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

Re: 75/100% bars question


This gets back to the thread D. Corbitt and others participated in earlier
this year regarding gamut--what is "legal," what is not, and whose standards
are we going by anyway?  

Color bars on videotapes are supposed to be used as a reference so that
different VTRs (in different facilities or not) can play the tapes back the
same way they were recorded.  Assuming everybody's monitors and VTRs are
properly maintained, this works out pretty well.  Needless to say, recording
100% color bars on a tape with ordinary 75% chroma--or vice versa--destroys
the usefulness of the bars, since the whole purpose is to properly play back
the recorded material (not the bars!), and in either case the chroma in the
bars doesn't accurately reflect the chroma levels in the program material.

100% bars is normally considered a transmission test--a transmitter (or an
encoder) should pass them without overdeviation, but no more.  Significant
compression and even some soft clipping may occur before you get to that
point, which changes the look of the material in any case.  75% bars is an
arbitrary level below which no clipping or compression should occur, and it
is advisable to keep chroma in that area to keep the look the same if the
material is going to be encoded and/or transmitted.  

Of course, not everything goes that route.  Some practicioners prefer to
create material beyond 75% chroma for inputting to graphics workstations and
the like, especially when the material is going to be output in print or on
film.  But anybody who actually generates "100%" chroma on a videotape that
is going to be encoded and/or broadcast is in for a surprise when they see
their stuff on the air/cable.

About the only thing you can do with material such as you describe is to
politely ask your clients whether the mismatch between bars and program
material is intentional or not, and if not, would they mind restriping the
tape (on the machine that originally recorded it) with proper bars.
 Otherwise, you haven't got a reliable reference to set
up to, and you'll just have to "wing it."

Christopher Bacon