Addressing matrix

So far, the matrix array has been treated in a purely static fashion. In actuality, the segment data enters in parallel from the drive circuitry and each digit is selected sequentially in time. After all of the digits have been addressed, the cycle is repeated. The temporal dependence of an arbitrary segment is presented in Fig. 15. The seg- 

Varying degrees of success have been reported for matrix addressing liquid-crystal displays. For displays using dynamic scattering, anywhere from 3 or 4 digits to 7 digits have been reported as the 

The cholesteric-to-nematic phase transition effects have also been utilized in matrix-addressed displays. Up to 28 lines have been scanned in the V V/S mode with a bias voltage of 35 Vrms and a contrast ratio of 15 1. The relatively large multiplexing capability is due to the long decay time produced by the bias voltage.  

The discussion until now has centered on the utilization of driving signals whose frequency components are much lower than the cutoff frequency. However, it is possible to implement useful multiplexing schemes with drive signals whose frequency components are both below and above the cutoff frequency. This approach will first be analyzed for dynamic-scattering devices. 

The two-frequency approach can also be used with field-effect materials. Bucher, Klingbiel, and Van Meter have shown that the low-frequency threshold voltage for the twisted nematic effect is increased by the superposition of a signal whose drive frequency is greater than the critical frequency where the dielectric anisotropy becomes negative. They claim that 

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Kaneko E Active matrix addressed displays Handbook of Liquid Crystals Vol 2A. Low Molecular Weight Liquid Crystals led D Demus, J Goodby, G W Gray, H-W Speiss and V Vill (New York Wiley-VCH)... 

Figure 13-18. Diagram of a simple pixel circuit for active matrix addressing of an OLED array. For a color display of N lows and M columns, this circuit must be reproduced Ny.My.7t limes.

Active matrix addressing can also be used in printer heads [628] and linear sensor arrays [629]. a-Si H TFTs are used to address a linear array of output or input transducers, respectively. In the linear sensor array, u-Si H photodiodes are used. 

X-ray imagers consist of a phosphor in direct contact with the surface of an array of a-Si H photodiodes. The device is a matrix-addressed array, in which each imaging pixel consists of a photodiode and a TFT [647-649]. These X-ray imagers are very suitable for static and dynamic imaging in medical diagnosis. 

Displays based on OLEDs may be addressed either passively or actively [124], and the drive requirements are quite different in each case. In passive-matrix addressing, the display is addressed one line at a time, so if a display has 480 lines then a pixel can only be emitting for... 

Many LCDs are based on active-matrix addressing, in which an active device circuit containing one or more TFTs is connected to each pixel. The TFT circuit at each pixel effectively acts as an individual electrical switch that provides the means to store display information on a storage capacitor for the entire frame time, such that the pixel can remain emitting during this entire time rather than for a small fraction of time, as is the case in passive addressing. 

Figure 5.5 Twisted nematic materials for active matrix addressing.

For high information-content displays, active-matrix (AM) pixel addressing provides improved display performance and reduced power consumption. In active matrix addressing each individual pixel is controlled by one or more thin-film transistors (TFTs). To date, most AM OLED displays have used polysilicon TFTs as the active elements, because they can provide sufficient current at low voltages and acceptable device dimensions, and they are capable of integrated drive electronics... 

V. Active-Matrix-Addressed Liquid Crystal Displays. . 127... 

In a small display it is feasible to individually connect each display element, or pixel, to an external driving circuit. As the number of display elements increases, however, the number of connections quickly becomes prohibitive. For example, a display with a resolution typical for a television set (300 X 300 pixels) would require 90,000 external connections. Matrix addressing reduces this number to 600 connections, a much more feasible number. 

The sharper thresholds required for the addressing of larger displays can be brought about by constructing liquid crystal cells with sharper transitions (see Fig. lc) (Boyd et al., 1982 Wilson et al., 1983), intrinsic threshold control, or by the addition of nonlinear electronic elements, extrinsic threshold control. Liquid crystal displays with gradual turn on such as DSM or GH LCDs always require extrinsic threshold control for matrix addressing. 

Various matrix addressing schemes are possible with two- and three-terminal devices. For more information, the reader is referred to an early paper of the RCA group (Lechner et al., 1971). Matrix addressing differs from multiplexing in that the LC can be driven at 100% duty factor, maintaining superior optical performance. 

Fig. S. Matrix-addressed liquid crystal display using an FET as an extrinsic threshold device. The capacitor (dashed) is optional.

A limited amount of trapping (either intrinsically or extrinsically) is helpful in that it ensures a deep OFF state of the device during the time at which other lines are addressed that is a (properly adjusted) hysteresis can serve as a memory element over the frame time (Ast, 1982a,b, 1983). This argument is not readily accepted by those who believe that a device must be inherently stable to function in a reliable manner. However, Luo et al. (1983) arrived at an identical conclusion in their analysis of the switching performance of CdSe transistors in matrix-addressed LC displays. It is very important that the trapping be noncumulative. Since the duty cycle of a... 

Direct, multiplex and active matrix addressing are the three electronic drive methods used to generate the appropriate voltage at a particular pixel of an LCD, see Figures 2.11-2.14. The size, shape and pattern of electrodes on LCD substrates are fashioned to be compatible with the chosen method of addressing. In directly addressed LCDs the desired pattern of pixel electrodes is created by etching on one surface. A non-pattemed back electrode on the second surface provides the electrical contact. LCDs with multiplexed addres-... 

Figure 2.16 Schematic representation of a high-information-content LCD with active matrix addressing provided by thin film transistors (TFTs)J ...

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