Abstract:
A projection display apparatus comprises a display device forming an image for projection to illuminate a display screen. A current source generates a current. A current sink is adjacent to the display screen and is coupled to the current source for receiving the current. A detector is coupled to the current source and the current sink for receiving the current and detecting a presence of the received current. Wherein a first condition, absent illumination by the projected image on the current sink, the current from the source divides and is coupled to the detector in accordance with a first ratio, the detector assuming a first state indicative of the absence of illumination, and in a second condition in accordance with an intensity of illumination by the projected image on the current sink, the current from the current source is received by the current sink in accordance with a second ratio and the detector, absent the received current, assumes a second state indicative of illumination on the current sink.

Description:
This invention relates to the field of image sensing, and in particular to the automated measurement of incident illumination of a sensor in a video projection display. 
     BACKGROUND OF THE INVENTION 
     In a projection video display, geometrical raster distortions result from the physical placement of the cathode ray display tubes. Such raster distortions are exacerbated by the use of cathode ray tubes with curved, concave display surfaces and the inherent magnification in the optical projection path. The projected image is composed of three scanning rasters which are required to be in register one with the other on a viewing screen. The precise overlay of the three projected images requires the adjustment of multiple waveforms to compensate for geometrical distortion and facilitate the superimposition of the three projected images. However, manual alignment of multiple waveforms is labor intensive during manufacturing, and without the use of sophisticated test equipment may preclude setup at a user location. Thus an automated convergence system is disclosed which simplifies manufacturing alignment and facilitates user location adjustment. An automated alignment system may employ raster edge measurement at peripheral display screen locations to determine raster size and convergence. However, at peripheral screen locations, sensor illumination levels may vary widely causing erratic or inconsistent detection of sensor signals thus a detector is required which obviates detection inconsistencies. 
     SUMMARY OF THE INVENTION 
     A projection display apparatus comprises a display device forming an image for projection to illuminate a display screen. A current source generates a current. A current sink is adjacent to the display screen and is coupled to the current source for receiving the current. A detector is coupled to the current source and the current sink for receiving the current and detecting a presence of the received current. Wherein a first condition, absent illumination by the projected image on the current sink, the current from the source divides and is coupled to the detector in accordance with a first ratio, the detector assuming a first state indicative of the absence of illumination, and in a second condition in accordance with an intensity of illumination by the projected image on the current sink, the current from the current source is received by the current sink in accordance with a second ratio and the detector, absent the received current, assumes a second state indicative of illumination on the current sink. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified front view of a projection video display. 
     FIG. 2 is a simplified block diagram of a video image projection display apparatus including inventive features. 
     FIG. 3A depicts various currents occurring during a sequence of time periods. 
     FIG. 3B illustrates a sensor detector output signal during the same time periods. 
     FIG. 4A shows an exemplary automated setup sequence. 
     FIGS. 4B and 4C depict various photo sensor signals and output signal  202 . 
     FIG. 5 shows an exemplary ambient illumination calibration sequence. 
     FIG. 6 shows an exemplary sensor calibration sequence. 
     FIG. 7 shows an exemplary sensor threshold calibration sequence. 
     FIGS. 8A and 8B are simplified schematic diagram showing inventive sensor detector arrangements. 
     FIG. 9 is a simplified schematic diagram showing an inventive digitally controlled current source. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a front view of a video projection display apparatus. The projection display comprises a plurality of cathode ray tubes with raster scanned images which are projected on to screen  700 . A cabinet supports and surrounds screen  700  and provides a picture display area  800  which is slightly smaller than the screen. Screen  700  is depicted with a broken line to indicate an edge area which is concealed within cabinet C and which may be illuminated with raster scanned images when operated in an overscan mode as indicated by area OS. Photo sensors are located adjacent to the periphery of screen  700  within the concealed edge area and outside viewed area  800 . Eight sensors are shown in FIG. 1, positioned at the corners and at the centers of the screen edges. Thus with these sensor positions it is possible to measure an electronically generated test pattern, for example peak video value block M, to determine picture width and height and certain geometric errors, for example, rotation, bow, trapezium, pincushion etc., and thereby align the displayed images to be superimposed one with the other over the whole of the screen area. Measurements are performed in both horizontal and vertical directions in each of the three projected color images thus yielding at least forty eight measured values. 
     Operation of the measurement and alignment system will be explained with reference to FIG. 2 which depicts in block diagram form, part of a raster scanned video projection display. In FIG. 2 three cathode ray tubes, R, G and B form raster scanned monochromatic color images which are directed through individual lens systems to converge and form a single display image  800  on screen  700 . Each cathode ray tube is depicted with four coil sets which provide horizontal and vertical deflection and horizontal and vertical convergence. The horizontal deflection coil sets are driven by a horizontal deflection amplifier  600  and vertical deflection coil sets are driven by a vertical deflection amplifier  650 . Both horizontal and vertical deflection amplifiers are driven with deflection waveform signals that are controlled in amplitude and waveshape via data bus  951  and synchronized with the signal source selected for display. Exemplary green channel horizontal and vertical convergence coils  615  and  665  respectively, are driven by amplifiers  610  and  660  respectively, which are supplied with convergence correction waveform signals. The correction waveform signals GHC and GVC may be considered representative of DC and AC convergence signals, for example static and dynamic convergence. However, these functional attributes may be facilitated, for example by modifying all measurement location addresses by the same value or offset to move the complete raster and achieve an apparent static convergence or centering effect. Similarly, a dynamic convergence effect may be produced by modification of the location address of a specific measurement location. Correction waveform signals GHC and GVC for the green channel are generated by exemplary digital to analog converters  311  and  312  which convert digital values read from memory  550 . 
