Improving speed
While the grayscale theory on the previous page works great, it really taxes the device that has to spit out the pixels. For a display of 32x16, like we use, 50Hz refresh rate and 32 grayscales, it has to spit out pixels at a rate of almost one megahertz. There's a little trick we can use to get that 50Hz minimum refresh rate down, though: When we actually turn on all pixels at the same time and then turn them off dependent on their brightness-level, we get a huge amount of flickering when we have a refresh-rate under 50Hz. The weird thing is: if we don't turn them all on at the same time but change that moment a bit for every led, the flickering becomes much less intense.
To show what I mean, here are two animated gifs which represent two LED-boards.
Both images have an average brightness of 50%.
The left image turns all the pixels on and off at the same time. The right one
spreads that moment between 4 pixels. Strangely, the right image is quite a bit
more easy on the eyes than the left one, although the per-pixel refresh rate is
actually lower in the right image.