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100% and 75% bars



Saw some interesting responses to the 75/100 bars thread and it makes
me wonder if there isn't some confusion as to what exactly these are
and if there isn't further confusion as to the white bar/flag on these
signals.  I hope I can summarize a little:

COLOR BARS generally come in one of 4 flavors:

75% FULL FIELD:
The Y-C-G-M-R-B-K bars are derived from combinations of 75% saturated
color signals (i.e., combinations of 75IRE R/G/B signals, ignoring
setup and assuming 0-100IRE scale on a parade display).  This yields
a white bar which is actually GRAY and color bars which do not exceed
+100IRE when viewd on a coded WFM.  This makes it hard to set luminance
levels, so a 100% W signal is ofetn added.

This sometimes takes the form of a split W bar; one half at 100 and the
other half at 75, as is often the case in Accom DNRs, or by simply
replacing the gray bar with 100%, as is the case with most Quantel kit.
(A 75% gray bar is of dubious utility, anyway.)

75% SMPTE BARS:
Similar to above, with 75% GRAY bar.  However, the filed is split, and
a 100% W flag is inserted as a luminance refernce. (I and Q are not
relevant to this discussion.)

100% FULL FIELD:
Same as 75% FF, but utilizing 100% RGB saturated signals to derive the
bars.  When viewed on encoded WFM or VEC, these signals go "off the 
scale".

100% SPLIT FIELD:
Generally the same as 100% FF, but with a flat field at the bottom half
of the display.

--------------

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.

Virtually all NTSC/525 material will have 75% CHROMA bars and as a
general rulke most PAL/625 will have 100% CHROMA saturation.  But,
you still need to be on the lookout for what the WHITE bar is, as
mentioned earlier, it can take on a few forms.  I suspect this is
what's confusing the setting up of playback tapes more than anything
else.

-bt