MaxCLL Alert

A Warning Light for HDR Brightness

A MaxCLL alert is a specific warning flag that appears during the technical quality control (QC) process of High Dynamic Range (HDR) video mastering. It signals a potential error or inconsistency with the MaxCLL (Maximum Content Light Level) metadata value embedded in the video file. This metadata is a crucial piece of information that HDR displays use to render the image correctly, so an alert indicates that the film might not be displayed accurately on consumer televisions.

Understanding HDR Metadata (MaxCLL and MaxFALL)

For the common HDR10 format, two key pieces of static metadata are included in the video signal to describe the content’s light characteristics:

• MaxCLL (Maximum Content Light Level): This value, measured in nits (a unit of brightness), informs the display about the brightness of the single brightest pixel in the entire film. This is often a small, specular highlight, like a glint of sunlight off metal or the filament of a lightbulb.
• MaxFALL (Maximum Frame-Average Light Level): This value informs the display about the highest average brightness of any single frame in the film. A shot of a bright, sunny sky would have a high MaxFALL.

An HDR television uses these values to perform tone mapping, an essential process where it intelligently adjusts the incoming signal to fit the specific peak brightness capabilities of its own screen. If a TV can only produce 1000 nits, but the MaxCLL metadata says a pixel is 4000 nits, the TV uses that information to compress the highlights gracefully without losing detail. A MaxCLL alert during QC means this critical data point might be missing or has been calculated incorrectly, which could lead to “clipped” (blown-out) highlights or an overly dim picture during playback.