Apple Final Cut Pro 7 - Manual (Page 1325)

The RGB Limit filter affects only RGB-equivalent values above the Clamp Levels Above
parameter value and below the Clamp Levels Below parameter value. The filter applies
up to three stages of level reduction at a time, as necessary:
• Clamping: Levels above the Clamp Levels Above parameter value are clamped to the
parameter value. Levels below the Clamp Levels Below parameter value are clamped
to the parameter value. For example, if the Clamp Levels Above slider is set to 100
percent, all RGB levels above 100 percent are clamped to 100 percent.
• Chroma desaturation: After clamping, any pixel with an RGB level above the Desaturate
or Darken Levels Above parameter value is desaturated until the red, green, or blue
channel reaches the parameter value. Desaturation is an effect easily understood in
Y
′
C
B
C
R
color space: the color difference channels are merely reduced. However, in RGB
color space, desaturation is achieved by reducing the level of the color channel with
the highest level while simultaneously increasing the levels of the two channels with
lower levels. This reduces saturation while maintaining the same luma level. A pixel
with equal RGB channel levels has no saturation (in other words, the pixel is
black-and-white only—like the luma channel in Y
′
C
B
C
R
color space) and therefore
cannot be desaturated.
• Luma reduction: Finally, for any RGB levels that still remain above the Desaturate or
Darken Levels Above parameter value after desaturation, luma levels are reduced so
they are at the level set by the Desaturate or Darken Levels Above slider. Reducing
luma levels in RGB color space is achieved by simply reducing the levels of all three
channels simultaneously.
Working with Analog Video
If you need to output video to an analog format, make sure you understand how digital
signal levels in Final Cut Pro are translated to analog levels.
How Analog Video Signals Are Measured
Analog video is measured using IRE units. (IRE originally stood for Institute of Radio
Engineers, which has since merged into the modern IEEE organization; the measurement
is a video-specific unit of voltage.) One IRE is 7.143 millivolts, but it’s easier to remember
that an analog video signal has a range of 1 volt, which spans 140 IRE units.
Synchronization pulses, which are necessary for an analog television broadcast, are located
between -40 and 0 IRE.
PAL and Japanese NTSC systems have luma (or Y
′
) values between 0 IRE (black) and 100
IRE (white), although higher values are possible. Signals above 100 IRE are considered
illegal for broadcast. North American NTSC systems set black at 7.5 IRE. This extra 7.5 IRE
is called setup or pedestal (because it pushes the entire image higher on the waveform
monitor).
1325 Chapter 78 Measuring and Setting Video Levels