Abstract:
A flat panel display is coupled with an electrostatic stylus driven digitizing panel to produce a display and digitizer system having an interference control feature. The interference control feature first determines the operating frequency of the flat panel display and/or the operating frequency of the stylus. The interference controller may then incrementally reduce the operating frequency of the flat panel display and/or the operating frequency of the stylus until the operating frequency causing the least interference is ascertained. Once the operating frequency causing the least interference is ascertained the interference controller selects new operating parameters for the display and digitizer system.

Description:
STATEMENT UNDER 37 CFR 1.71(D) AND (E) 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF INVENTION 
     The present invention is generally related to flat panel displays operably utilized in conjunction with an electrostatic pen operated digitizer, and more particularly related to a method and apparatus for reducing pen/stylus signal interaction. 
     BACKGROUND AND OBJECTS OF THE INVENTION 
     Electrostatic, stylus driven digitizing panels are commonly configured with a flat panel display such that the display scan signal is coupled to the digitizing panels channel inputs. This causes the signal emitted from the display surface to capacitively couple with the digitizing panel&#39;s sensing layer. This in turn has the highly undesirable effect of introducing uncertainty in stylus position determinations so as to reduce the effective resolution of the display and digitizer system. 
     Flat panel displays typically have a scan signal with a square waveform of between 15 kHz and 20 kHz. The frequency of a typical pen or stylus is between 100 kHz and 200 kHz. Because the typical display horizontal scan signal is wide band, some harmonics of the signal fall within the normal pen or stylus frequency range. Since designers rarely have the opportunity to select the pen or stylus and display scan frequencies this interference is difficult to design around. Even where the designer has full control over pen/stylus and display frequencies the pen frequency often has an associated tolerance (e.g., where the pen frequency is not crystal controlled) which causes frequency uncertainties. This problem is compounded where different display resolutions (and frequencies) are utilized. 
     It is, therefore, a primary object of the present invention to provide a method and an apparatus for reducing pen or stylus and display signal interactions. Another object of the present invention is to provide a method and apparatus for dynamically adjusting the display horizontal scan frequency so as to reduce pen or stylus and display signal interactions. A further object of the present invention is to provide a method and apparatus for allowing a user to optimize the display horizontal scan frequency so as to reduce pen or stylus and display scan signal interference. Yet a further object of the present invention is to provide a method and apparatus for automatically adjusting the horizontal scan frequency of a display and digitizer system whenever signal interference is detected. 
     SUMMARY OF THE INVENTION 
     The present invention teaches a method and apparatus for reducing horizontal scan frequency interference in a display/digitizer system. In accordance with the invention, a flat panel display is coupled with an electrostatic stylus driven digitizing panel to produce a display and digitizer system having an interference control feature. The interference control feature in a preferred embodiment may first determine either or both the operating frequency of the flat panel display and the operating frequency of the stylus. The interference controller may then incrementally adjust (reduce or increase) the operating frequency of either or both of the operating frequency of the flat panel display and the operating frequency of the stylus until the operating frequency causing the least interference is ascertained. Once the operating frequency causing the least interference is ascertained the interference controller selects new operating parameters for the display/digitizer system. 
     In preferred operation the present invention may be constructed from a flat panel display, a digitizing panel, and a controller operably connected to the display. The controller may control the display&#39;s horizontal scan frequency, display a target on the display, prompt a user to hold the stylus to the target, incrementally adjust the horizontal scan frequency, ascertain the horizontal scan frequency with the least interference, and adjust the horizontal scan frequency to the frequency having the least interference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention is described hereafter, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a diagrammatic perspective drawing of certain components of a display/digitizer system; 
     FIG. 2 is a flow diagram illustrating a preferred embodiment of the present invention; 
     FIG. 3 is a graphical illustration of the dependence of the σ xy  standard deviation which corresponds to the noise calculation uncertainty of five different filter cutoff frequencies; and 
     FIGS. 4A and 4B are graphical illustrations of a probability curve for a randomly selected pen wherein the coordinate noise level is higher for the corresponding level on the horizontal axis, FIG. 4A illustrates the noise level without application of the present invention and FIG. 4B illustrates the corresponding noise level with application of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The following co-pending and co-owned United States Patent Applications are incorporated herein by reference: (1) Cancellation of Common-Mode Signals in Digitizing Tablet, U.S. patent application Ser. No. 08/192,078 (filed Feb. 4, 1994); and (2) Compensation of Stylus Signals in Digitizing Tablet, U.S. patent application Ser. No. 08/286,720 (filed Aug. 5, 1994). In the interest of providing a full and complete disclosure there is annexed hereto Appendix A which provides an exemplary driver program for reducing the interference in a display/digitizer system. 
