CRT Device Light Versus Input Signal Characteristic Function

The present invention relates to a system and a method for measuring the light from a cathode ray tube device as a function of the input signal to that device.

A portion of the disclosure of this patent document contains material to which a claim of copy right protection is made. The copyright owner has no objection to the facsimile reproduction of any one of the patent documents or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but reserves all other rights whatsoever.

CRT devices particularly of the multi-phosphor gun type require a number of settings for variables such as white balance (R, G, B screen controls), black balance (R, G, B bias controls), contrast and black level (brightness) which ultimately determine the CRT's light response to an input signal. In U.S. Pat. No. 3,962,722 entitled "Color Television Setup Apparatus and Method" by W. S. Ciciora there is disclosed a factory production-line system for the adjustment of controls on television receivers to obtain consistent CRT output. Test patterns are displayed on the CRT screen and are measured by photosensitive devices at the system input. The outputs of the system are connected to bidirectional motors temporarily attached to the shafts of the controls of the receiver to adjust the controls as a function of the light measurements. Prior to the aforementioned patented invention, the adjustment of the various controls was performed by an operator in response to a "go", "no-go group" of lights which received inputs from the face of the CRT. Two additional patents of interest are U.S. Pat. No. 4,700,218 entitled "Automated System for Adjustment of the White Balance in a Signal Displayed on the Screen of a Color Cathode Ray Tube", by P. N. Thomsen et al. and U.S. Pat. No. 4,706,108 entitled "Automatic Setup System for Controlling Color Gain, Hue and White Balance of TV Monitor" by T. Kumagai et al. Both of the inventions covered in the adjustment of some CRT device parameters in accordance with stored reference values. The stored values are then used to provide adjustment signals for automatically adjusting one or more of the controls associated with the CRT to bring the value of the control to some predetermined standard value.

Another patent of interest is U.S. Pat. No. 4,415,921 entitled "Automatic Calibration System for Video Displays in Vision Testing" by P. Mulvanny et al. The method of that patent relates to a procedure for calibrating a video display to obtain a desired light level in response to an input level. Of main concern is the ability to repeatably adjust to the selected values of brightness and contrast. One of the prime uses of the invention is to provide a degree of standardization to vision tests in order to provide statistically constant data that can be utilized to detect vision problems.

The present method and apparatus determines the light output from a cathode ray tube device through a characteristic equation as a function of the device input signal. Parameters of the equation are determined by a two stage process once the measurement system, CRT device and signal generator are configured. The measurement system is controlled by a computer and software program. The program directs a signal generator to output a plurality of discrete signal values. For each signal, the program records a value from a light meter. The light meter's sensor is rigidly positioned to collect the light output from the CRT at specific and repeatable CRT-screen regions. The number of signal values and their relationships are requested as a part of the program operation. The final series of monochrome or multichannel (for example red, green, and blue) signal values and light readings are placed into a light table which fulfills stage one (Light Table Collection) of the system process.

Stage two (Light Table Analysis) evaluates each parameter of the characteristic equation [see Equation (1)]in an automatic operation or by the exercise of controls that allow an operator to analyze the error surface occurring between the actual light measured and the theoretical light predicted by the characteristic equation.

FIG. 1 graphically illustrates the CRT device's characteristic equation. Note that the log-log axis shows that the gamma (.gamma.) is associated with the slope as an exponential or power operator while the device gain G is a linear operator on the input signal. The device gain change the resolution (level-to-level discrimination) and range of the signal. The offset O starts taking control of the function when the signal gets into a low range.

The CRT device characteristic function is based on equation (1) which mathematically describes FIG. 1. Preferred value of parameters K, .gamma., G, and O are computed, based on a statistical regression and optimization technique. This occurs in the stage two process. The stage two process also allows trial values to be tested so that regions of minimum error, brought about by optimized values, can be fully evaluated by the operator for any satisfaction. criteria desired. The CRT device's characteristic (predicted light) equation is:

where

L=phosphor-light output

K=a specific constant which converts a specific unit of signal into a specific unit of light

S=input signal to the device

G=device gain

O=device offset

and

.gamma.=an exponential relationship between a s drive signal and its light output.

From the foregoing, it can be seen that a primary object of the present invention is to model specific light characteristics of a CRT device as provided by signal processing circuits from the device input to the CRT and the light output characteristics of the CRT.

It is a further object of the present invention to provide a method for simulating the response of a CRT device to an input signal.

Another object of the present invention is to establish the repeatability of the measurement of the parameters of the characteristic equation.

It is another object of the present invention to provide a method and an apparatus for more accurately determining the characteristics of a cathode ray tube device in response to an input signal.

These and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein like characters indicate like parts and which drawings form a part of the present specification.

FIG. 1 is a graph illustrating the CRT device's characteristic equation.

FIG. 2 is a block diagram illustrating the system of the present invention.

FIG. 3 is a flow chart illustrating the method of the present invention.

The graph of FIG. 1 has been previously discussed under the paragraph, "Summary of the Invention".

