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[TIG] What is Legal?
Subject: Time: 10:11 AM
OFFICE MEMO What is Legal? Date: 5/20/96
Fellow TIG members,
I think SMPTE, the EBU, and our Post Production industry need to come up with
guidelines and recommended practices for exactly what constitutes "LEGAL" for
recorded video signals be they on a D-1 or Composite format, 525 or 625. It
appears some facilities are referring to the Diamond Pattern of the Tektronix
WFM601 as a Pass/No-Pass test for legality. I personally disagree and will
contest this as a valid reason for accepting or rejecting a tape. The Diamond
Pattern is used to determine Gamut Limits NOT Transmission Legality. Our in
house legal test here at Manhattan Transfer has to do with the permissible
levels for transmission over the air on normal analog broadcast transmitters.
We don't use either RGB (Diamond Pattern) or D-1 (CCIR 601) "Legal" signal
levels although these are important also. We feel here at MTE that something
is "ILLEGAL" if it can cause a television broadcast transmitter to
overmodulate. This can produce harmonics which will spill over into adjacent
channels. Also, the signal should not cause the transmitter to got to ZERO
modulation and Shut Down the carrier (Video Level too High). With those
criteria in mind we use the test of not creating any signal that will have to
be limited at the transmitter to make it "Legal for Transmission". Clipping
or limiting the signal by the broadcaster will change the look of the program
material that was so carefully created in the telecine, edit, or graphics
suite. For Commercial and Long Form clients this is not acceptable so we
prelimit the signal at our facility in the D-1 domain. That way any esthetic
decisions are made with the gamut limits already built in and the client will
not have any big surprises when he/she sees his/her product over the air.
But, using the Diamond Display will not limit the signal to a "Legal for
Transmission" condition. With 100% saturated color bars the Diamond Display
will be quite happy but the transmitter will not. Clipping of an unlimited
100% color bars signal to make it "Legal" will severely change the look of
We have had tapes rejected by other facilities because their tests for
legality are different than ours. As far as I know there is no industry wide
standard. I think it's time we worked on this. What do all of you think on
Dave Corbitt <dc at mte.com>
Manhattan Transfer / Edit, Inc.
545 Fifth Ave.
New York, NY 10017