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Re: Chroma Levels

Anne Masucci wrote

> How do I know when I have illegal chroma in the video?
> Is is when the color goes outside the boxes?"

No.  It is when the chroma exceeds the "maximum" (can of worms) peak
voltage amplitude of the composite signal as represented on a waveform
monitor, not a vectorscope.  Unfortunately, the reason that the subject of
invalid chroma remains unprecise, is because of the many subjective opinions
of what constitutes excessive chroma even in light of given parameters of the
NTSC system and the FCC broadcast transmission standards.

Whether it be right or wrong, you will certainly find that many individuals in
post production facilities create their own standard of what is or isn't "legal"
chroma.  This can vary all over the place.  In some cases it may be based
on a misunderstandings of the system.

> Is there still a limit to how much chroma can tolerated on the broadcast
> sytem?

If you define the broadcast system limit as the maximum allowable positive
and negative peak voltage amplitude, the answer is yes.

> When looking at the vector, when is the chroma illegal?  I always
> thought it is illegal when it goes beyond the little boxes.  Then I was
> told that chroma is illegal if it goes outside the circle."

Here is the reason for this common confusion.  Although one relies on
a vector representation as a guide for "relative" hue and saturation in
color correction, when we are speaking in terms of "illegal", the + and
- peak voltage (or IRE) "video" levels are now in question.  This is not
the same as peak to peak chroma modulation amplitudes.  Viewing
chroma only, whether it be in vector form or hi-pass, does not indicate
peak composite video levels since the luminance component is left out.

> Could someone advise what overall NTSC levels are permissible ?

The NTSC system was designed such that 100% "saturation" of colors
(.7v of R,G,B) can cause peak chroma modulation to reach a maximum
peak video level of 133 IRE in the positive direction and to -33 IRE in the
negative direction.  Now the question becomes, what is the purpose for
limiting the color gamut (chroma), from the established system levels?

The answer, like many, is that it evolved from early transmission practices.
Television transmission is such that the video is inverted so that peak white
(100% IRE) would occur at 12.5% from power cutoff while sync tip is at
maximum power.  Consequently the input video levels would be scaled
to maximize overall luminance information sacrificing higher saturation
chroma levels to hopefully achieve better signal to noise.  Afterall, high
saturations reaching 100% were rather rare, especially back 20 years ago.
This was the reason for the practice being established to limit peak chroma

Unfortunately limiting peak chroma, in either direction, destroys the
integrity of the video signal.  Even though analog VTR's (remember those?)
also had difficulties with both positive and negative maximum chroma levels,
by today's standards those problems no longer exist.  Additionally, the video
input to transmitters is usually scaled (yet another variable) to fit more
of the chroma information in for more accurate color reproduction.  Even
so, most facilities will still limit + peak chroma probably on an average of
120 IRE to help avoid being pulled over by the many anxious aging
chroma police.

Michael C. Kaye

thanks to Allan Taylor for support of the TIG in 1997
TIG subscriber count is 881 on Mon Nov  3 06:56:31 PST 1997
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