     An input display signal selector selects, by means of bus  951 , between two signal sources IP 1  and IP 2 , for example a broadcast video signal and an SVGA computer generated display signal. Video display signals RGB, are derived from the display video selector and electronically generated message information, for example user control information, display setup and alignment signals and messages generated responsive commands form controllers  301 ,  900  and  950  coupled via buses  302  and  951 , may be combined by on screen display generator  500 . During automated sensitivity calibration or convergence alignment, controller  900  sends commands via a data bus  302  to controller  301  which instructs video generator  310  to generate an exemplary green channel calibration video test signal AV comprising an exemplary black level signal with a rectangular block M having a predetermined video amplitude value. Controllers  900  and  301  also position block M to illuminate exemplary sensor S 1  by determining horizontal and vertical timing to position block M within the scanned display raster or by moving the scanned raster, or a part of the scanned raster containing the marker block M. Green channel test signal AV is output from IC  300  and combined at amplifier  510 , with the green channel output signal from on screen display generator  500 . Thus, the output signal from amplifier  510  is coupled to exemplary green cathode ray tube GCRT, and may include display source video and or OSD generated signals and or IC  300  generated calibration video test signals AV. 
     Controller  301  also executes a program stored in program memory  308  which comprises various algorithms. To facilitate an initial setup adjustment controller  301  outputs a digital word D on data bus  303 , which is coupled to a controllable current source  250 . The digital word D represents a specific current to be generated by current source  250  and supplied to sensors S 1 - 8  and sensor detector  275 . 
     To facilitate adjustment and alignment of the three color images, setup block M is generated as described previously and coupled to exemplary green CRT. In FIG. 1 test pattern, block M is shown approaching sensor S 1 , and as previously mentioned each sensor may be illuminated by the timed generation of the marker block within a video signal projected with an overscaned raster, or by positioning the scanned raster such that marker block M lights sensor S 1 . With certain display signal inputs, for example computer display format signals, substantially all of the scanned area may be utilized for signal display thus operation with an overscaned raster is largely precluded. During operation with computer display format signals, raster overscan is limited to a nominal few percent, for example 1%. Hence under these substantially zero overscan conditions exemplary sensor S 1  may be illuminated by raster positioning of block M. Clearly, individual sensor illumination may be facilitated with a combination of both video signal timing and raster positioning. 
     Each sensor generates an electron flow which enables conduction in a substantially linear relationship to the intensity of the illumination incident thereon. However, the intensity of illumination at each individual sensor may vary greatly for a number of reasons, for example, the phosphor brightness of each individual CRT may be different, there may be lens and optical path differences between the three monochromatic color images. As each CRT ages the phosphor brightness declines, furthermore with the passage of time, dust may accumulate within the optical projection path to reduce the intensity of illumination at the sensor. A further source of sensor current variability results from variations in sensitivity between individual sensors and their inherent spectral sensitivity. For example, in a silicon sensor, sensitivity is low for blue light and increases through the green and red spectrum to reach a maximum in the near infra red region. Thus, it may be appreciated that each individual sensor may conduct widely differing photo generated currents. Hence, to facilitate stable, repeatable measurements, it is essential that these sensor current variations are individually measured and a detection threshold set for each sensor and illuminating color. Thus, having determined the peak sensor current, which is directly proportional to the intensity of illumination, individual sensor detection threshold values may be stored to permit the subsequent detection of a lit or unlit sensor to occur at a consistent amplitude point of each sensor current, for example at approximately 50% amplitude value. 
     With reference to FIG. 2, video generator  310  is instructed by control logic  301  to generate an exemplary green video block M having an initial non-peak video value and positioned on a substantially black or black level background. Similar video blocks with non-peak video values may be generated in each color channel, which when generated simultaneously and superimposed at the screen produce a white image block on a substantially black background. Thus, an exemplary green block M is generated by video generator  310  and coupled via amplifier  510  to the green CRT. The video generator  310  is controlled by the micro controller  301  to generate the green block M at a horizontal and vertical screen position such that a specific sensor, for example, sensor S 1 , is illuminated by green light from block M. Illumination of the sensor results in a photo generated current Isen, as depicted in FIG.  2 . 
     The widely differing photo generated sensor currents described previously are advantageously compensated, calibrated and measured by means of an inventive control loop  100  depicted in FIG.  2 . Sensor detector  275  is depicted in circuit block  200  of FIG.  2  and is shown in greater detail in FIGS. 8A and 8B. In simple terms a reference current Iref is generated by a digitally controlled current source  250 . The reference current is supplied to both exemplary opto sensor S 1  and sensor detector  275 . In the absence of sensor illumination, sensor S 1 , represents a high impedance and consequently diverts an insignificant current, Isen, from reference current Iref. Thus the majority of reference current Iref, is coupled to sensor detector  275  as current Isw. Current Isw biases detector  275  such that the output state is low, which is chosen to represent an unlit or unilluminated sensor. When sensor S 1  is illuminated, photo generated charge causes the sensor to present a lower impedance and shunt a greater current Isen, from reference current Iref, thus diverting current Isw from sensor detector  275 . At a particular illumination level, sensor S 1  diverts sufficient current from sensor detector  275  to cause it to switch off and assume a high, nominally supply voltage potential, which is chosen to be indicative of a lit or illuminated sensor. The output from sensor detector  275  is positive going pulse signal  202  which is a coupled to an input of digital convergence IC STV2050. The rising edge of pulse signal  202  is sampled which causes horizontal and vertical counters to stop thus providing counts which determine where in the measurement matrix the lit sensor occurred. 
