[Tig] Neophyte with calibration questions

David L. Tosh dlt
Tue Apr 29 05:33:51 BST 2003


>To me this begs a question:  How is this really defined?  That is, 
>whats the real definition of "just visible" in the case of these two 
>bars?  What is the difference in their origin signal and how 
>(quantitatively) should that be represented by the display?
>Why shouldnt I just try to generate a full screen of the "blacker 
>than black" bar, measure that, then generate the "merely black" bar 
>and measure that, then define the specific signal difference I'm 
>looking for?
>Pardon me if I'm posing naive questions, but I'm really chasing, 
>perhaps in vain, quantitative definitions for "that monitor looks 
>too dark"

You can learn some interesting things about the precision of your 
monitors and some perceptual quirks by "playing" with a good switcher 
(vision mixer.) Generate your own blacker than black patch on a 
perfectly black background. Add another nearly black shape. With a 
digital component switcher you can generate up to -6.8 IRE of 
negative black -- you actually want to use about half of that 
magnitude. Make the nearly black the same magnitude but brighter than 
true black. (Use a good WFM and match the patches on SMPTE Bars if 
your switcher passes the negative blacks.)

Change the size and position of the PLUGE patches and see if your 
monitors can hold the distinction between actual light output cutoff 
and anything electrically greater than zero. Wipe to another picture 
and notice what happens. Wipe on 100% white and you have a real 
torture test of the monitor's electrical black level stability.

The purpose of PLUGE was to set the CRT's no-light cut off point 
exactly at the electrical representation of "zero black." Anything 
blacker than black should not be visible. The smaller the difference 
between blacker than black and its complementary not quite black, the 
more precisely you can set the middle, black cut off value. (And NTSC 
composite and digital component each need to have been setup on a 
monitor that their respective blacks -7.5% and 0% each achieve CRT 
zero cut off.)

For demonstration purposes, you can generate text at blacker than 
black that says "You shouldn't be able to see this!" and other text 
that says "This text should always be visible." and achieve your own 
custom PLUGE. (I think a THX DVD setup feature on "Episode I" did 
something similar.)

You can sometimes get access to a good paint program to do these 
exercises (I did all this with a Quantel DPB-7000 a loong time ago.)

-- 

---
David Tosh
<dlt at earthlink.net>





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