Beyond The Bars

Brightness and contrast can be adjusted by looking at two regions of your bars (Fig. 1). If your bars appear to be darker or lighter than each other, there’s a problem and you need to adjust your saturation and phase to line things up (Fig. 2).

Color bars also allow us to adjust phase and saturation. Both SMPTE and ARIB patterns have several 75% brightness color bars lined up. These bars have different colors, but are all the same brightness. When phase and saturation are off in a monitor, the bars will appear to have different brightness relative to each other. Most production monitors give you a "blue only" or "mono mode" function, which takes the different colors out and lets you just look at the brightness of each bar. If your bars appear to be darker or lighter than each other, then there’s a problem; adjust your saturation and phase to line things up.

Great, now we have brightness, contrast, saturation and phase all lined up. So, isn’t that enough? The answer is still no because we haven’t adjusted the white point and color primaries of the image at all yet. The phase and saturation adjustments only account for the brightness of colors relative to each other; the individual colors and white can still be far off. I’ll explain how to adjust for this below.

DO I REALLY NEED A PROBE TO CALIBRATE MY MONITOR?

The short answer is "yes." We humans are very adaptable to our environment; our brains will adjust white balance on the fly. Don’t believe me? If you’re reading this inside under regular tungsten or fluorescent light, take a minute and look outside (during the day). Doesn’t it look blue out there? Keep looking outside for a while until your eyes adjust to the brightness, wait a minute or so, and now look back inside. Doesn’t it look orange? Our flexibility is a great thing, but it makes us very bad at judging a key element of monitor calibration: white point.

The vast majority of monitors and televisions today conform to the Rec. 709 (ITU-R BT.709) standard, which specifies a white point for monitors at D65 or 6500 Kelvin. Above is a chromaticity diagram, which shows the Rec. 709 gamut (the black triangle), and in the center you’ll see D65, the white point.

Note that, on the diagram, the coordinates of D65 fall at x = 0.3127 and y = 0.3290. These are important coordinates because, if your monitor doesn’t produce white at (or close to) these numbers, you have a problem. Similarly, red, green and blue all have coordinates specified by the Rec. 709 standard. Our eyes won’t be able to tell us if our monitor hits those numbers; we need a probe for that.

I HAVE A COMPUTER MONITOR PROBE; CAN IT CALIBRATE MY PRODUCTION MONITOR?

Yes, well, sort of. Most computer monitor probes are designed to analyze a computer monitor’s white point and primaries. That’s exactly what we want to adjust on a production monitor, so that seems good, at first. The probe itself can do exactly what we want; the problem is that a computer monitor probe works with software designed to create a monitor profile inside your computer. That is, it creates a profile to adjust the output of your computer to offset for any problems with the monitor. The monitor itself undergoes only small adjustments, usually: brightness, contrast, etc. All the corrections are made on the computer output, so if the screen is too blue, your profile adds red; if the screen is too green, the profile adds magenta, etc. Many of these software tools will even let you calibrate to Rec. 709. This is great if your monitor is connected to your computer monitor output, but if you have a production monitor or if your computer is hooked up to an SDI IO card, then this isn’t going to work.

To calibrate a production monitor, you need software that works with an external signal generator (a generator that makes color bars or patches on a monitor) and can read the result on a probe. The software will let you know all the adjustments that need to be made at the monitor level to get everything in spec. If the results are off, you make adjustments at the monitor level. Most production monitors give you the ability to adjust RGB gain and bias, which are settings to adjust the white level of the monitor. The probe, software and generator make all this possible. At AbelCine, we use SpectraCal’s CalMAN software to do the job. It works with a range of probes and generators, and produces excellent results. However, there’s a range of more and less expensive options out there. For the budget-conscious, check out the Datacolor Spyder4TV HD software, which uses the inexpensive Spyder4 probe and makes color bars with a DVD or Blu-ray player.

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