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Home: Video University Forums: Tech Q & A:
Color Bars-Does our article have an error?

hlanden
The Dean / Moderator

Sep 4, 2005, 9:34 AM

Post #1 of 6 (1072 views)
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Color Bars-Does our article have an error?	Can't Post

Our Color Bars article at

http://www.videouniversity.com/tvbars2.htm

elicited a question from Glen C in Canada. He writes

"I may be wrong here but I believe the right way to set contrast on a
broadcast monitor is to set it to a specific brightness.

It's 30ftl (foot lamberts) for SMPTE / NTSC, and 25 ftl for PAL I
believe."

Can anyone tell us if he's correct?

Thanks
Hal
Produce Profitable Special Interest Videos

videobear
Veteran

Sep 4, 2005, 1:52 PM

Post #2 of 6 (1052 views)
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Re: [hlanden] Color Bars-Does our article have an error? [In reply to]	Can't Post

Hal,

Glen C. has the correct value for the standard illumination (light output) from a monitor or television set. This value is measured by a photometer at a distance of one foot from the center of the screen, with the monitor displaying a full-screen 100 IRE test image.

However, the light output value is NOT the contrast. They are two different measurements.

Since very few people have the means to measure light output, setting the brightness by noting the point at which the 100 IRE chip starts to bloom is the best method for most, as described in the VU article. From there, setting the black level as described (using the three black pluge chips) will properly set the contrast range.

That's my take on it. Any folks with some broadcast engineering experience care to weigh in?

Regards,
Doug Graham
Panda Productions

glennchan
New User

Sep 5, 2005, 3:33 PM

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Re: [videobear] Color Bars-Does our article have an error? [In reply to]	Can't Post

Hi,
This is my first post here and I was the person who sent in that email.

1- Just to clear up some possible confusion:
By contrast, I mean the contrast control on a monitor. I believe this controls white level.
By brightness, there's a brightness control on monitors that controls black level.

The (contrast control) determines how bright a full-screen of 100 IRE would be, so that's what I mean by setting contrast.

2- I've been wondering: What exactly is blooming? How does it look like?

On one site, I've read that you can tell a display is blooming if white starts turning a different color (because one of the electron guns has maxed out). I have tested this myself and do not experience it on my PVm-1354Q or my consumer television in my living room. Or at least, the blooming is incredibly subtle when brightness and picture/contrast controls are cranked.
I tested this with a full field of 100IRE, with cyan magenta and yellow patches. If the monitor is blooming, one of the patches should disappear. Needless to say, they do not.
AWH: How to Calibrate a Television FAQ

3- From what I can figure out of setting white level:

Too low:

White no longer appears white, but some sort of grey.

If the monitor gets really dark, you don't have color perception at that light level. You probably won't get THAT low.

Excessive glare on monitor makes setting black level sketchy.

Too high:

If your monitor is new, you may want to run it slightly dimmer as to extend monitor life. Greater brightness will make the phosphors get dimmer faster, and then you have to increase brightness to compensate as the monitor ages.

Geometry distortions. Turn the contrast control all the way down and then all the way up. You may notice there is a subtle distortion in the geometry of the monitor- the brightest white bar in the bottom left (100IRE) will actually change in shape slightly. If you do not see this, it may help to take a piece of paper and hold it up to the monitor so that you see only a sliver of the 100IRE bar (static should help stick the piece of paper to the monitor). Start with the contrast control all the way down and then slowly increase it until you see its size change. Set contrast to the point just before the white bar begins to change in shape.

If the monitor appears almost blindingly-bright then it could cause eye strain. If your eyes hurt, turn the brightness/contrast/picture down.

(This post was edited by glennchan on Sep 5, 2005, 4:47 PM)

videobear
Veteran

Sep 6, 2005, 9:01 AM

Post #4 of 6 (989 views)
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Re: [glennchan] Color Bars-Does our article have an error? [In reply to]	Can't Post

Quote

The (contrast control) determines how bright a full-screen of 100 IRE would be, so that's what I mean by setting contrast.

2- I've been wondering: What exactly is blooming? How does it look like?

On one site, I've read that you can tell a display is blooming if white starts turning a different color (because one of the electron guns has maxed out)

The contrast and brightness controls interact with each other. Brightness raises the level of the WHOLE picture, not just the black level. Contrast controls how far apart the whitest and the blackest parts of the signal are. You can see this if you feed your color bars to a waveform monitor, and observe the display as you adjust the monitor controls.

Blooming is the loss of detail in either colors or whites. It's kind of hard to see with color bars, since there is little detail to start with. A waveform monitor is the best tool to use. But in general, you should not turn your brightness control past the point where the edges of the large white chip in the pluge area (the bottom of the pattern) begin to appear softer.

I don't think you will see a color change in most cases, since all of the electron guns will be putting out the same level to create white, and should max out at the same time.

Regards,
Doug Graham
Panda Productions

glennchan
New User

Sep 6, 2005, 9:20 PM

Post #5 of 6 (959 views)
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Re: [videobear] Color Bars-Does our article have an error? [In reply to]	Can't Post

Hi Doug,
Thanks for the explanation on what blooming is.

When I first read the calibration article way back when, I wasn't exactly sure what blooming was. Perhaps there should be an explanation of what it is?

2- How are you supposed to judge blooming on color bars pattern?

I find that putting a piece of paper on screen until you see a sliver of 100% white really aids in seeing where the 100% bar begins to change. I haven't heard of this being a standard practice, but perhaps it is better than trying to set the monitor by eye?

Or should one use another test pattern, like small text or the same test pattern you'd use for convergence.

videobear
Veteran

Sep 7, 2005, 10:03 AM

Post #6 of 6 (954 views)
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Re: [glennchan] Color Bars-Does our article have an error? [In reply to]	Can't Post

Quote

I find that putting a piece of paper on screen until you see a sliver of 100% white really aids in seeing where the 100% bar begins to change. I haven't heard of this being a standard practice, but perhaps it is better than trying to set the monitor by eye?

Or should one use another test pattern, like small text or the same test pattern you'd use for convergence.

Those are both good ideas!

Regards,
Doug Graham
Panda Productions