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Re: 100% and 75% bars



<< To take issue with some data that was presented, 100% bars CANNOT be
 transmited by NTSC transmitters; the peak excursion of the Y bars will
 cause overmodulation.  Therefore, only 75% bars are used for trans-
 mission tests.  However, it is possible to have MOMENTARY excursions
 of chroma up to 100% saturation levels and still get by.  It is for
 this reason that we often let chroma levels up to about 120IRE go
 out; assuming it's a transient thing. >>

I'd like to quote the following from "Television Measurements--NTSC Systems,"
published by Tektronix.  It's in their excellent series of booklets,
obtainable from your local Tek rep, which are packed with information and
more effective than melatonin!

"There are two basics types of NTSC color bar signals in common use.  The
terms '75% bars' and '100% bars' are generally used to distinguish between
the two types...

"The 75%/100% nomenclature specifically refers to the maximum amplitudes
reached by the Red, Green, and Blue signals when they form the six primary
and secondary colors required for color bars.  For 75% bars, the maximum
amplitude of the RGB signals is 75% of the peak white level.  For 100% bars,
the RGB signals can extend up to 100% of peak white...

"Both 75% and 100% amplitude color bars are 100% saturated.  In the RGB
format, colors are saturated if at least one of the primaries is at
zero...zero signal level is at setup (7.5 IRE) for NTSC.

"In the composite signal, both chrominance and luminance amplitudes vary
according to the 75%/100% distinction.  However, the ratio between
chrominance and luminance amplitudes remains constant in order to maintain
100% saturation.

"Color bar signals can also have different white bar levels, either 75% or
100%.  This parameter is completely independent of the 75%/100% amplitude
distinction, and either white level may be associated with either type of
bars."

I worked for a couple of years installing and testing transmitters--microwave
relay units high on the roofs of midtown Manhattan skyscrapers--and we used
100% color bars to make sure the chroma compression and clipping would occur
somewhere beyond the nominal 75% range.  Of course we never showed our
customers anything but 75% bars online!  100% color bars equates to about 130
IRE of chroma on yellow, followed closely by cyan and green, which would most
assuredly attract the FCC's attention if one applied the signal directly to
the modulator of a NTSC TV broadcast transmitter.  Hence my previous remarks
about somebody being in for a big surprise if they put 100% bars on a
videotape they expect to see on the air!

Christopher Bacon