How to calibrate your equipment using color bars
Calibration is probably the most important term that you will learn on Production Apprentice, but what does the term calibration actually mean? To calibrate, you set your equipment to known standards. For audio, the standard calibration tool is the 1kHz tone. If your device is generating 1 kHz tone – someone else can read that measurement and adjust their equipment accordingly. For video, we use the standard test pattern commonly known as color bars.
The Society of Motion Picture and Television Engineers (SMPTE) created this test pattern to help calibrate color video monitors. I’m sure that you’ve seen them before – they’re hard to miss – but do you know how to actually use them? In this post, we’ll take a close look at how to set up your video equipment according to SMPTE’s standard.
Connecting your devices
Connecting one device to another is obviously the first step in the calibration process. I’m sure that you already knew that. If you didn’t, the rest of this article will get exponentially harder. For the purposes of making this simple, we will discuss connecting a camera to a field monitor. Just remember that you should calibrate every piece of equipment that you use. This means monitors that you use in your edit suite, monitors that you hook up to VTRs or cameras – everything!
The professional way to connect devices is to use the highest quality connection possible. Most of the time this will be a digital connection, like HD-SDI but sometimes you will have to connect an analog component (YPbPr) cable to you camera and monitor. Of course the old NTSC composite cable is also a choice – but if you can use a higher quality connection I would strongly urge you use that instead.
Adjusting the monitor to the camera’s color bars
Every camera that is worth its weight should have a setting for bars. If your camera doesn’t, this step will be a little difficult. (Actually, it won’t work for you at all – but read along anyway to understand the theory.)
Some cameras don’t have SMPTE bars they have something called full field bars. The difference is in the lower 1/3rd portion of the image. Full field bars are missing something called the PLUGE (explained below) and the chip for 100% white. Also, a 7.5 IRE black stripe is added to the right side.
If you have this type of test pattern in your camera or edit system, you’re not out of luck. The only difficulty here would be adjusting brightness and contrast – there’s actually no real way to explain how to do this without using a waveform monitor. A waveform monitor is one of those things that you should try to have on hand at all times. They’re expensive, I know, but they are really essential to a good image. In a future post, I will explore some waveform/vectorscope solutions that won’t kill you. Until then, continue on with the lesson.
Warm up your equipment
After you have the camera’s color bars turned on, turn on your monitor. Make sure that you’re seeing the camera’s bars on the monitor and then allow the monitor to warm up for a few minutes. This will give your monitor a chance to get up to its normal operating temperature. Now, this was more important in the old CRT days when all of the monitors had tubes that would change brightness after they heated up for a while. Nowadays most of the monitors in use are LCD monitors that have less of an issue with this problem. Nevertheless, it’s a good idea to warm up your display to simply get it up to operating temperature so you don’t have to re-calibrate later – so go get some coffee and relax for a minute.
Adjusting the brightness of the image
The first place to start your calibration is with the brightness and contrast. At the bottom right of the color bar pattern you will see three small black lines (highlighted above) called the PLUGE which stands for Picture Lineup Generating Equipment. The PLUGE bars correspond to specific IRE values of the image. Left to right they read 3.5, 7.5 and 11.5 IRE on a waveform monitor.
Ok, here’s the part where I tell you again that it would be a good idea to have a waveform monitor handy, but let’s be serious here. They’re expensive. Some LCD monitors have them built-in, but even those are expensive. You have to get used to understanding how to properly “read” your PLUGE.
Adjust the brightness control on your monitor until the lightest bar on the right is barely visible. If it’s not visible, turn the brightness up until it becomes visible. There should be no real difference between the left bar of the PLUGE and the middle bar of the PLUGE. There should be a difference or a dividing line between the middle bar and the right bar. Why do we set it this way? Well, the black level of NTSC video is 7.5 IRE and that corresponds to the middle bar of the PLUGE. Since the 7.5 IRE mark contains the lowest detail in the image, anything below that (like the PLUGE bar on the left) will simply disappear. Anything above 7.5 IRE (like the PLUGE bar to the right) will begin to look more like grey than black. This is why we want to just barely see the right bar when we dial in the brightness on the monitor.
The next step is to set the contrast control for a proper white level. To do so, turn the contrast all the way up on the monitor. The white bar (highlighted above) will look as though it is bleeding into the surrounding chips of the bars – this is referred to as blooming. Now turn the contrast down until this white bar just begins to get sharp edges again.
The blooming is definitely more prominent when using CRT monitors and will, therefore, probably not respond as much as is highlighted above when using an LCD monitor. The contrast control is definitely subjective, so don’t be concerned if you are using an LCD monitor and you can’t quite see the difference. Setting the knob in the middle (at the 12 o’clock position) will usually work just fine.
Setting the colors
Now we’re ready to set the colors. With the monitor’s “blue-only” switch enabled (the screen becomes black and white) start to adjust the hue and chroma values until the color chips match the color bars in intensity.
As you adjust the hue and chroma values on the monitor, you will be able to watch as the chips start to match up with the bars. Adjust them both until you get it perfectly even.
So what do you do if your monitor doesn’t have a “blue-only” switch? The old idea is to simply get a piece of CTB (color temperature blue) gel – the densest one you can find – and hold it up to the monitor. In theory this should “filter” out the reds and the greens of the monitor leaving you with something similar to the “blue-only” image above. I have only tried this once and even though it worked pretty well, it was a giant pain in the neck. My advice to you is to use the blue gel technique at first, but learn how to read the colors without the gel. It will only take a few tries until you start to recognize when the colors are off. Of course the easiest solution is to simply buy a better monitor – and while you’re at it, get one with a waveform built in.
16 x 9 SMPTE Color Bars
Since 2002, SMPTE has designated a newer version of their standard color bars that works with 16×9 video.
The new test pattern has an additional 40 percent gray bar on each end and there are new sample areas that include a smooth gray scale ramp in the center area. There isn’t much of a difference in how you use the newer 16×9 SMPTE bars so I decided to demonstrate with the older version because most professional cameras still utilize that type of pattern – it just happens to scale wider a bit.