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Two Frequency, Compensated Threshold Multiplexing of Liquid Crystal Displays

IP.com Disclosure Number: IPCOM000080050D
Original Publication Date: 1973-Oct-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 60K

Publishing Venue

IBM

Related People

Alt, PM: AUTHOR [+3]

Abstract

The use of one or more full cycles of alternating voltage at a frequency, f(1), below but close to the cutoff frequency, to excite a dynamic scattering liquid crystal display cell in a scanned matrix array has been suggested. This use of an elevated operating frequency increases the threshold voltage, and thereby the available overdrive, which in turn increases the number of lines which may be scanned.

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Two Frequency, Compensated Threshold Multiplexing of Liquid Crystal Displays

The use of one or more full cycles of alternating voltage at a frequency, f(1), below but close to the cutoff frequency, to excite a dynamic scattering liquid crystal display cell in a scanned matrix array has been suggested. This use of an elevated operating frequency increases the threshold voltage, and thereby the available overdrive, which in turn increases the number of lines which may be scanned.

In principle, 3:1 selection should yield a larger scanning capability than 2:1, but this advantage may not be realized in practice, since this type of operation has a unique worst case waveform for the "off" condition which must be accommodated for proper operation. Accommodating this condition severely impairs the scanning capability by reducing the available overdrive.

By use of a second voltage, V(2), at a frequency, f(2), higher than cutoff, the inherent scanning capability of 3:1 selection at elevated frequencies (but which are below cutoff) is achieved. This two-frequency results in higher scanning capability, and thus larger matrix displays than can otherwise be achieved with dynamic scattering.

Fig. 1 shows a portion of a display matrix in which information carrying waveforms are applied to the columns simultaneously, while the rows are scanned in sequence repetitively. Maximum applicable selection voltages relative to the threshold voltage V(TH) are shown for each axis for 3:1 selection. Worst case "on" condition and "off" condition waveforms are shown, representing the net voltage across a cell. The former appears when all cells in a column are "on," the latter when "on" and "off" cells alternate on a column. The frequency which predominates in the worst case "on" waveform is f(1), that in the worst case "off" condition is f(1)/2. Since the threshold, which increases with increasing frequency, is lower for f(1)/2, the maximum applied voltage must be less than the value which would cause undesired excitation of cells in the worst case "off" condition. This reduction in voltage to accommodate the worst case "off" condition, results in reduction of the overdrive in the worst "on" condition and consequent reduction in the scanning capability.

By application to each column of a voltage V(2) at frequency f(2) higher than cutoff in approximately the amount needed to adjust the threshold of the cells in that column, scanning capability is maximized. This new signal is separately determined for each column automatically according to the pattern of "on" and "off" cells on that column, and is added to the regular column voltages. The effective amount of V(2) will be least or zero for all cells "on," and greatest for alternation of "on" and "off." Ideally, V(2) would vary with intermediate conditions so that the resultant threshold remains independent of the on-off pattern. In practice, some compromise may be required to obtain an optimum cost- performance pr...