Referring to FIG. 2, the system 10 configuration of components incorporates two cathode ray tube (CRT) monitors 14 and 20, each having a rigid bracket or positioning device 34 for attachment and repeatable positioning of a light meter 26's sensor to the face of the tube. Once the configuration of system 10 is selected and assembled, as generally depicted in FIG. 2, it is controlled by the human operation of a computer 30. In the system 10, computer 30 is of the Digital Equipment Corporation PDP-11 family running under RSX11M. Here, system 10 provides signal generators 12 and 18 that are compatible with the CRT device or encoder. An appropriate incoder 22 outputs a signal consistent with a corresponding standard monitor 20. Switches 16 and 24 allow convenience and are for the purpose of providing paths for characterizing multiple types of CRT devices such as those shown in system 10. Under computer 30 program control, interface 32 directs the signal generator to output a specifically channeled (for example red, green, blue or gray), specifically sized and positioned video-image rectangle, and specific signal levels. Also under program control, interface 28 receives the light meter 26's measurement of light from the CRT.

A single evaluation of the characteristic function parameters is conducted with switches 16 and 24 in a set position, and with a single generator and a single monitor. Once the light table of stage one (See FIG. 3, block 52) is complete, a different or similar system 10 component configuration is important in statistical analysis of parameter values.

Industry standard generator 12 can provide a signal to standard monitor 20 via the standard encoder 22 or directly through switch 24. It should be noted that standard as used herein implies convention to a system such as NTSC, PAL, and SECAM. Generator 12, being an industry standard, permits a comparison to be made with a host signal generator under actual environmental (hardware terminations, temperature, earth's magnetic field, etc.) conditions. By operating the standard generator 12 to provide a full-field-level-selective output signal, it is possible, through proper interface to standard encoder 22, to obtain discrete monochrome (for example red, yellow and gray) signal levels for comparison to those from a host generator path through the same encoder 22.

Use of different radiance type light meters, with no other change to system 10, may poduce a change in parameters K, G and O but, theoretically, not in parameter .gamma.. Use of different luminance type light meters should not produce a change in any parameter. However, because all of the aforementioned parameters are finally determined statistically, the uncertainty and tolerance of all system components will affect actual parameter evaluation.

Use of different signal generators may provide different units of signal control (such as volts, IRE units, digital levels corresponding to the total number of bits, etc.) These different units will affect the numerical value but not the meaning of parameters K, G and O but should not, in theory, affect the numerical evaluation of parameter .gamma..

Computer 30 and interfaces 32 and 28 do not directly affect the numerical evaluation of any parameter. They simply serve as convenient and time-saving system components in the operation of system 10. The purpose of the interface can also be carried out manually by a human. The computer calculations would then be carried out on any computational device capable of regression and optimization as explained in a following description which accompanies FIG. 3. In a practical application of this invention, the computer and interface automation is mandatory. In periodic applications such as maintenance and troubleshooting or other special investigations like quality analyses, the light output resulting from a specific signal channel, signal level, rectangle size, and rectangle position could be accommodated with a partially or totally manual operation of system 10.

Positioning of the light meter sensor by apparatus 34 is critical to the integrity of this invention. CRT devices are nonuniform in their light output at any tube-screen-region of active video. The optical coupling and geometry of the light meter sensor material and housing to any particular screen-region is also critical to the integrity of this invention. The uncertainty and tolerance of this mechanial aspect will bias the numerical values of the parameters but will assure the integrity of their use.

Referring now to FIG. 3 wherein the flow chart of the process, using the system 10, is depicted with four major flow steps identified by blocks 50, 52, 54 and 56. Once the operator of the system 10 satisfies all of the hardware setup conditions of block 50, the computer programs proceed to fulfill tasks as labeled in blocks 52 and 54 which in turn work to output documents as specified in block 56. Below each block of FIG. 3 is a list of the major items accomplished in each step. This format serves as a total representation of the process of system 10's operation which is to obtain the characteristic function parameters and to document all data used to obtain their values.

Block 50, hardware setup, insures that the major components of system 10 are ready to function. This step also includes a text of computer 30's interfaces to the signal generators 12 or 18, whichever is to be used, and to the light meter 26. Testing here simply means that the system 10 components provide technically reasonable results. It is to be expected that nominal values would result when all equipment is operated at the manufacturer's specified conditions. Of course, operation of the components at the borders of their specifications or in a particular condition of setup is an application of this invention for establishing criteria for troubleshooting limits (such as margin testing) or quality control limits (such as gamma or other characteristic function parameter tolerance).

Block 52 identifies stage one of the process called light stable collection. Stage one can be conducted purely manually, partially manually and partially by computer program, or completely by computer program. The method of control is entirely application dependent. For example, some applications of this invention can use signal generators with no hardware connection to computer 30. In applications of this nature, the system 10 operator must manually place the generator at a signal level and on a channel as called out by the stage one operational process. Since stage one must address many applications, it will be described herein in as general a manner as possible rather than presenting one or many similar programs or flow charts for controlling its operation. It is assumed in this writing that stage one does use a computer program and general computer peripherals. All information used in conducting stage one such as the type generator and type device, controls, and the complete light-to-signal response is recorded electronically in a coputer file for use in stage two and also as final documentation information. The types are recorded because a specific type generator with a specific type device requires a proper video drive equation for stage two's mathematical integrity.