     The sensor current is advantageously measured by controllably increasing reference current Iref until sensor detector  275  switches to indicate loss of sensor illumination. The value of reference current that caused detector  275  to indicate loss of sensor illumination is representative of the level of illumination incident on the sensor. Thus this current may be processed and stored as a sensor and color specific threshold value. The stored reference current value differs between sensors and from color to color, but detector switching is equalized to occur for illumination values of down to one half of the measured Isen switching value. 
     Various currents occurring over a sequence of time periods, for example TV frame periods, are shown in FIG. 3A while FIG. 3B illustrates the sensor detector output signal during the same periods. Upon initiation, at time period t 1 , the reference current Iref is controlled by a digital word D 1 , generated by control logic  301 , in response to micro controller  900 . Digital word D 1  is output with a value equal to 255 which results in a minimum value for reference current Iref. In addition, at time period t 1  exemplary sensor S 1  is not illuminated, and any current Isen is generated by dark current or leakage effects. Thus, the minimized reference current Iref is largely conducted by sensor detector  275  as current Isw, which causes output signal  202  to assume a low or essentially zero volt state, indicating an unlit sensor. At time period t 2 , video block M is generated and positioned to illuminate sensor S 1  causing a photo current Isen to be generated or conducted. Thus, reference current Iref is divided between photo sensor S 1  and sensor detector  275 . However, the magnitude of sensor current Isen 2  is greater than current Iref, thus detector  275  is starved of current causing it switch states. Thus, output signal  202  assumes a high voltage potential which is sampled by control logic  301  and indicates the presence of sensor illumination via bus  302  to micro controller  900 . As a consequence of detected illumination, at time period t 3  micro controller  900  instructs controller  301  to output a new digital word D 2  which causes the reference current to increase to Iref 3 . However, even with an increased reference current the sensor current Isen is sufficient to starve detector  275  of current, as depicted by Isw 3  and thus detector output  202  to continues to indicate a lit sensor. Control logic  301  senses output signal  202  indicating the lit sensor and via micro controller  900  generates a further current control word D 4  which increases reference current to a value of Iref 4 . This increased reference current is sufficient to supply both the sensor S 1  and detector  275 , thus the detector switches and the output signal  202  assumes an essentially zero volt state which indicates to logic  301  and micro controller  900  that sensor S 1  is now unlit. Thus by progressively increasing the reference current, the sensitivity of the sensor signal detector is reduced, and a value Iref 4 , is reached which is representative of the maximum sensor illumination. The value of current Iref 4  is halved and stored as IrefB in memory  305  for use during subsequent measurement of exemplary sensor S 1  when illuminated by an exemplary green marker block M. The reference current is halved to facilitate sensor detection when sensor illumination is reduced to approximately half intensity as a consequence of dirt obscuration or diminished CRT output. 
     FIG. 4A shows an exemplary automated setup sequence which is initialed at step  10 . At step  100  all sensors are evaluated for the ingress of unwanted illumination. Adjustments, detailed in FIG. 5, are made to compensate for unwanted spurious sensor lighting, which if successful enables the automated setup to continue to step  200 . However, if the unwanted sensor illumination is beyond a range of compensation, the setup sequence is terminated and an on screen display message is generated at step  675 , advising of test termination and suggesting that ambient illumination of the display screen be reduced by reducing room lighting or closing window curtains. 
     At step  200 , which is shown in detail in FIG. 6, the sensors are tested to determine their ability to be illuminated by the exemplary marker block M. Once again if the test is successful the automated setup continues to step  400  detailed in FIG.  7 . However, if each sensor fails to respond to illumination by marker block M the setup sequence is terminated and an on screen display message is generated advising of test termination and suggesting that ambient illumination of the display screen be reduced by reducing room lighting or closing window curtains. 
     Thus, having compensated for unwanted sensor illumination, step  400  measures the sensitivity of each sensor for each position and color. Once again, measurement failure at step  400  terminates the auto setup sequence with an OSD message indicating the failure and suggesting possible remedies. With the successful completion of step  400  the automated sequence progresses to step  600  where electronically generated patterns are measured to enable image registration to be performed. 
     FIG. 4B depicts photo sensor signals generated under various conditions. Signal  100  depicts a generally constant amplitude signal generated by unwanted illumination of a photo sensor and is shown with a broken line to portray a constant presence to which wanted photo sensor signal responses, for example signals  200  and  400 , are added. FIG. 4C depicts detector  275  output signal  202  on the same time axis as FIG.  4 B. During the control sequence depicted in diamond box  100  of FIG. 4A, the detection threshold is adjusted such that ambient light generated signal,  100  of FIG. 4B is too small to exceed threshold  101 , AMBI. SENS. VAL. Thus no output signal  202  is generated by detector  275 , as depicted in FIG.  4 C. 
     Signal  200  of FIG. 4B depicts a wanted photo sensor signal, for example, generated by green illumination. Signal  200  is illustrated with an amplitude which is insufficient to exceed threshold signal  100 , thus the extinguishment of ambient illumination detection has also extinguished detection of a lit green sensor. In diamond box  200  of FIG. 4A, the sensor is tested to determine if it can be illuminated by an exemplary marker block M. Thus the marker video amplitude is incremented, depicted by AMP. INC. to a value which permits the sensor signal to exceed threshold  101 , and generate a corresponding output signal  202  of FIG.  4 C. 