     In an exemplary embodiment the present invention may be constructed from and include, for example, the SYM93C2000 WriteTouch™ Interface Controller. This controller is available from Symbios Logic, Colorado Springs, Colo. The WriteTouch™ is capable of stylus events by measuring the corner currents injected by the stylus. Differences in the corner currents reveal the location directly under the tip of the stylus. In touch mode, an active indium tin oxide sensor is biased with a voltage, and the measured corner currents are the additional currents required to feed the added finger capacitance. The sensor panel includes a glass writing surface with an under coating of indium-tin-oxide (ITO). Preferably a polyester spall shield is attached over the ITO coating in order to prevent shattering if the glass is broken. The underside of the shield is also coated with a grounded ITO layer in order to shield the sensor from LCD electrical noise. The top layer of the glass writing surface is provided with a grounded fine grid ITO coating pattern in order to eliminate hand effects. The active ITO layer is slightly resistive and divides the stylus (or finger) current among four corner wires. The corner current signals carried by these wires are proportional to the ITO conductivity between each individual corner and the finger location. 
     While the WriteTouch™ provides a currently preferred implementation of the system of the present invention, it will be appreciated, a system of the present invention may be assembled which utilizes a membrane, infrared, electromagnetic, or resistive based touch screen sensors or the like. 
     The source code of a driver program for use with Microsoft® Windows 95® is appended hereto as Appendix A. This program, by way of example only, may be utilized to implement the system of the present invention where a general purpose computer running Windows 95® is utilized with the WriteTouch™. It will be appreciated that the invention of the present invention may be implemented with other operating systems and, likewise, with dedicated computers. 
     In operation, an exemplary embodiment of the present invention may be constructed with a computer, for example, having an IBM PC type architecture and a touch screen connected to the computer as an I/O device. The touch screen may be, for example, a Symbios Logic® SYM93C2000 controller, a Scriptel Corporation (Columbus, Ohio) WriteTouch™ electrostatic sensor panel, and a software driver to interface the SYM93C2000 to an operating system such as Microsoft® Windows 95®. The computer, for example, may operate application environment software such as Microsoft® WinPad® in conjunction with an additional driver such as set forth in Appendix A. 
     Appendix A is a driver program (Microsoft® Windows 95® Message Interface) which allows operation of a currently preferred embodiment of the present invention. The invention is related to an electrostatic, stylus driven digitizing panel. A typical solution for this kind of panel is shown in FIG. 1. The panel 102 is built out of a resistive plate covered with a nonconductive layer. The stylus 104 has a built in AC voltage source with an output connected to the stylus tip. When the tip touches the plate it capacitively couples to the resistive layer causing electric current to flow to the corners of the panel 102. The stylus 104 position is based on the corner current ratio. 
     As previously noted, one of the problems existing in the above solution is the coupling of the display horizontal scan signal to the channel inputs. The signal emitting from the display surface capacitively couples to the digitizer sensing layer, thus, introducing uncertainty to the stylus 104 position determination and reducing system resolution. The display scan signal is a square waveform of typically 15 kHz to 20 kHz in frequency. The typical stylus 104 operating frequency is between 100 kHz to 200 kHz. The scan signal, however, is a wide band signal which contains many harmonics which fall into the pen frequency range. Moreover, the harmonics are spaced only by 15 to 20 kHz so the probability of the digitizer 102 operating bandwidth overlapping one of the harmonics is relatively high. If at the time the digitizer 102 is added to the system, the designer has full control of the stylus 104 and scan frequencies, the interference of both can be avoided. However, the stylus 104 frequency may be fixed, i.e., non adjustable. In addition, the stylus 104 frequency may have an associated tolerance creating an uncertainty as to the actual frequency (e.g., where the stylus frequency is not crystal controlled. Additional problems arise whenever different display resolutions (and frequencies) are selected. The display scan frequency may also not be known to the system designer of the digitizer is an add on device temporarily mounted on the top of the display 100. In this situation, there is some probability that the system will fail to work properly due to the display scan signal interference. 
