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The Wikipedia entry for the history of the LCD during the 1970s and 80s, states that twisted nematic (TN) field effect LCDs were available in the 1970s. Later, in 1983, super-twisted nematic (STN) structure for passive matrix addressed LCDs was invented.

From Passive and Active-matrix, it is stated that STN requires constant refreshing:

STN LCDs have to be continuously refreshed by alternating pulsed voltages of one polarity during one frame and pulses of opposite polarity during the next frame.

whereas, apparently, the TN appears not to require refreshing, see the description of Twisted Nematics. Without wishing to quote the entire description, there is no mention of refreshing, or field reversal:

A voltage of about 1 V is required to make the crystal align itself with the field, and no current passes through the crystal itself. Thus the electrical power required for that action is very low.

So, assuming that the Wikipedia entry has not omitted something, with TN LCD was the refreshing mechanism required or not?


As an aside, if it was not, why then was it required for STN?

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    \$\begingroup\$ Yes they always did. \$\endgroup\$ – Brian Drummond Dec 26 '16 at 23:36
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    \$\begingroup\$ When you don't refresh, you can see a ghost image which rapidly fades away. The difference between TN and STN is the amount of twist to individual LC molecules. \$\endgroup\$ – Janka Dec 26 '16 at 23:37
  • \$\begingroup\$ @Janka - So, is the wiki entry on the TN omitting the fact that the polarity of the field requires frequent reversal, because as it stands, it sort of implies that only a constant DC voltage needs to be applied to the terminals, as opposed to an alternating signal, with opposite polarity, as described for the STN. \$\endgroup\$ – Greenonline Dec 26 '16 at 23:41
  • \$\begingroup\$ @BrianDrummond - thanks for the quick reply. referring to my comment above, so the wiki entry is missing some information? \$\endgroup\$ – Greenonline Dec 26 '16 at 23:42
  • \$\begingroup\$ Yes, it obviously misses that information. You cannot drive any LCD with DC. \$\endgroup\$ – Janka Dec 26 '16 at 23:47
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All LCDs have to be driven with AC. When you use DC instead, you get a ghost image which fades away rapidly. The difference between TN and STN is the amount of twist each LC molecule does, nothing more.

You can have an active matrix with TN and STN, but TN has nearly died out, what's advertized as "TN panel" is STN in fact. For low-temperature applications, TN is better than STN and may be still available.

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