Adjusting PWM Refresh Rates for High-Speed Cameras

To shoot clean slow-motion video of LED sources, your PWM refresh rate must be far higher than your camera frame rate.

When your PWM refresh rate is high enough, slow-motion shots look like continuous light instead of a flickering strobe. The game is to push PWM high enough, then lock camera frame rate and shutter speed into a sweet zone where flicker and banding vanish.

PWM vs Refresh: Two Clocks Running the Show

Every LED in your rig is strobing on and off. That strobe speed is the PWM frequency, and the on-time inside each cycle (duty cycle) is what actually sets brightness in most PWM LEDs.

Frame rate and refresh rate are different. The controller sends new image or DMX data at a relatively low rate, while the LED driver re-lights each pixel or LED many more times per second. On LED screens, the 1,920-3,840 Hz figure on spec sheets is that driver refresh rate, and it is what the camera actually sees.

For high-speed cameras, PWM is the first thing to check. If the camera is sampling while the LEDs are in the "off" slice of the PWM cycle, you get rolling bands, exposure pulsing, and visible ripple even on an expensive LED wall or a rig of PAR lights and moving heads. High-refresh LED panels help, but if PWM is too slow, the camera will still see the strobe.

Choosing a Target PWM Frequency for High-Speed Capture

As a rough starting point, aim for a PWM frequency many times higher than your camera frame rate, then confirm with a test shot. The faster the frame rate, the higher the PWM needs to be, and extreme slow motion leaves very little margin.

Stage and rental LED screens marketed as high-refresh often sit around 1,920-3,840 Hz. These are great for phones and standard broadcast video and show up constantly in LED display spec sheets, but they can still band at high frame rates and extreme slow motion.

Pro video fixtures for high-speed work push PWM into tens of kilohertz so each camera exposure spans many PWM pulses. That turns the light from a discrete strobe into an effectively continuous wash, even when you are firing off microsecond-level shutter speeds.

Nuance: lower PWM can make fades feel slightly smoother on some fixtures, but that is a trade against on-camera flicker, so when in doubt, prioritize the cameras and live with a tiny bit of dimming "grain".

Tuning the Rig: Fixtures, Drivers, and Cameras

Start on the light side. Choose fixtures and pixels with published PWM specs, whether they are LED walls, PAR lights, or moving heads; for camera-heavy shows, target rated PWM of 4 kHz and up, and do not be shy about demanding that from vendors of "high-refresh LED screens" and drivers built for broadcast and XR.

Many pro fixtures expose PWM as "LED frequency" in the menu, letting you scroll through multiple kilohertz options; some fixtures provide front-panel control of LED frequency so you can bump it up until rolling bands disappear on your reference monitor.

Now sync the camera. For general LED flicker you want frame rate and shutter that are harmonically related to the PWM or mains flicker frequency, so each frame integrates a consistent slice of the waveform. Slower shutters (1/120 s instead of 1/2,000 s) can average over many PWM cycles and calm down banding when super-high PWM is not available.

When you are shooting extreme slow motion, treat light like a precious resource: keep dimmer levels higher so PWM duty cycles are not razor-thin, add ND filters to control exposure, and consider purpose-built high-speed video lights instead of driving generic house LEDs at 5% and hoping for the best.

Quick Field Tests to Chase Down Flicker

• Run your hero camera at target frame rate and shutter, then sweep the fixtures' PWM/LED frequency presets while watching a calibrated monitor until banding and ripples disappear.
• Use your cell phone's slow-motion mode aimed at the wall or fixtures; dark rolling bands instantly reveal low PWM or bad refresh interactions.
• Pan quickly or shake your head while looking at LED sources; if you see dotted or dashed streaks, PWM is still too slow or dimmer levels are too low for your camera plan.
• For big LED walls, check multiple colors and brightness levels; some driver and scan combinations behave differently at low gray levels than at full white.
• Lock in the "clean" combinations of fixture mode, PWM setting, frame rate, and shutter in your show bible so the next operator can recall them in seconds.

Record the PWM setting, frame rate, and shutter speed that test clean for each fixture, and you can reuse them at every filmed show.