P.W.M.

Pulse Width Modulation

PWM is in general the best way to operate a LED screen. The way 'old fashioned' screens are operated is by constant current.

The difference between P.W.M and constant current?

First of all we'll have to understand the fact that led's give different colours at different current value's. When the current is stable, so are the colours.

In case of 'constant current' the voltage going through each led is always "on". In case of P.W.M. the voltage is turned off and on real fast (about 8000 times per second). In the example below you can see how it works. When the line is on top, the current is on...when the line is below, the current is off.  The longer the period of the line 'on top' the more brighteness you'll get.