     Signal  400  of FIG. 4B depicts a wanted photo sensor signal generated for example by blue illumination. The sensor signal clearly exceeds threshold  101 , thus the sequence of diamond-box  400 , of FIG. 4A, is followed to determine the peak sensor signal value, which is equal to the reference current required to extinguish detection. This peak value amplitude value is halved and stored as the blue detection level or threshold. 
     FIG. 5 shows an exemplary adjustment sequence which detects the presence of unwanted sensor illumination and automatically adjusts a sensor detection threshold to progressively compensate for ambient lighting until the unwanted illumination fails to be detected. Thus the amount of ambient light compensation sets a maximum range of compensation available for each sensor position and color combination. Ambient illumination compensation starts at step  110  with a blank raster generated and sensor detector sensitivity set to a maximum value at step  120 . In the exemplary detector of FIGS. 2 and 9, sensitivity or switching threshold may be set to any one of  255  values determined by digital word D generated by controller  301 . Maximum sensitivity occurs with a digital word value of 255 which results in a minimum current Iref. Thus, with a blank raster, video block M is not generated and any lit sensor must be illuminated by unwanted light. At step  130  a delay is introduced to allow detection of a lit sensor to occur at step  140  and be reported to convergence micro computer  900  within a display frame period. If step  140  tests YES, the sensor is lit by unwanted light and the sensor detector sensitivity is reduced at step  150 . In response to an instruction from micro computer  900 , coupled via bus  302 , controller  301  outputs a new value for word D. At step  160  a test is performed to determine that the decremented sensitivity value is greater than zero. A YES at  160  forms loop  165  which continues to decrease detector sensitivity until test  140 , lit sensor, indicates NO signifying that the effect of unwanted sensor illumination has been compensated or nullified. The NO at step  140 , causes the sensitivity value which extinguished ambient light detection to be stored as AMBI. SENS. at step  170 . The NO at step  140  also results in the sensitivity value being tested, at step  175 , to determine if the value is less than a predetermined value n. A NO at step  175  results in the generation of an on screen display message suggesting that ambient illumination is of a sufficiently high level which may result in subsequent setup failure. For example, blue sensor signal levels may exceed the remaining sensitivity control range, or conversely a green sensor signal may not be of sufficient amplitude to exceed the ambient light extinguishment threshold. Thus restarting the auto sequence is suggested with reduced ambient illumination. However, although the advisory OSD message is generated responsive to the NO at step  175 , the ambient illumination compensation test is completed as indicated at step  185 , for example by means of an OSD message, and the automated sequence continues to initiate marker brightness calibration at step  210 . 
     However, if step  140  continues to test YES as loop  165  successively reduces detector sensitivity and eventually step  160  will test NO, indicating for example that word D is equal to zero. Since with minimum detector sensitivity, a lit sensor continues to be detected, hence the unwanted illumination is excessive and beyond the range of compensation thus the auto sequence is terminated at step  180 . At step  190  an on screen message is generated to advise the user that incident screen illumination must be reduced to permit automatic setup to be performed. 
     FIG. 6 illustrates an exemplary sequence which determines that the sensor detector can detect a sensor signal produced by marker block M generated and displayed in each of the three display colors. The sequence starts at step  210  and at step  220  the sensor detector sensitivity is set to the value, AMBI. ILL. stored at step  165  in FIG.  5 . An exemplary marker block M is generated with a predetermined video amplitude value, of for example step  12 . Marker block M is displayed on a green CRT and positioned on the display surface such that when projected, it will illuminate a sensor, for example sensor  1 . A delay, for example one display frame period, is applied at step  230  to allow time for a lit sensor to be detected. If the CRT display is scanned with an interlaced format, block M may be detected in either of the interlaced fields comprising the display frame period, hence step  230  allows for sensor detection of block M in either display field. A test is performed to detect a lit sensor at step  240 , where a NO causes step  250  to increase the brightness, or video amplitude of marker M from the initial predetermined value. At step  260  the marker amplitude value is tested to be less than 15 with a YES forming a loop  265  back the delay  230 . Thus loop  265  progressively increases the video amplitude of marker M by one amplitude control step until sensor  1  is detected as lit and step  240  tests YES. When the sensor is lit the corresponding brightness, or marker amplitude value is stored at step  270  as BRI. VAL. 
     However, if the lit sensor test  240  continues to test NO, eventually a maximum value will be reached for the video amplitude where test  260 , BRI. VAL. will equal 15. Thus, NO at test  260  results in the termination of the automatic calibration sequence at step  360  and the generation of an on screen display message at step  370 , indicating that the intensity of illumination incident on the exterior screen surface must be reduced to permit the automatic sequence to be reinitiated. 
     With sensor  1  lit at step  240 , a further test is performed at step  280  to determine if all eight sensor positions have been evaluated. A NO at step  280  causes step  290  to reposition block M on the CRT face to illuminate exemplary sensor  2 . The video amplitude of marker block M is set to the predetermined, exemplary amplitude value 12, at step  300  and the lit sensor loop  265  is rejoined at delay  230 . Thus as for sensor position  1  loop  265  is traversed until step  240  tests YES, to detect a lit sensor, or until maximum video amplitude is reached and step  260  tests NO terminating the calibration sequence as described. If step  240  tests YES, a second loop  285  is formed where amplitude video amplitude values are stored, at step  270 , for marker M at sensor positions  2  through  8 . 
