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Re: Correction



Karl,

I know that if tape operators were told to use zero setup they would know
how to do it. They are a fine bunch just like telecine personel and members
of TIG. 

What I meant was that setup should not be used in the production and post
-production phases. If most other countries can do it so can we. The
problem is that the FCC rules say you must have between 7.5 ire units, 2.5
ire of setup to be compliant in an NTSC transmitter. That's in the FCC
reqs. So therefore it makes sense to put setup into the image when it is
transmitted.

The problem with setup is that it is easy to put in but difficult to take
out. Other countries that use tapes produced in NTSC (with setup) do not
bother to take setup out (because of the difficulty in taking it out) so
what they transmit in their countries (who don't use setup) an image with
milky blacks compared to their TV images. Japan as an example does not use
setup and do not try to remove it.

Contrary to popular belief the blacks will not clip. 75% color bars
properly setup, the chroma extend below the reference black down to -16 ire
units. Setup is added to the video image by adding a (scaled) horizontal
blanking pulse, pushing the image up 7.5 ire. Because that horizontal
blanking pulse can and does vary from place to place it is not a simple job
of subtracting it out. Also it has a rise time of 140 20 ns (nanoseconds)
that tends to slightly differ from place to place due to the 20 ns
tolerances. (If a 150 ns pulse were removed with 130 ns pulse, both within
the 20ns tolerance, you end up with what I call "snivey's" at reference
black going in a positive direction. Not a clean reference black.)

The NTSC transmitter, using negative polarity, transmits the image as an
amplitude modulated signal. 100% modulation is sync tip which has been
clipped in the transmitter to go from the blanking level of 75% modulation
to 100% modulation. (The incoming signal has -40 ire units of sync but the
clamp in the transmitter slightly clips that.) Reference black and blanking
level should be at 75% modulation like it was in the very begining of
television (no setup) and also what most other countries use today. But 7.5
ire setup was added a long time ago to the image (and the FCC rules) that
reduces the modulation from our blanking level of 75% down to a reference
black of 70% modulation. Peak white or 100 ire units modulates the
transmitter at 8%. If the video level should go to 110 ire units we only
have 1.7% of carrier left. At 112 ire we only have 0.3% carrier left. At
112.5 ire there isn't any more carrier left and the sound will buzz on most
TV sets. Bear in mind waveform monitors if properly calibrated are only
accurate to 2% and transmitters are allowed a tolerance of  2.5% to be
legal.

"Keyed clamps" clamp on the back porch of the signal in blanking level.
They do not clamp at reference black. So you see that if we eliminate setup
and use the blanking level as our reference black everybody gains. If setup
is inserted at the transmitter we would gain in post-production and
production a REAL not variable (2%) reference black.  The eye is very
critical in the blacks because thats where gamma correction is the highest.
When you adjust your monitor brightness control on 75% bars with "PLUGE"
you are biasing the CRT to the "Kine Cut Off" and yes, that clips the
blacker than blacks because you can't see them.

Sorry for the lengthy reply but that's why, in one of my other comments, on
TIG I said the "telecine operator indirectly controls" the transmitter. 
PAL, SECAM, and DTV does not require setup nor does component video (YUV,
RGB, etc.).

I'm glad your company watches levels because as you know it is very
critical to the overall sucess of the TV industry and more than 75% of what
we view on TV starts with a film to tape transfer.

Jim Mendrala
Real Image Technology, Inc.


---
Thanks to Blake Jones for support of the TIG in 1998.
No product marketing allowed on the main TIG.  Contact rob at alegria.com
928 subscribers in 37 countries on Tue Feb 17 00:57:05 PST 1998
complete information on the TIG website http://www.alegria.com/tig3/