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Methods to Minimize the Effects of LCD M-Signal Noise on the Operation of an Attached Sensor Screen

IP.com Disclosure Number: IPCOM000122689D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 98K

Publishing Venue

IBM

Related People

Stein, F: AUTHOR

Abstract

Electromagnetic radiation from LCDs may negatively affect the performance of touch and sensor screens because of the close proximity of the touch and sensor to the LCD. Prior art in the area of electromagnetic compatibility engineering includes the use of shielding materials between the radiating source (LCD) and the interfered device (sensor screen). Such shielding has the detrimental property that it reduces the level of light transmission reaching the viewer and may cause other optical defects to the display. This article describes means of minimizing the effects of the LCD noise through electronic circuitry. The invention disclosed here is particularly applicable to sensor system described in U.S. Patent 4,686,332.

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Methods to Minimize the Effects of LCD M-Signal Noise on the Operation
of an Attached Sensor Screen

      Electromagnetic radiation from LCDs may negatively affect
the performance of touch and sensor screens because of the close
proximity of the touch and sensor to the LCD.  Prior art in the area
of electromagnetic compatibility engineering includes the use of
shielding materials between the radiating source (LCD) and the
interfered device (sensor screen).  Such shielding has the
detrimental property that it reduces the level of light transmission
reaching the viewer and may cause other optical defects to the
display. This article describes means of minimizing the effects of
the LCD noise through electronic circuitry.  The invention disclosed
here is particularly applicable to sensor system described in U.S.
Patent 4,686,332.

      One of the external interfaces to an LCD is an AC signal
typically called the M-signal for AC drive signal. The M-signal
changes polarity at a constant frequency to prevent the liquid
crystal material from becoming polarized. Because this signal is
wired to all the drivers of the LCD, and causes a polarity reversal
throughout the LCD, it generates a large amount of electromagnetic
noise.  This noise interferes with the operation of the capacitance
measurement circuitry, as well as the stylus signal.

      Several different circuits were developed to solve the
interference to the capacitance measurement system used to detect
finger touches.  Because the M-signal is relatively slow signal,
there is generally enough time between transitions to perform the
capacitance measurement.  The touch sensor control processor
generates requests for touch (capacitance) measurements.  Options
1-4, below, describe the several ways to prevent the M-signal from
interfering with these measurements.

      Option 1 - The capacitance measurement can be held off until
just after an M-signal polarity reversal has occurred. This is the
simplest method, but since sometimes there may be enough time before
the next M-signal transition to perform a touch measurement, it
reduces the touch sampling rate more than necessary.

      Option 2 - The capacitance measurement is only held off until
an M-signal has occurred only if there is not enough time to perform
th...