PWM stands for Pulse-Width Modulation. It is a method for using a switching device (one that can only be on or off) to create what appears to be analog changes in level. This is done by switching on and off at a high speed and varying the proportion of the time for each. For example, of the switch is on and off for equal periods, the result appears to be halfway between fully on and fully off.
To create a simple PWM dimmer output with the 8-bit (256 step) resolution of a DMX channel, a digital circuit continuously counts backwards from 255 down to zero, over and over again. On each restart, the switch is turned off. When the counter reaches the level of the channel, the switch is turned on. As long as this happens fast enough, you will see the average rather than a series of blinks or flickering.
The human eye stops seeing flicker and starts seeing smooth changes in level when the frequency gets up to 60 cycles per second, or 60Hz. Some people still see an odd trailing effect in their peripheral vision until the frequency is considerably higher, usually 500Hz or more.
Film and video cameras are more sensitive, because of the way they capture a series of still images (which works for the same reasons that PWM works!). It is important that several PWM switch cycles happen within the time that a single frame is being captured by a camera. Otherwise, some frames will see it and others will not, and this creates visible artifacts.
Increasing the number of values in the countdown will increase how long it takes to get through one cycle, but also provides more steps and higher resolution. Thus, reducing resolution allows higher PWM frequencies, while lower frequencies allow higher resolution. In general, dimming resolution is favored when eyes are viewing a dimmer directly, while frequency is favored when effects are being captured for film or video.
The PWM frequency on RC4 devices can be changed using the Set Buttons to find the optimal frequency for any given application. RC4Magic devices default to 615Hz (optimal for human eyes), while LumenDim and W-DIM devices default to 5kHz (optimal for typical video capture). All devices in all families can be set to 77Hz, 615Hz, 1.2kHz, 5kHz, 10kHz, 20kHz, and 40kHz. High frequencies provide lower resolution but are used for the very high frame rates that generate slow-motion imagery.