[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Ms. Masucci and Chroma

	I believe this question has been worked over pretty thoroughly in
the past here on the TIG but I will jump in and put in my 2 cents.  There
is a lot of myth about this issue, some of it because the industry folklore
has been passed on incorrectly through the years.

	What "Broadcast Legal"  refers to is the maximum allowable overall
amplitude of signal that can safely be put out over the air with a
composite video signal amplitude modulating the RF carrier.  As your video
signal gets larger the carrier amplitude gets lower, that's the way
transmitters work. So the sync signal, which is the largest negative going
signal is actually sent out with the greatest amplitude carrier.  This
inverted modulation was initially done ages ago to improve the TV
receiver's ability to get a locked picture even if the signal was horribly
noisy due to a faraway transmitter. It really has nothing to do with the
vectorscope but everything to do with the flat response display on your
composite video waveform monitor.

	If the positive going signal exceeds 133 IRE as displayed on your
wfm then the carrier will go to ZERO amplitude which is the whole reason
for being concerned about "Legality".  Broadcasters are not allowed to go
to ZERO carrier, by law, thus the issue of legality. For practical reasons
it is not good policy to allow the signal chroma peaks (riding on top of
luminance, remember this is the whole signal that is transmitted, not just
the chroma envelope) to exceed around 118 IRE and for the Luminance levels
to go above 100IRE.  If you exceed these limits then your signal will be
clipped at the transmitter and the look you worked so hard to create will
be seriously compromised.  It will also, in all likelyhood, be bounced by
the QC Police at whatever station or network is transmitting your product.
For a different reason negative going chroma excursions should not go below
-20IRE so as to not confuse the sync separator in the home receiver
(although I doubt if there is a receiver left in the world that does not
have a low pass filter in the sync separator).

	Now the reason that the vectorscope is not the right device to
measure Broadcast Legal is because it only shows you the Chroma.  Without
the Luminance signal added to the Chroma you have no idea if what is
displayed exceeds the overall amplitude maximums of the transmitter.  Some
highly saturated colors with relatively low luminance values are perfectly
acceptable such as Red or Blue, but almost always high Luma signals with
high Chroma (such as a bright Yellow) will jump off the WFM scope and get
clipped reducing the displayed chroma very noticeably.  The vectorscope is
however a good device to see if what you are creating will exceed the
recordlimits of the dreaded 3/4" approval copy going to the agency.  These
*&%$?#$ machines have been the standard we are held to for far too many
years.  They are incapable of broadcast quality bandwidth, signal to noise,
and chroma levels due to their color under recording system.  If there is a
deity he/she will see the wisdom in finally designating these devices to
the scrap heap.  Limiting yourself to what can fit on a 3/4" deck is a big
handicap relative to the much larger gamut that is transmittable.

	To insure that what you are creating is within bounds the best
device to have is a video legalizer.  Various manufacturers make such a
device, often called a Digital Proc Amp.  We use the HP QA-100 here but
there are other devices on the market.  Make sure if you get these things
that it legalizes the D-1 signal for eventual encoding, not for RGB legal
or 601 legal, although these are important too. Another day.

	If you are still living in an analog environment in your suitres
then get a good analog proc amp to keep everything within reasonable limits.

>Anne Masucci wrote
>*	" 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?....deletia....

* Dave Corbitt <dc at mte.com>	    *
* Director of Telecine Engineering  *
* Manhattan Transfer / Edit, Inc.   *
* 545 Fifth Ave			    *
* New York, NY 10017		    *
* vox (212) 687-4000		    *
* fax (212) 687-8023		    *

thanks to Rich Zabel of Tekniche for support of the TIG in 1997
TIG subscriber count is 884 on Fri Oct 31 05:32:39 PST 1997
mailinglist digest available.... unsubscribe via a message to
'telecine-request at alegria.com' with Subject: unsubscribe