     When marker amplitude values have been determined and stored for all eight sensor positions, step  280  tests YES causing a color selection test to be performed at step  310 . Since the sequence initiated with the green CRT step  310  tests G YES which at step  320 , terminates the green marker display and causes a red marker to be generated, displayed and positioned on a red CRT to illuminate sensor position  1 . At step  300  marker video amplitude is set to the predetermined, exemplary amplitude value 12and loops  265  and  285  are successively traversed storing amplitude values, at step  270 , for each of the eight red sensor positions. Thus, sensor count step  280 , tests YES causing step  310  to test R YES which results in step  320  terminating the red marker display and causing a blue marker to be generated, displayed and positioned on a blue CRT to illuminate sensor position  1 . Again the marker video amplitude is set at step  300  and loops  265  and  285  are successively traversed storing amplitude values, at step  270 , for each of the eight blue sensor positions. With the completion of sensor eight, step  280  tests YES and color selection test  310  tests B YES. Thus step  340  indicates that the brightness or video amplitude test has been successfully completed for all colors and sensor positions and that sensor sensitivity calibration may be initiated at step  410 . In summary, completion of the sequences depicted in FIG. 5 compensates for unwanted display surface illumination by establishing a sensitivity value for lit sensor detection that precludes the unwanted light. The sequences shown in FIG. 6 establish that, with the sensitivity value necessary to prevent spurious measurement of extraneous light, each sensor can see markers generated in each color. 
     The widely differing photo generated sensor currents described previously are measured and stored by use of the exemplary sensor calibration sequence shown in FIG.  7 . In simple terms the sequence shown in FIG. 7 determines a maximum value for each sensor output signal and then establishes a sensitivity value, or detector switching threshold, which allows detection to occur at substantially half the peak value for each sensor signal. Sensor sensitivity calibration is initiated at step  410  and at step  420  the detector sensitivity is set to the value determine and stored as AMBI. SENS. at step  165 . A green marker block M is generated with the previously determined video amplitude value BRI. VAL. read from step  270 . Block M is positioned on the green CRT to illuminate sensor position  1  when projected. For the reasons discussed previously a delay  430  is included to provide time to detect a lit sensor condition occurring in either field of an interlaced scanning system. 
     Step  440  tests to determine if a sensor has been lit, clearly since the initial sensitivity at  420  was set to AMBI. SENS., the highest sensitivity value which excluded detection of extraneous light, sensor  1  should see green block M and detect it as lit. The YES at  440  causes the sensitivity value, currently AMBI. SENS., to be reduced at step  450 . This reduced sensitivity value is tested at step  460  to determine if the value is greater than zero, thus indicating that the range of sensitivity control has not been exceeded. A YES at  460  forms loop  465  which comprises steps  439 ,  440 ,  450   460  that are circled iteratively until step  440  detects an unlit sensor and tests NO. However, loop  465  may be traversed without detecting an unlit sensor condition until step  460  tests NO indicating that the sensitivity range limit has been met. A NO at step  460 , indicates at step  470  that the calibration sequence has failed and causes an on screen display message to be generated at step  475  which indicates that the level of extraneous, incident screen illumination is still too great to permit automated setup. 
     As described above, loop  465  successively reduces sensitivity until an unlit sensor is detected and step  440  tests NO. The sensitivity value which caused an unlit condition is halved and stored at step  445 . Following storage of the 50% sensitivity value, step  480  testes to determine if the current sensor position is number  8 . Step  480  initiates a sensor position loop  485  comprising steps  490 ,  500 , loop  465  and step  445 . Since step  420  initiated the sequence at sensor position  1  in the green channel, test  480  testes NO which causes step  490  to move block M to sensor position  2 . At step  500  the video amplitude value is read from storage (step  270 ) for this block position and color and applied to the block. The detector sensitivity is restored to the AMBI. SENS. value stored at step  165 , and loop  465  is initiated at delay step  430 . As described, loop  465  is iterated until step  440  detects an unlit sensor, or step  460  indicates a sensitivity value limit. It is assumed that an unlit sensor condition is achieved and step  440  tests NO causing, the corresponding sensitivity value for green sensor position  2  to be halved and stored at step  445 . Thus having established the 50% value for sensor position  2  both sensitivity loop  465  and sensor position loop  485  are completed and step  480  tests NO initiating loop  485  for sensor position  3 . Thus loops  465  and  485  are circled until step  480  tests YES indicating that the current sensor position is  8 , indicating that all sensors for the green channel have been calibrated. 
     The YES at step  480  causes a color test to occur at step  510  where the current color under calibration is tested, which in this exemplary green sequence results in G. YES. The G. YES at step  510  causes step  520  to switch from green to red marker block generation with the red marker positioned on the red CRT to illuminate sensor position  1 . At step  500  the video amplitude of the red marker block is set to the position  1  value, (stored at step  270 ), and the detector sensitivity is restored to the AMBI. SENS. value stored at step  165 . Following initialization of the red block parameters at step  500 , iterative adjustment loop  465  is initiated at delay step  430 . As described, loop  465  is circled until step  440  detects an unlit sensor, or step  460  indicates a sensitivity value limit. It will be assumed that an unlit sensor condition is achieved and step  440  tests NO causing the corresponding sensitivity value for red sensor position  1  to be halved and stored at step  445 . Thus having established the 50% value for sensor position  1  both sensitivity loop  465  and sensor position loop  485  are completed and step  480  tests NO. Loop  485  is reinitiating moving block M to sensor position  2  at step  490 , and at step  500 , the ambient sensitivity and red position  2  block brightness values are set. Again loops  465  and  485  are iterated until step  480  tests YES indicating that the current sensor position is  8 , indicating completion of red channel calibration. 
