Passive matrix display

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Scheme of the passive matrix

A passive matrix display or PMLCD (short for passive matrix liquid crystal display ) is a matrix display in which the individual pixels are controlled passively (without additional electronic components for the individual pixels). In contrast, screens with active pixels controlled by transistors are referred to as active matrix displays .

Problem

Each pixel could be controlled individually via individual conductor tracks, which also happened with the first versions of the so-called seven- segment displays for displaying digits. However, there are only a few points (segments) per display compared to a screen that obeys the VGA standard and requires 640 by 480, i.e. 307,200 pixels.

This type of display occurs, for example, on the Casio TV-970 mini television .

construction

The solution is the so-called matrix control of the image points: each point is at the intersection of a column and a row, which are vapor-deposited in the form of transparent conductor tracks on the glasses of the liquid crystal cell. In the example above, that would be 480 rows on one sheet of glass and 640 columns on the other. If you now apply a voltage to such a row-column pair, an electric field is created exactly at the crossing point. It is now possible to address all 307,200 pixels using 640 + 480, i.e. only 1120 lines, which considerably reduces the technical effort. This type of pixel control is called a passive matrix.

Crosstalk

A number of problems are associated with passive matrix technology which place relatively narrow limits on its application. For example, an electric field is created not only at the selected intersection points, but also unintentionally along the active row and column at other intersection points that cannot be controlled. This unwanted field is weaker than the one at the selected crossing point and the molecular reorientation in the liquid crystal only takes place above a threshold value for the field strength: But if you want to display many gray levels on the screen, you can only achieve this through a slower transition in the molecular reaction. Instead of the sharp threshold value, a broader range of differently strong reactions of the LC material to small changes in the field strengths is then required. But then the weak field along a row or column as a gray scale may possibly even show, resulting in reduced contrast of the display - this is called a special type of crosstalk (English crosstalk ).

consequence

It seems that one has to make a decision and either build displays with a low resolution, but many gray levels, or those with high resolution, but few gray levels. If you try to implement both in a single passive matrix display, you have to pay with a reduced contrast. The contrast ratio of TSTN displays ( Triple Super-Twisted Nematic LCD ) with video resolution is roughly between 10: 1 and 15: 1, i.e. H. a selected pixel is 10 to 15 times as bright as one that is not selected.

Impulse control

In order to be able to control all pixels of such a matrix, the static observation made up to now for a row-column pair is not sufficient. It must z. B. all lines cyclically controlled one after the other (English scanned ) and the image contents are fed in parallel via the columns for each line. This leads to a pulse-shaped control of the pixels with higher voltage amplitudes than in the static case. Crucial for the optimization was the realization of Peter J. Wild , that when impulsive, periodically repeated activation of the RMS (English root mean square , RMS) of the voltage differences prevail.

literature

  • P. M Alt, P. Pleshko: Scanning limitations of liquid-crystal displays . In: IEEE Transactions on Electron Devices . tape 21 , no. 2 , 1974, p. 146-155 , doi : 10.1109 / T-ED.1974.17884 .
  • J. Nehring, A. R Kmetz: Ultimate limits for matrix addressing of RMS-responding liquid-crystal displays . In: IEEE Transactions on Electron Devices . tape 26 , no. 5 , April 1979, pp. 795-802 , doi : 10.1109 / T-ED.1979.19495 .

Individual evidence

  1. ^ Peter J. Wild: Liquid Crystal Display Evolution - Swiss_Contributions . Engineering and Technology History Wiki , accessed June 30, 2017
  2. ↑ PJ Wild: Matrix-addressed liquid crystal projection display. In: Digest of Technical Papers, International Symposium, Society for Information Display. 1972, pp. 62-63.