On my train to work there is a dot-matrix sign which has the words 鈥淐alling at鈥� in a Roman font, but the station names below it in italic font. However, if you take a photo of the sign all the words appear Roman (see photo, top). If the photograph catches the sign at the correct moment, you can see that the italic font is created by the dots on the sign lighting up with some clever on-off timing that fools the eye (see photo, bottom). All the dots are in vertical and horizontal rows. How is this italic effect created and, more intriguingly, why bother?
鈥� The italic effect that the questioner describes is not created deliberately. It is an accidental result of the way the dot-matrix display works, through a technique called multiplexing.
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The number of LEDs in the display is large 鈥� easily in the thousands 鈥� and it would be expensive to use a separate electronic driver to control each one. Instead, the lights are controlled by column and row. Each LED is illuminated by powering its column and row drivers at the same time 鈥� a bit like the game Battleships. And although the display appears to be illuminated steadily, it is actually flickering at a high speed, with only one row lit up at a time.
This is fine until it becomes necessary to show a moving line of text. To avoid the words distorting in this case, the display would have to flash its entire sequence of rows 鈥� perhaps 25 of them 鈥� before the letters changed position. By using this method, the text would appear to move only very slowly.
If the text is to speed up, the display only has time to illuminate a few rows (or maybe even just one) before the position of the letters changes. When the subsequent row flashes up, the text has moved a little to the left or right, and this leads to the apparent sloping lines.
The alternative is to refresh the display one column of LEDs at a time, but this would make the distortion even worse. Instead of an acceptable italic slant, the characters would appear to be squashed or expanded and would not be legible.
The reservation displays above seats in some trains demonstrate the problem, with the text seeming to move infuriatingly slowly. There could be several reasons for this, but one may be that the displays are driven a column instead of a row at a time, meaning that fast-moving text cannot be accommodated at all.
David Gibson, Leeds, West Yorkshire, UK
This article appeared in print under the headline 鈥淪loping off鈥