     The YES at step  480  initiates step  510  which determines the current color being calibrated and results in R. YES which in turn causes step  530  to switch from red to blue block generation with the blue marker block positioned on the blue CRT to illuminate sensor position  1 . At step  500  the blue block parameters are initialized for position  1  and iterative adjustment loop  465  is initiated via delay step  430 . As described previously, loops  465  and  485  are circled and successively determine 50% sensitivity values for each sensor position when illuminated by the blue CRT. However, since sensor currents for blue illumination are significantly larger than those of red or green, calibration of blue lit sensors may be more rapidly calibrated with larger control steps increments than those of red or green. Once again step  480  tests YES, indicating completion of eight sensor positions, and color test  510  tests B. YES, which in turn indicates at step  540  that sensor sensitivity calibration has been successfully completed for all sensor positions in each display color. At step  550  convergence pattern edge detection is initiated. 
     As was described with respect to FIG. 2, the reference current Iref divides or is shared between sensor detector  275  Isw and the exemplary sensor S 1 , Isens. With light incident on sensor the majority of reference current Iref is conducted by the sensor. However as the control sequence ( 60 ,  70 ,  130 ,  140 ,) increments the current, a value is reached where the sensor current requirement is equaled and surplus current is available to cause detector  275  to switch, producing NO at step  70  indicating that the sensor is unlit. The sensor is still lit but the control sequence has now established the peak sensor current and caused the sensor detector to switch. Hence the NO at  70  initiates a test to determine if the threshold or reference current at step  60  is set to minimum, the starting sequence condition. If step  80  tests NO, the current generated by step  60  and caused detector switching, is decrement or halved responsive to step  85 . 
     Having reduced the threshold or reference current, detector  275  switches to indicate a lit sensor at step  70 . The YES at  70  causes step  85  to be tested at step  130 . Since step  85  decrement current Iref, to establish a detector switching threshold current Ithr, step  130  tests YES and the nominally halved peak current value, Ithr, is stored at step  150 . Step  160  tests to determine if the calibration sequence if to be repeated, by YES which loops to step  20 , or by NO which ends the calibration sequence. Thus the automated sensor calibration sequence produces digital values corresponding to detector switching threshold currents Ithr which are stored for each sensor position and color. The halving of the peak sensor current value advantageous establishes a switching threshold which is substantially the same for each sensor position and color thus minimizing potential sensor pulse width detection variations during subsequent deflection signal processing. 
     A sensor detector is shown in FIG.  8 A and comprises a current source transistor Q 2  with a digitally controlled emitter network, depicted as resistor R 1 , coupled between the emitter and the positive supply. The digitally controlled emitter network is shown and described with reference to FIG. 9 which also includes the detector of FIG.  8 A. However, in simple terms resistor R 1  may be set to binary related values between 200 ohms and 50 K ohms. Thus resistor  11  and the potential at the base of transistor Q 2 , determine the magnitude of a reference current Iref, generated at the transistor collector. The constant current Iref is divided to form current Isen, which is coupled via a ferrite inductor FB 1  to a photo detector S 1 , for example a photo transistor, and current Isw, which is coupled to the base of an NPN transistor Q 3 . The base of transistor Q 3  is coupled to ground by a capacitor C 1  which forms a low pass filter with ferrite inductor FB 1  to attenuate high frequency energy resulting from, for example horizontal scanning frequency signals or high voltage arc components that may cause spurious circuit operation or component damage. The emitter of transistor Q 3  is grounded and the collector is connected to the emitter of NPN transistor Q 4  to form a cascode connected amplifier. The base of transistor Q 4  is biased by a voltage divider formed by resistors R 2  and R 3 . Resistor R 2  is connected to the positive supply and resistor R 3  is connected to ground with the junction of the resistors biasing the bases of transistors Q 2  and Q 4  to 2 volts when the base emitter junction of transistor Q 4  is not conducting. The collector of transistor Q 4  generates an output signal  202 , which indicates the illuminated state of sensor S 1 , i.e. lit or unlit, for coupling to a digital convergence integrated circuit, for example type STV2050. 
     The sensor detector of FIG. 8A operates as follows. The reference current Iref is divided forming a sensor current Isen and a detector or switch current Isw. When sensor S 1  is unlit it represents a significant impedance consequently current Isen is insignificantly small comprising, for example, leakage and dark current. Thus with current Isen insignificantly small, the majority of current Iref is directed to the base of transistor Q 3  as current Isw. Current Isw causes transistor Q 3  to turn on and saturate, forcing the collector to assume a nominally ground potential of Vcesat, approximately 50 milli volts. Hence, the emitter of transistor Q 4  is nominally grounded via the saturated collector emitter junction of transistor Q 3 , and transistor Q 4  is turned on causing the collector to assume a potential of nominally 100 milli volts or (Q 3  Vcesat+Q 4  Vcesat). The collector of transistor Q 4  forms output signal  202  where nominally zero volts indicates an unlit sensor condition and the nominal supply voltage represents a lit sensor. 
