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Dynamic Threshold Comparator

IP.com Disclosure Number: IPCOM000084278D
Original Publication Date: 1975-Oct-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 59K

Publishing Venue

IBM

Related People

Beidl, JR: AUTHOR [+2]

Abstract

In a gaseous discharge display device having a light pen, with or without graphic capability, detection of specific areas of the display by the light pen presents several problems, since the gas panel is not continuously reintensified in a scanning mode as the more conventional cathode-ray tube. One method employed for selection of a unique point by a light pen utilizes a selective sustain signal, which precedes the normal sustain on the coordinates associated with the field of view of the light pen and detection of the sustain signal by the photodetector in the light pen.

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Dynamic Threshold Comparator

In a gaseous discharge display device having a light pen, with or without graphic capability, detection of specific areas of the display by the light pen presents several problems, since the gas panel is not continuously reintensified in a scanning mode as the more conventional cathode-ray tube. One method employed for selection of a unique point by a light pen utilizes a selective sustain signal, which precedes the normal sustain on the coordinates associated with the field of view of the light pen and detection of the sustain signal by the photodetector in the light pen.

Light pulses generated by the light pen on detection can vary from 50 millivolts (0.050 volt) to 1.5 volts, depending on the light level in the field of view of the optical sensor which varies with the ambient light, as well as the light level resulting from the number of light cells energized in the sensor field of view. The reference level for the comparator can vary as much as 400 millivolts compared to the 50 millivolt level of the lowest input signal to be detected, such that the reference light level can change by a factor of 8 compared to the detection level. A final problem associated with the detection circuit herein described is the hysteresis characteristic of the comparator and signal source, wherein the reference voltage varies as a function of temperature.

The system herein described provides a dynamic reference voltage to a comparator that is equal to an instantaneous DC offset voltage of the input signal, plus or minus the offset voltage of the comparator. The circuit operates in the following manner, reference being had to Figs. 1 and 2 which illustrate the circuit configuration utilized and the waveforms of the corresponding signals, respectively.

Referring to the drawings, an input signal representative of light pen detection is applied from line 11 as a positive input to comparator 13. The term "positive" is used to indicate the relative relationship between signals, rather than designate a specific polarity. Capacitor C1, when charged, provides a DC offset signal which is applied through transistor Q1 to the second input 15 of comparator 13. This input signal is also identified as signal A in the waveform which is illustrated in Fig. 2. Thus capacitor C1 provides the reference level to the comparator 13 and is normally charged to the level of the input signal as described hereinafter.

When the signal difference corresponding to a predetermined value such as 5 to 10 millivolts exist between the inputs of the comparator 13, the comparator trips and provides a readout which is identified as output signal B, shown in Fig.
2. The resultant output is also applied through inverter 17 and its output line 19 to condition logical AND circuit A1. The compara...