Technical: Understanding The Broadcast Safe Filter

Posted on by Larry

[This article was first published in the September, 2010, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe.]


For those of you interested in a more technical look at the Broadcast Safe filter, Dennis Couzin sent me the following article that he wrote. (Dennis is the technical assistant to a video documentarian based in Germany.)

In the past, I’ve just used the default settings of this filter. It is nice to know what these settings do.

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FCP Broadcast Safe Filter custom luma limiting controls

Dennis Couzin — Oct. 2010

Here’s how Apple describes the three control parameters for custom luma limiting with its FCP Broadcast Safe Filter.


Clamp Above: All luma values above this parameter value are clamped to the parameter value. The lower you set this value, the more clipping occurs at the top of the waveform.


Max. Output: Compresses the range of values between the Start (Threshold) and Clamp Above values so the maximum output value is the value set here. Setting this value higher than the Clamp Above value has no effect. Values below the Start (Threshold) value clip the signal.


Start (Threshold): Defines the lowest value affected by the Custom Luminance Limiting controls.


I read the words and reread them and reread them and couldn’t understand what the first two controls did. Can you?


The underlying problem is that Apple’s description of Clamp Above is circular. It’s first sentence says literally:


All luma values above Clamp Above are clamped to Clamp Above.


It’s useless to describe the one technical notion “Clamp Above” in terms of a second related technical notion “clamp to”. So I experimented with the controls to find out how they worked.


Call the three controls clamp, max, and start. Assuming the normal ordering of levels:


clamp ≥ max ≥ start


I found the filter does this:

#1 All lumas above clamp become max.


#2 All lumas between start and clamp are linearly squeezed into the range start to max.


#3 All lumas below start are unchanged.


Armed with these three facts, let’s review Apple’s descriptions. #3 says what Apple says about Start (Threshold). #2 says what Apple says about Max. Output. But #1 does not say what Apple says about Clamp Above. #1 says something about the two different controls clamp and max, whereas Apple’s statement is about Clamp Above and clamp to (if that’s saying something).


Enough with words! Another way to describe what the three controls do is graphically. Again assume the normal ordering of levels,


clamp ≥ max ≥ start.


The graph shows the relation between input and output. The result is a broken line. Starting at (0, 0), output exactly equals input until we get to the point (start, start). Then output increases more slowly than input until we get to the point (clamp, max). Then output remains constant at max. So the Broadcast Safe filter can do a crude, broken-line version of a roll-off filter.



The filter does unobvious things when the controls are set not obeying the normal ordering of levels. But the filter does nothing that can’t also be done with a normal ordering of levels. So we can keep to the normal ordering and say we now understand the filter.


For fun I’ve made an Excel sheet to produce the graphs from the control settings, whatever they may be. It can be downloaded from:


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