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Safe Color Limits with Waveform and Vectorscope Information
836
Safe Color Limits with Waveform and
Vectorscope Information
Broadcasters or distributors often issue safe color limits for video levels. Video levels outside
safe color limits are generally known as “illegal” A program with illegal levels might be rejected
on technical grounds or the image quality might suffer with further processing. You might want
to set reasonable limits for yourself even if you are not aware of specific limits that you must
meet.
In Y (luma only) waveforms, reference white of 100% corresponds to a digital level of 235, an
NTSC level of 100 IRE, and a PAL level of 700 mV. White excursions up to 108% are
technically possible.
In Y (luma only) waveforms, reference black of 0% corresponds to a digital level of 16, an
NTSC level of 7.5 IRE, and a PAL level of 0 mV. Black excursions down to –8% are technically
possible.
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With both white and black levels, further signal processing after output from your Avid editing
application might clip the peaks in your material. In addition, delivery specifications might
require you to limit the white peaks to a lower level and the black peaks to a higher level.
Use Y Waveform to see the black and white levels of your image. Sometimes, particularly with
white levels, keeping the white peaks within the 100% limit does not produce a pleasing level for
the rest of the image. This is particularly common with backlit subjects, where the sky or a
window is in the background and the lighting on the foreground is insufficient. In these cases,
you might want to adjust for the foreground and leave the background too bright.
Chroma peaks are easiest to see on the vectorscope. The theoretical maximum is the circle
around the outer edge, but to be safe you might like to keep vectors closer to the center than the
75% color bar squares.
Saturated bright or dark colors might have very low or high luma values, together with a lot of
chroma. Even if neither luma nor chroma alone is excessive, the combination can be illegal. For
example, vivid yellow and cyan in an image can produce composite levels that are too high, and
those from vivid blue might be unacceptably low. The YC Waveform is a good way to see how
far these levels extend. In general, avoid levels above approximately 120 IRE or 850 mV, and
those below –20 IRE or –200 mV.