QEX November

of the above requirements. This, together with the improved understanding of the ordering of dyes in liquid crystals which is emerging from current research [29, 30], should contribute to the future success of such dved displays.

7 Applications of liquid-crystal displays

Any of the effects described above can be used to make a simple, directly driven reflective or transmissive display, i.e. one in which each display element has its own connection to the drive electronics and the voltages applied to these produce a desired voltage difference at ON elements and zero voltage difference at OFF elements. Such displays, in twisted nematic form, are frequently found in digital watches, clocks and voltmeters which use seven electrode segments and a counter electrode to produce each digit. Guest-host displays are now also finding their way into such products. The main reasons for this lie in the low voltage and power requirements of the LCD, its legibility in bright ambient light and its low cost. It is true that some problems remain, such as its sluggish response at low temperatures (due to the increased viscosity of the liquid crystal) but recent work, e.g. in producing low-viscosity liquid crystals [31] and in identifying a fast turn-off mechanism [32], is progressively eliminating or alleviating these problems.

This move towards LCDs is also observed when more complex displays are considered. These may have several thousand elements, and it ceases to be practicable to make connection to each element to give each one an individual signal at an appropriate time of its own. Under these conditions it becomes necessary to matrix address the display or to time multiplex the drive voltages. The problems inherent in this approach and the optimum methods of implementing it are the subject of an excellent review by Clark [33] and it would be pointless to recapitulate them here in detail. In summary, however, the problem arises from the fact that OFF elements do not have zero voltage difference and that a finite ratio of Von to Voff exists for any drive method and tends to unity as the display complexity increases. The state of the art is a commercially available, 5 V, twisted nematic display module capable of showing four lines of 40 alphanumeric characters with a restricted viewing angle over a temperature range of about 0-50°C [34], This is shown in Fig. 10. Looking slightly to the

THIS IS A MATRIX-ADDRESSED LIQUID CRVS-|TAL DISPLAY MODULE USING TWISTED HEMATIC MODE. 40X4 CHARACTERS CAN BE DISPLAYED BY MODIFIED 5X7 DOT MATRIX WITH CURSOR.

Fig. 10 Photograph of a commercially available twisted nematic, matrix display made by Toshiba

This can display 4 lines of 40 alphanumeric characters over a temperature range of 0 to 50"C and includes electronic drive circuitry operating from a single 5 V supply with a power consumption of 150 m W [Photograph: Toshiba Corporation]

future, workers at Hitachi [35] have demonstrated a matrix-addressed, twisted nematic television display having 120 x 160 elements and 16 levels of grey scale. This performance is achieved at the expense of peak contrast and viewing angle and by using a complex electrode structure so that each column of the matrix display requires four separate connections. It represents, nevertheless, a very significant achievement.

Multiplexing or matrix addressing becomes even more

I EE PROCEEDINGS, Vol. 130, Pt. 1, No. 5, OCTOBER 1983

difficult when the dyed-phase-change is considered since the presence of Voff can reduce the contrast of the display and make the turn-off of previously ON elements intolerably slow. Two-way multiplexing has been demonstrated, however, and it is expected that a dramatic improvement will be achieved in the near future.

Higher complexity twisted nematic displays having up to about 20 lines of characters may be obtained in the future using a 2-frequency matrix addressing technique, also reviewed by Clark [33]. Owing to a dielectric relaxation exhibited at low frequency by some liquid-crystals, a high-frequency voltage may be used to oppose the effect of a low-frequency drive voltage due to the change in sign of Ae (Fig. 11). Using this method, workers at Seiko [36] in great use in permitting the design of liquid-crystal vehicle instruments which can operate over a wide enough temperature range to be useful and have a small enough