The first task of stage one is to select the allowed options which are provided by the system 10's configuration of major components. The configuration mode number identifies the specific type of generator and device so that stage two uses the proper drive equation. In FIG. 3, three modes are defined, more are possible. The idea is to label each mode so that data is properly formulated in stage two. Another option is to select the generator's signal series and the number of channels. It is assumed that the generator and CRT device will characterize three channels identified as RGB for red, green, and blue. An industry standard generator may force specific levels as opposed to a host digital generator which would allow, for example, a logarithmic series or a series to test the linearity of a low range. FIG. 3 identifies two signal units although there can be many more. Here again, a control procedure is devised so that stage two maintains its mathematical integrity and the entire process is properly documented. The option of selecting the size and location of the video image rectangle is strictly a matter of investigating the behavior of various CRT screen regions. Along with the attachment of the light meter and its light gathering properties, the rectangle specification allows an analysis of CRT screen uniformity. As the person who serves as the system-component selector considers the operation of stage one, specific options must be allowed for reasons such as convenience, integrity, and testing. For example, two options given for testing are the number-of-readings to be taken at each light level and the reading-delay time (the time between application of the signal and the actual measurement). The minimum reading-delay time can only be determined, practically, in automatic stage one operations. Parameters which result from strict changes to one option, one value at a time, will provide troubleshooting and quality control tolerances.

Preforming the light table collection of stage one is accomplished by randomly selecting a single channel and associated signal value from the series provided and reading the light meter until all signal values and options are satisfied. In automatic operations, the coefficient of variation (CV) statistic is computed. If the CV is greater than a minimum, an optional value, the process of light collection is halted to allow a retesting of that light-versus-signal measurement. The number of significant digits which are saved for the light meter reading is a matter of light meter precision. Appendix E contains an example of a representative light table. The precision of numbers is noted.

Block 54 identifies stage two, light table analysis, of the process. Stage two is conducted by operation of a computer program which is listed in Appendix A through Appendix D and which is itemized in FIG. 3. The program, shown written in FORTRAN, allows the operator to determine optimum values for gain (G) and offset (O) in equation (1) for each channel by a manual or by an automatic mode of operation.

The program receives light-versus-signal and associated data from stage one in the form of a file. This type of inforamtion can be input manually in cases where it must be transported in written form. Program options allow the operator to specify the channels to be analyzed, the regression limits (the full series from start to end or a subset of the series), back substitution limits (full series or a subset), and type of analysis to perform (automatic or manual optimization). Also, some program setup questions are presented to maintain integrity with stage one and to finalize the documentation of the entire process such as configuration mode and video image rectangle size and position.

In the manual mode of optimization, the program requests initial (seed) gain and offset values to test. The video drive (VDR) is calculated as determined by the configuration mode (which ultimately defines VIN) and the signal units (VDRDAC or VDRIRE). A least squares regression, performed on the log measured (statistically from stage one) light versus log VDR, results in a slope [gamma (.gamma.)] and intercept [constant (K)] used in equation (1). The values of gamma, K, gain and offset are then used to calculate theoretical (predicted) light values for each signal of the selected channel. The predicted light values and measured light values are used to form a percent difference (PD) for each signal. The sum of PDs for the selected channel, called New-Sum, is reported to the operator. The operator can repetitively enter gain and offset values until satisfied with the New-Sum value; hence, a manual mode of optimization. The operator can request full documentation of results. An example of the documentation is disclosed in Appendix E.

In the automatic mode of optimization, the program requests a seed value for gain and offset, and a comparison value (epsilon). A New-Sum, NSO, is calculated for the seed offset and gain. The first parameter, gain or offset, to be varied is incremented and another New-Sum, NS1, is calculated. The sign of the difference between the two New-Sums (NS1-NS0) is used to determine the sign of the increment which drives the New-Sum toward a minimum value. With the direction of the increment determined, the program continues in that direction by establishing an Old-Sum (OS) and by testing against epsilon and the New-Sum (NS). OS is normally established from the previous pass but, in some cases, when NS is greater than OS, it is held. If the absolute difference between OS and NS is greater than epsilon and NS is greater than OS, then the iteration is ended.

Having obtained an optimum value for the first parameter, the program would be finished with the optimization mode if it were HGMO (hold gain, minimize offset). In the two other cases, the optimized value of the first parameter and the seed value for the second parameter are used as the starting point of a final pass through the optimization process as just described. When the second parameter value is optimized, the analysis of the channel is complete. After each channel is optimized, gain and offset values are reported to the operator. As with the manual mode, the operator can request full documentation of results.

While there has been show what is considered to be the preferred embodiment of the present invention, it wil be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as may fall within the true scope of the invention. ##SPC1## ##SPC2##