     Referring now to FIG. 2, a flow diagram of a preferred embodiment of the present invention is shown. In a preferred embodiment, the scan signal interference is eliminated by adding an initial calibration step to the digitizer operation. The calibration has to be performed only once for a given system-pen-display resolution combination (i.e., calibration need only be repeated upon changing resolution or stylus). During the calibration, the display scan frequency is dynamically changed in a small range and the digitizer noise is measured. Finally, the scan frequency which gives the best performance of the digitizer is set. 
     In operation, a preferred method of reducing interference in accordance with the invention is to dynamically modify the flat display 100 horizontal scan frequency so as to reduce stylus 104 and scan signal interaction. In an exemplary embodiment a user is asked to touch the digitizer panel 102 with the stylus 104 and hold the stylus steady briefly. During this brief time period, the digitizer software driver (e.g., Appendix A) re-programs the video controller chip to slightly modify the scan frequency and measures the digitizer noise for the new frequency. This procedure is repeated several times (typically 5 to 12) and each time a different scan frequency is programmed to the controller chip. At the end, the frequency for which the lowest digitizer noise has been measured, is set. Typically, the scan frequency is modified within about ±5.0% range. This change in the frequency does not cause any visible changes in the image quality. 
     As shown in FIG. 2., the calibration algorithm is initiated by the software program, and list of display frequencies is generated. The user then contacts the stylus to the digitizer panel. The display is driven at the first frequency, and the noise present in the digitizer output signal for that frequency is measured. If the present frequency is not the last frequency, then the display is driven at the next frequency. The noise in the digitizer output signal at the next frequency is measured, and the process is repeated for all succeeding display frequencies until the last frequency is reached. Subsequent to the noise in the digitizer output signal at the last frequency being measured, operation of the display at the display frequency producing the least amount of noise in the digitizer output signal is selected at which point the calibration algorithm then terminates. 
     During the calibration process, the noise in the digitizer output signal is determined in the following manner. The software driver calculates the digitizer noise level for each frequency. This is accomplished by collecting a number of samples of the X and Y coordinate and estimating the position standard deviation σ xy  (which represents the noise level) using the following formula: ##EQU1## 
     The number of X and Y coordinate samples taken is important. Too few samples results in a σ xy  having a high variance level. Too many samples results in an unnecessarily long calibration process. Determining the right number of samples is critical and should include an analysis of the level of X and Y filtration obtained before data is sent to the host computer. The heavier the filtration the more samples are necessary (heavy filtration causes samples to be significantly correlated, and more samples are necessary to calculate noise with a sufficient confidence level). FIG. 3 illustrates the dependence of the σ xy  standard deviation (which corresponds to the noise calculation uncertainty) on the number of samples used in the calculation. FIG. 3 illustrates five curves showing the dependence for five different coordinate filter cutoff frequencies. 
     FIG. 3 is valid for 100 samples per second coordinate sampling rate. If the sampling rate is different proper scaling must be done. For instance, for the sampling rate of 200 coordinate pairs per second the frequency labels on the graph should be changed as follows: 1 Hz to 2 Hz, 3 Hz to 6 Hz, 5 Hz to 10 Hz, and 10 Hz to 20 Hz. 
     The effectiveness of the described method is illustrated in FIGS. 4A and 4B. This data assumes the frequency of the stylus had a tolerance of ±8.0% and the display scan frequency was 15 kHz. The horizontal axis shows the coordinate noise level. The vertical axis shows probability. The curve shows the probability the randomly selected stylus coordinate noise level will be higher than the corresponding value on the horizontal axis. FIG. 4A illustrates the absence of the method of the present invention. FIG. 4B illustrates the results obtained with the application of the invention of the present invention. During the calibration process, the best scan frequency was chosen from eight (8) frequency points tested. The eight (8) frequency values were uniformly distributed with ±5.0% of the nominal scan frequency range. 
     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes and modifications in form and details may be made therein without departing from the spirit and scope of the invention according to the following claims. ##SPC1##