1A 12 10

Ae>0

Ae<0

frequency. Hz

Fig. 11 Diagram of the low-frequency dielectric relaxation in £u exhibited hy a so-called 2-frequency nematic liquid crystal The resulting change in the sign of Ac = ctt — eL at the crossover frequency f allows alternating voltages of frequency f > f, to oppose the effect of those having / <$ ft when both are applied simultaneously. Alternatively, bursts of low (500 Hz) and high (100 kHz) frequency voltages may be applied in sequence to drive a display ON and OFF rapidly at repetition rales up to 100 Hz [Reference 31]

Japan have demonstrated a prototype twisted-nematic matrix display capable of presenting up to 8 lines of 64 alphanumeric characters over a temperature range of about 0-40°C using temperature compensation of the drive voltages. The choice of suitable 2-frequency materials is very limited at present, however, and although materials are being developed, this and other problems make it likely that some time will elapse before such displays become generally available. The method may, however, find particular applicability in the case of dyed displays in order to substantially improve their present, poor level of multiplexibility.

8 Alternatives to conventional multiplexing

It has been shown by Nehring and Kmetz [37] and Clark, Shanks and Patterson [38] that, if the information patterns to be displayed can be restricted, the intractable problems inherent in multiplexing can be significantly ameliorated or even completely avoided. For example, if only a few elements in a matrix column are to be distinguished, then significantly higher ratios of Von to Voff may be obtained. In particular, it has been shown that a novel addressing method, utilising the correlation properties of pseudorandom binary sequences, may be used to obtain the equivalent of direct drive (zero voltage difference at OFF elements) in complex liquid-crystal oscilloscope matrix displays [39] and analogue meter or clock displays [40] where the information presented is the position of a hand or hands relative to a scale. A commercially produced digital storage oscilloscope with a 126 x 256 dyed-phase-change matrix display is shown in Fig. 12. This method does not degrade with display complexity, and the direct-drive capability of the analogue meter may be of

IEE PROCEEDINGS, Vol. 130. Pi. I. No. 5, OCTOBER 1983

Fig. 12 Commercially available digital storage oscilloscope with 10 cm x 6 cm dyed-phasechange liquid-crystal display

The 126 x 256 matrix display is driven directly from 15 V CMOS logic circuits and uses a nonmultiplexed correlation addressing method to give a single- or dual-trace display

[Photograph: Scopex Instruments Ltd.]

Fig. 12 Commercially available digital storage oscilloscope with 10 cm x 6 cm dyed-phasechange liquid-crystal display

The 126 x 256 matrix display is driven directly from 15 V CMOS logic circuits and uses a nonmultiplexed correlation addressing method to give a single- or dual-trace display

[Photograph: Scopex Instruments Ltd.]

number of connections to be manageable. Directly driven, full circular PPI radar displays have also been achieved using this method [41] and a prototype is illustrated in Fig. 13.

Fig. 13 Prototype 120 x 60 element polar co-ordinate radar display developed at RSRE. Malvern

The full circular display uses nonmultiplexed correlation addressing of a novel matrix format [41]. It employs the dyed-phase-change effect and also incorporates an internal reflector which eliminates parallax effects (Photograph J Glasper. RSRE. Malvern. England]

Fig. 13 Prototype 120 x 60 element polar co-ordinate radar display developed at RSRE. Malvern

The full circular display uses nonmultiplexed correlation addressing of a novel matrix format [41]. It employs the dyed-phase-change effect and also incorporates an internal reflector which eliminates parallax effects (Photograph J Glasper. RSRE. Malvern. England]

Another method of avoiding the limitations imposed by multiplexing complex displays is to address the display optically. Workers at Hughes Research Laboratories [42] have demonstrated a projection display system using a liquid-crystal light valve. This contains a reflective light-blocking layer between the liquid crystal and a photo-conductive layer. Light is imaged from a CRT onto the photoconducting layer to allow more or less of an applied alternating voltage to be developed across the liquid-crystal layer using capacitive coupling through the blocking layer. The nonemissive image thus formed in the liquid crystal is then projected onto a screen or can be used with laser illumination in optical-signal processing applications.

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