     With transistor Q 3  saturated, the emitter base potential of transistor Q 4  is reduced from nominally 2 volts, due to the resistive divider R 2  and R 3 , to a voltage of about 0.7 volts formed by the base emitter junction voltage of transistor Q 4  and the saturation voltage of transistor Q 3 . Since the base of current source transistors Q 2  and cascode transistor Q 4  are joined, as depicted by bb of FIG. 8A, the bias at the base of transistor Q 2  is also reduced to nominally 0.7 volts. This change in transistor Q 2  base potential results in constant current Iref increasing by about three times. 
     When sensor S 1  is lit it generates or sinks current in proportion to the intensity of the incident illumination, thus sensor current Isen increases rapidly. Since the reference current Iref is constant, current for the lit sensor (Isen) is diverted from the base current (Isw) of transistor Q 3 , causing the transistor to turn off. With transistor Q 3  off, transistor Q 4  is turned off causing the collector to rise to the supply voltage, generating an output signal  202  of nominally 3.3 volts amplitude indicating a lit sensor. As described previously, with transistors Q 3  and Q 4  turned off, the base bias of current source transistor Q 2  reverts to the potential determined by the resistive divider (R 2  and R 3 ) with the result that the magnitude of constant current Iref is decreased by approximately 66%. Thus, the reduction in reference current Iref advantageous sustains or latches the lit sensor condition by establishing a lower switching threshold for terminating detection and indicating a sensor off or unlit condition. In summary the advantageous sensor signal detector permits detection over a range of sensor signal amplitudes represented by predetermined reference current Iref and set by resistor R 1  or digital word D. In addition the advantageous feedback coupling provides for a sensor off or unlit detection threshold value which has a substantially constant percentage value for any predetermined reference current Iref value. Hence the detector with hysteresis provides a first detection level set by a current Iref and a second detection level resulting from the changed reference current. Thus detector with feedback prevents intermittent or ragged edge illumination of sensor S 1  from toggling the state of output signal  202  and generating multiple spurious measurements within the digital convergence integrated circuit. 
     An alternate arrangement which provides different detector thresholds is shown in box  275 A of FIG.  8 A. The circuitry of box  275 A is inserted, breaking coupling bb to the base electrode of transistor Q 2 , as depicted by the wavy lines. The alternate threshold arrangement operates as follows. Resistors R 2   a  and R 3   a  form a potential divider which biases the base of current source transistor Q 2  to about 2 volts. A transistor Q 2   a  forms a switch which is activated by the collector potential of transistor Q 4 . When sensor S 1  is unlit transistor Q 4  is turned on and the collector is substantially at ground potential which turns on transistor Q 2   a  coupling resistor R 3   b  in parallel with resistor R 3   a . Thus, the bias at the base of transistor Q 2  is changed between nominally 0.7 volts and 2 volts. The lower base voltage provides a lit detection threshold current and the higher voltage reduces the detection threshold current to sustain the detected state with a reduced intensity of illumination. 
     A further sensor detector is shown in FIG.  8 B and functions as follows. A PNP transistor Q 2  functions as a controllable source of current Iref. The emitter of transistor Q 2  is connected to a positive supply +V via a variable resistor R 1 . The controllable current source depicted by transistor Q 2  and variable resistor R 1  are shown in greater detail in FIG.  9 . The base of transistor Q 2  is coupled to a voltage divider formed by resistors R 2  and R 3 , where resistor R 2  is connected to the positive supply +V and resistor R 3  is connected to ground or the return side of supply +V. The junction of the voltage divider resistors is also connected to the base of an NPN transistor Q 4  which is connected in a cascode arrangement with an NPN transistor Q 3 . 
     The collector of current source transistor Q 2  is connected via an inductor FB 1 , for example a ferrite choke, to a collector electrode of an opto sensing device S 1 , for example an opto transistor. The emitter of opto sensor S 1  is connected to ground, and the base region is exposed to receive incident photo illumination as depicted by marker M. The collector of transistor Q 2  is also connected to a diode network where diode D 1  is connected in parallel with a pair of series connected diodes D 2  and D 3 . The cathode of diode D 1  is connected to the anode of diode D 2  and the anode of diode D 1  is connected to the cathode of diode D 3 . The cathode of diode D 3  is connected to ground, and the cathode of diode D 1  is connected to the collector of transistor Q 2 . In addition the collector of transistor Q 2  is also connected via a resistor R 6  to the base of PNP transistor Q 5 , which with PNP transistor Q 6 , forms a differential amplifier. Thus as described with respect to FIG. 3A, current Iref is divided between sensor S 1  and diode network D 1 , D 2  and D 3 . When sensor Q 1  is unlit, current Iref, becomes current Isw, which is conducted via series connected diodes D 2  and D 3  to ground. Diode D 1  is normally reversed biased and provides a protective path to ground for negative transient effects such as, arcs, EMI etc. Thus, current Isw generates a voltage across diode network D 1 , D 2  and D 3  of approximately 1.2 volts which is coupled to the base of transistor Q 5 . The collector of transistor Q 5  is connected to ground and the emitter is connected to the emitter of transistor Q 6  which is connected to the positive supply +V via resistor R 9 . The base of transistor Q 6  is connected to the junction of resistors R 7  and R 8 , which form a voltage divider, where resistor R 7  is connected to the positive supply +V, and resistor R 8  is connected to ground. Thus resistors R 7  and R 8  bias the base of transistor Q 6  to a voltage of approximately 0.3 volts. Since transistors Q 5  and Q 6  are configured as a differential amplifier, and the base of transistor Q 5  is held at 1.2 volts, the approximately 0.3 volt base bias causes transistor Q 6  to be turned on and transistor Q 5  turned off. Thus current from the positive supply +V, coupled via resistor R 9 , is divided between collector load resistor R 10  and the base of cascode connected transistor Q 3 . Since the base emitter impedance of transistor Q 3  is considerably less than the value of load resistor R 10 , the majority of the collector current is coupled to ground via the base of transistor Q 3 , which turns on and saturates. Thus with transistor Q 3  saturated, the emitter of transistor Q 4  is brought within a few milli volts of ground and responsive to the nominal base bias of 2 volts transistor Q 4  turns on and saturates. Hence a current flows via resistor R 4  and the collector transistor Q 4  assumes a potential of 2×Vcesat or approximately 50 milli volts. In addition the collector of transistor Q 4  forms detector output signal  202  which is coupled to controller  301  of exemplary convergence integrated circuit STV 2050. 
