Monochrome and color passive-matrix LCD screens
Switching from passive-matrix to active-matrix LCDs was a big deal for many. However, not everybody in the mid-90s had enough money to afford a color TFT panel in a new laptop as the price premium was still about $1000. Color passive-matrix screens were a solution for those with tight budged.
640x480 monochrome passive-matrix screens needed about 1.800 transistors to control all pixels (n*2+m for dual-scan). Color active-matrix screens (TFT) with the same resolution needed 300.000 transistors as each pixel is controlled by three transistors (3*n*m). That’s why these screens were so expensive.
Color passive-matrix screens (DSTN) were a good low-cost alternative for everybody who wanted colors “on the road”. This technology needed only about 5.000 transistors for the resolution of 640x480 (2*3*n+m). The result was not exceptional but it was good enough for many productivity applications where slow response rate (300-700 milliseconds) was not a problem.
Unfortunately, quality varied a lot between DSTN screens. Although some had a very crisp picture with shiny colors on par with today’s cheap TN panels, some suffered from significantly poorer contrast, washed colors and color bleeding.
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The machine on the right is in fact my first own PC compatible laptop. I hated the screen so much back then and I envied others their color-screen portables. The machine on the left has the best DSTN panel I’ve ever seen in the laptop. Although both machines use the passive-matrix technology the difference in picture quality is huge.
UPDATE: (1) I was not completely right with the transistor counts – for passive-matrix displays, there will be at least twice as much transistors (one on Vcc and one on GND for each row/column to output both logic levels 0 and 1). (2) I also forgot to mention that passive-matrix screens don’t have transistors in the panel itself. There are special chips on a separate board which makes the solution much cheaper in comparison with TFT (which has multiple layers with tiny transistors inside the panel).
