How to calibrate a broadcast monitor to color bars (NTSC)
An uncalibrated color bar test signal will look something
like the image below:
Steps to calibrate a monitor:
- Turn on the monitor and leave it on for at least a few minutes to let it
warm up.
- Feed color bars to the monitor. You should not use images
or video files downloaded from the Internet as there is a good chance that
the levels will not be converted correctly due to Quicktime gamma issues,
incorrect levels conversions, RGB color space limitations, etc. etc. Use color
bars generated from the source that you are interested in (e.g. your NLE).
Most current NLEs will generate proper color bars by default (except Sony
Vegas 8; see my
article explaining color space conversions in Sony Vegas).
- Using a s-video signal instead of composite will make the process easier.
- Set the lighting in your room to what you normally use. e.g. do not turn
the lights off if you normally leave them on. The PLUGE adjustment (explained
later) was intended to compensate for ambient room lighting reflecting off
the monitor.
- Set the contrast control ("picture" on some monitors) to its middle/detente
position.
- Calibrate chroma/saturation/color and then phase/hue. Use the "blue
only", "blue gun", or color check feature on your monitor.
If your monitor does not have this feature, set the chroma and phase settings
to their middle position and then skip this step (also see the
sidenote on why the 'blue gel trick' doesn't work). With blue only engaged,
adjust the chroma setting until the vertical pair of bars on the very right
match and have no dividing line in between them where the red circles in the
image below are.
- Then adjust the phase setting until the bars left and right of center match.
- The result should look something like this:
All four columns have an even brightness with no dividing lines visible.
Note that some monitors such as the Sony PVM CRTs have an "auto adjust"
feature which will adjust phase and chroma for you automatically so steps
6-8 are not necessary on such monitors. However, you will still need to set
brightness and contrast.
*Please not that digital interfaces (e.g. SDI, DVI, HDMI) do not require calibration
of the hue and phase. As the signal is digital and not analog, the calibration
will not drift out of alignment. If the monitor does allow for chroma and
phase adjustments, set the knobs to their middle position. Broadcast CRTs
fed with a SDI signal should still be calibrated for brightness and contrast
per the instructions below.
Turn off the blue only setting.
- Calibrate brightness (black level). Focus on the PLUGE bars in the lower
right-hand corner of the screen. The leftmost PLUGE bar is lower than black
level, the middle PLUGE bar is exactly at black level, and the rightmost PLUGE
bar is slightly higher than black level. Adjust black level until the left
and center bars just merge together and the right bar is still visible.
 |
 |
| Incorrect |
Correct. *These diagrams are exaggerated for illustration. |
- Calibrate contrast (white level). If a CRT is being overdriven, the electron
beam will lose focus and/or geometric distortions in the picture will result.
To make this easier to spot, I would suggest placing a piece of paper on the
monitor until you can just see a sliver of the 100 IRE white chip in the bottom
left. Play with the full range of the contrast setting to see which direction
the geometric distortions occur.
Turn the contrast setting all the way down and place your piece of paper (or
other reference) on the monitor so that you see a sliver of white. Adjust
the contrast setting until you see the sliver begin to grow larger. Then back
off on the contrast setting. Note that some consumer monitors will defocus
or distort throughout the entire range of the contrast setting.
Take the piece of paper away and look at the 100 IRE chip. To your eye, this
chip needs to appear white and not greyish. If it appears grey, you should
adjust the contrast setting higher until it appears white. It is better to
have geometric distortions than to have grey-ish whites.
- You're finished!
Possible issues
Color bars do not calibrate no matter what
If you can't calibrate the color bars no matter what, then there are possibly
issues with the signal being converted. If doing HD<-->SD conversions,
some equipment will not properly handle the difference in the luma coefficients.
The current Rec. 709 based HD formats, the obsolete 1035i format, and Rec. 601
based SD formats all use a different set of luma coefficients. The conversion
must apply the right matrix to handle the difference in the luma coefficients,
otherwise gross color inaccuracy will result. Some consumer displays cannot
do the right conversion or need to be set to use the right set of luma coefficients
(but should you really be using consumer equipment anyways?).
If you are monitoring with a CRT in both HD and SD, you should check that the
color bars line up in both HD and SD. One potential trap is using the easy setup
presets in Final Cut Pro to set FCP to downconvert HD to SD so that you can
monitor a SD image. When I last tried this, FCP did not convert the values correctly!
While this bug may be fixed by the time you read this, do use color bars to
check that all your conversions are being done properly.
7.5 IRE setup (applies to NTSC countries except Japan)
Note that the majority of consumer, prosumer, and even some professional DV
equipment will not perform a proper digital->analog conversion and put black
level at 0 IRE instead of 7.5 IRE. If you have multiple analog signals connected
to your broadcast monitor, the monitor's calibration may be correct for only
some of the sources if they differ in black level. Keep an eye out for this.
Accurate monitoring may require re-calibrating the monitor for the second source.
More information on this issue can be found in the article 7.5
IRE setup demystified.
Other Approaches
Why the blue gel trick doesn't work
Some sources advocate using a blue (or magenta) filter if your monitor does
not have a blue only setting. The blue filter in this case would theoretically
emulate the blue only setting on the monitor. I have tested this trick and found
that it does not work very well at all. Using a professional broadcast monitor
with a blue only setting, I compared:
- Calibrating the monitor using the blue only setting.
- Setting the knobs to their middle position.
- Using the blue gel trick.
The accuracy of the calibration is in the order above. The accuracy of the
blue gel trick is worse than setting the knobs to their middle position.
The reason why it does not work is because the red and green phosphors emit
small but meaningful amounts of light in the blue spectrum. Therefore blue light
gets through when the desired behaviour is no light getting through.
The other approach to setting black level
How to do it:
- Turn off all the lighting in the room and block off outside light if it
is bright.
- On a CRT, enable the widescreen/16:9 setting and underscan. Adjust black
level until the middle PLUGE bar and surround area merges with all the area
beneath it.
If the display does not have such a setting, then adjust PLUGE normally except
with the lights turned off.
The difference is that this calibration will not compensate for the effect
of glare. Some people prefer this style of calibration.
Ideally, ambient light on the monitor would be as low as possible.
You can achieve this by painting the side and black walls of a room black (or
hanging black fabrics over these walls). The area surrounding the monitor should
be a neutral grey color and lit. Put some distance between the monitor and the
wall and aim lighting behind the monitor.