     With transistors Q 3  and Q 4  saturated, the base of transistor Q 4  assumes a potential of [Vbe(Q 4 )+Vcesat(Q 3 )] or approximately 0.65 volts. Thus, the current generated by current source transistor Q 2  is increased and held at nominal current of (3.3 v−0.7 v −0.6 v)/R 1 . 
     When sensor Q 1  is illuminated photo generated current or conduction occurs, causing current Isen to increase and current Isw to decrease. At some level of sensor Q 1  illumination, sufficient current Isens is diverted from series connected diodes D 2  and D 3  and conduction ceases, with the result that the base of transistor Q 5  assumes the voltage at the collector of sensor Q 1 . Thus, the base of transistor Q 5  brought to a lower potential than that of transistor Q 6  causing the transistor pair to switch. Transistor Q 5  turns on and conducts current from resistor R 9  to ground and transistor Q 6  turns off removing base current from cascode transistor Q 3 , which turns off together with transistor Q 4 . The collector transistor Q 4  then rises to the supply potential +V, and output signal  202  assumes a positive potential indicative of a lit sensor condition. With transistors Q 3  and Q 4  turned off, the potential at the base of transistor Q 2  rises to about 2 volts, as set by resistors R 2  and R 3 . Thus the change in base emitter potential of transistor Q 2  causes reference current Iref to reduce to approximately one third, with the consequence that intermittent or ragged edge illumination of sensor Q 1  within a range of about three to one is prevented from toggling the state of output signal  202 . In addition the action of a differentiator formed by capacitor C 1 , resistor R 5  and transistor Q 7  couple a positive transient to the base of transistor Q 6  which holds the transistor off and prevents further switching by the transistor pair for a time period determined by the differentiator time constant. 
     FIG. 9 shows a digitally controlled current source, for example as depicted by block  250  in FIG. 2, or as variable resistor R 1  in FIGS. 8A and 8B. The digitally controlled current source is illustrated in FIG.  9  and shown coupled to the sensor detector described and shown in FIG. 8A. A digital control word D is generated by controller  301  and comprises 8parallel data signals D 0 -D 7 , representing from least to most significance respectively. The individual data bits are coupled via series connected resistors R 1 , R 3 , R 5 , R 7 , R 10 , R 13 , R 16  and R 19  to the bases of corresponding PNP transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6 , Q 7 , and Q 8 . The emitter of transistor is connected to a positive supply +V and each collector is coupled via a resistor to the emitter of a PNP transistor current source, Q 9 . Thus the current sourced by transistor Q 9  is controlled by emitter resistor R 22  and the parallel combination of the digitally selected resistor network. The collector resistors R 2 , R 4 , R 6 , R 8  and R 9 , R 11  and R 12 , R 14  and R 15 , R 17  and R 18 , R 20  and R 21  are selected to have values of resistance which increase in a binary sequence. For example, the parallel combination of resistors R 20  and R 21  approximate to 400 ohms, and resistor combination R 17  and R 18  approximate to 800 ohms. Thus digital word D 0 -D 7  can select resistance values between 200 ohms, with all transistors turned on, and 51.1 kilo ohms, due to resistor R 22 , with all transistors turned off. Digital word D 0 -D 7  has voltage values of zero and 3.3 volts, with resistor selection occurring when a data bit has a zero volt value, and no resistor selection when the bit has a 3.3 volt value. 
     As described previously for FIG. 8A, constant current source transistor Q 9  generates a collector current Iref which is divided between, or directed to, sensor S 1  when lit and switch transistor Q 10  when sensor S 1  is unlit. However, as previously described the intensity of sensor illumination varies greatly from sensor to sensor and from color to color, hence so too does the sensor current Isen. Thus to establish a consistent switching point for each sensor combination requires that individual reference currents are determined, stored and used for each sensor. Furthermore, as previously described for FIG. 8A, the inventive switching hysteresis resulting from cascode connected transistors Q 10  and Q 11  advantageous changes the digitally determined reference current to ensure consistent sensing, or detection, of the sensor signal. In simple terms the combined result of the digitally determined reference current and the sensor detection switching hysteresis may be considered to locate the sensor lit threshold at approximately half the peak sensor signal amplitude, with the sensor off, or unlit threshold being set dynamically, by the advantageous hysteresis, to approximately one third the sensor lit threshold value. Thus detector turn on is consistently maintained at about half the sensor amplitude with sensor illumination imperfections largely suppressed and thereby unwanted toggling of sensor detector output signal  202  is prevented.