (overscan) where test lines are generated for the black current loops during the offset loop L 2 . During the start-up phase, the gain loop L 1  is used to check if the picture tube  70  is (almost) warm or predict the start-up curve. The level of the test pulses at the RGB outputs can be chosen by software with the WP register  64 , the CL register  62  and the status gain measurement register  60 .  
         [0038]    The television manufacturer can do one simple check to release the picture. At a certain test voltage, free to be chosen by software, the feedback current of the picture tube  70  is checked. The PTW (Picture Tube Warm) register  52  becomes “1” when the feedback current exceeds the chosen offset current of 150 or 220 μA. After reloading the memory setting, the picture can be released with or without an additional fixed delay period.  
         [0039]    Referring now to FIG. 2, for better prediction, using stored start-up curve data, two crossing points are generated by performing two checks (e.g. at test 1 and test 2). Thereafter using the measurement results of test 1 and test 2, the optimum release moment (tx delay) and conditions of the picture tube are calculated. Test 1 and test 2 are checks on PTW with different preset values (stored and determined via a software algorithm) to control the gain multiplier  20 .  
         [0040]    The curves of FIG. 2 show different release moments of the picture after the television is switched “ON” on a warm and cold picture tube  70  with suppression of the aesthetic imperfections. Curve C 100  is an exemplary start-up curve for a warm CRT. Curve C 110  is an exemplary start-up curve for a cold CRT.  
         [0041]    When performing test 1 and test 2 on a warm CRT, points P 100  and P 102  are created. The difference in time between points P 100  and P 102  is time T 1 . As can be readily seen, if the manufacturer wanted to release the picture when the characteristics of the picture tube  70  reached point P 103 , the circuit  10  would release the blanking after time T 1  delay (optimum release moment), wherein the time T 1  delay is the difference in time between point P 102  and point P 103 .  
         [0042]    When performing test 1 and test 2 on a cold CRT points P 100 ′ and P 102 ′ are created. The difference between points P 100 ′ and P 102 ′ is time T 2 . As can be readily seen, if the manufacturer wanted to release the picture when the characteristics of the picture tube  70  reached point P 103 ′, the circuit  10  would release the blanking after time T 2  delay (optimum release moment), wherein the time T 2  delay is the difference in time between point P 102 ′ and point P 103 ′ on the cold CRT start up curve C 110 .  
       Start-Up Procedure  
       [0043]    Referring now to FIG. 3, the general flowchart of the start-up procedure in accordance with the present invention is shown and begins at Step  100 . At Step  100 , a switch “OFF” condition is determined. Step  100  is followed by Step  105  where the gain setting is stored. Step  105  is followed by Step  110  where a switch “ON” condition is determined. Step  110  is followed by Step  115  where the picture is blanked during start-up as the television is switched “ON”. Step  115  is followed by Step  120  where a test voltage or test line is applied during vertical interval (overscan) to stabilize the cutoff via the offset loop L 2 . When applying the test voltage or test line, LPG is set to “1” and the gain of the loop is fixed such that the preset value register  56  is set equal to the preset value stored in external memory+“x”. The EGL is set to “1”; and the test lines are automatically generated.  
         [0044]    Step  120  is followed by Step  125  where the PTW becomes “1” and the memory settings are reloaded. Step  125  is followed by Step  130  where the picture is released and the blanking is removed without any additional delay. The preset value in the preset gain register  56  equals the original preset value. Furthermore, the EGL register  54  is set equal to “0” (optional) and the LPG register  58  is set equal to “0”.  
         [0045]    In an alternative embodiment, the blanking can be removed after a predetermined fixed delay established by the manufacturer in lieu of without additional delay.  
         [0046]    During normal operations, the external memory settings are equal to the last results of the status bits. Only during the start-up phase will the preset gain value differ from the status bits to ensure that the feedback current exceeds the offset current so that the PTW register  52  becomes “1”.  
         [0047]    Referring now to FIG. 5, there is shown a general flowchart of a start-up procedure in accordance with present invention using the curves of FIG. 2 that begins at Step  200 . At Step  200 , the picture is blanked during start-up. During start-up, the television has been switched “ON”. Step  100  is followed by Step  205  where a first test voltage or test lines is applied during vertical interval (overscan). When applying the test voltage or test line, LPG is set to “1” and the gain of the loop is fixed. The preset value equals the preset value+“x”. The EGL is set to “1” and the test lines are automatically generated. Step  205  is followed by Step  210  where a second test is performed using a different preset value. Step  210  is followed by Step  215  where an optimum release moment tx and conditions of the picture tube  70  are calculated.  
         [0048]    During test 1 at Step  205 , a first test voltage or test line during vertical interval (overscan) is applied. This is automatically done with LPG=1 and EGL=1 and the preset value equals the preset value+“x1”. During test 1, the PTW register  52  is checked. During test 2 at Step  210 , a second test voltage or test line during vertical interval (overscan). Here, the preset value is equal to the preset value+“x2”. Again during test 2, the PTW register  52  is checked.  
         [0049]    Numerous modifications to and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the embodiment may be varied without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.