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Re: Ms. Masucci and Chroma



<< *	" My first question is, has anyone used a lightning display for  color
correction, and if so, do you prefer it to the "traditional"  vector and
waveform, and why?  Also, how do I know when I have illegal  chroma in the
video? Is is when the color goes outside the boxes?"  >>

Would just like to mention that the Lightning Display was created by
Tektronix several years ago for use in setting up component (Y,Pr,Pb) video
systems.  As it does not work for RGB, nor is it so convenient to use for
determining when colors have become invalid, the Diamond Display has
superceded it for many purposes.  Since the Diamond and Lightning Displays
are intended to be component video alternatives to the traditional composite
Vector Display, neither can be considered a direct replacement for a
vectorscope -- and none of them are replacements for a waveform monitor.

It also appears that there is some confusion over what the terms "illegal"
and "invalid" mean in video, just as there is in life.  Video usually starts
out as three voltages, one for red, green, and blue.  These voltages, which
translate into the color gamut of the particular system, have specific
ranges.  For example, in NTSC they range between 714 millivolts maximum and
54 mV minimum with setup (or 0 mV without setup).  In PAL, the range is from
700 mV to 0 mV.  To be "legal," the RGB values have to be within the limits
set by the standard in use.

Well, it turns out that RGB is not the easiest way to handle video.  Y,Pr,Pb
and composite are a lot more efficient, but the price paid is a loss of color
gamut.  (Who says there's anything new about video compression ?!?)  So it is
possible to have "legal" RGB that cannot successfully be encoded or
transcoded into other formats.  Such signals are said to be "invalid."
 Obviously, all "valid" signals are also "legal," but the reverse ain't
necessarily so.  

<< Can anyone tell me how US colourists calibrate their vectorscopes using
only 75% SMPTE bars ? >>

The 75% or 100% designation refers to the RGB values used to make the bars.
 100% bars is used for setting up camera encoders, among other things, but
they contain some levels that are too high to transmit.  75% bars represents
a fully broadcastable signal.  NTSC vectorscopes have switches to change the
calibration  for the type of bars being used; it's the same graticule either
way.

<< Could someone advise what overall NTSC levels are permissible ? No black
below 7.5 IRE - No white over 100 IRE - OK that covers Luminance in NTSC.
 But what about Chroma levels ? >>

This really depends on the client and what they intend to do with the
finished product.  Generally speaking, no more than 110 IRE maximum on the
chroma highlights and no more than 20 IRE below zero on the chroma blacks
proves to be an adequate formula in many cases.  As was mentioned elsewhere,
a clipper or "proc" amp should be used to make sure nothing out-of-range
accidently gets recorded.

Christopher Bacon



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