HDMI Monitor Calibration - Frequently Asked Questions
Q - Why do I need to calibrate my monitor?
A - The color of light in a room affects your perception. Think of a white sheet of paper. If the light in the room is blue, the sheet of paper will look blue. But if the light is red, the paper will look red. In the same way, the colors you see on your monitor will depend on the color of the lighting in the room. The level of lighting in the room also plays a role.
Manufacturers of televisions, broadcast monitors, and computer monitors make assumptions about the lighting in your room when adjusting their factory default settings. Computer monitors are factory adjusted to be used in brightly lit rooms such as offices. Broadcast monitors are designed for the lighting typically found in living rooms. Manufacturers of broadcast monitors take extra care to make sure their red, green, and blue are as close as possible to those specified by ITU-R Recommendation BT.709 (also known as Rec 709). Television manufacturers, on the other hand, are more interested in having their televisions stand out from the crowd in the showroom than in having exact colors or shades of grey. More often than not, when comparing the colors on a broadcast monitor to the default setting on a digital television, you'll find that the television is too bright and the colors are too saturated.
In addition, variations in manufacturing processes result in variations of the actual shades of red, green, and blue from monitor to monitor.
Q - Why do HDMI monitors and televisions require more adjustments than broadcast monitors?
A - Broadcast HD-SDI monitors are manufactured to match the Rec 709 specification. The only adjustments that need to be done on them are those required to adapt their picture to the environment in which that are used. For example, in a bright room your eyes will have a harder time perceiving the 2% black pluge bar than in a very dark room. You can adjust the monitor by increasing the black level (brightness) until you can see the 2% black pluge bar. Similarly, other controls on the broadcast monitor are there to adapt the picture on the monitor to its environment, not to compensate for errors in the manufacturing of the monitor.
In the case of analog component signals, some compensation for transmission errors might be required even on a broadcast monitor. But for the discussion here, transmission errors are not an issue because HDMI monitors and televisions are digital.
Because computer monitor default settings are not designed to display video in a living room, more parameters need to be adjusted to match the Rec 709 specification. One would think that television default settings would closely match those of a broadcast monitor. However, because television manufacturers set up their television to stand out from the crowd in the showroom, they also require more adjustments to match the Rec 709 specification.
Q - Why is it so important that my monitor follows the Rec 709 spec?
A - In order to ensure that the picture a video camera is filming is going to match the picture the viewer is ultimately going to see, the camera and monitor need to digitize and display colors in exactly the same way. Otherwise for example, the viewer might see a dark green apple when in reality the apple was light green. That is why there is a specification, ITU-R Recommendation BT.709 (also known as Rec 709), that defines how a HD monitor should display color. It specifies, among other things, a gamma response of 2.22, the white point, and the exact primaries to be used.
Q - What is the white point?
A - There are several parameters in the Rec 709 spec that define how a monitor should display colors. One of them is the actual color the monitor should display when displaying white. It sounds odd, but white can actually take on slightly different hues. If we take a white piece of paper and look at it in sunlight at noon and in sunlight at sunset, our eyes perceive a slightly different hue of white. The one at sunset will be more orange, but our brain will still see both as white. Rec 709 specifies the exact hue white should have. It's roughly what the white sheet of paper looks like under sunlight on an overcast day at midday. This white is neutral, neither blue nor yellow.
Q - What is the gamma of a monitor and why is it important?
A - On a Rec 709 video display, the intensity of the light generated by the display is not linearly proportional to the input signal. For example, given an 8-bit RGB image, white is R,G, and B at 255 (100%) and 50% grey is R,G, and B at 128. This 50% grey pixel will generate a light intensity on the screen that is NOT half as bright as the white pixel. The function that describes the response of the monitor to the input signal is called a gamma function. This is an important characteristic. If a display does not have the proper gamma response, shadows and highlights will not be rendered properly.
Q - What are primaries?
A - Monitors display colors by using a combination of Red, Green, and Blue. Since it's possible to have different shades of Red, Green, and Blue; Rec 709 specifies the exact color of Red, Green, and Blue to use. These are the primaries of the monitor.
Q - My HDMI TV has a blue-only mode, is that useful to adjust my white point?
A - No. It's true that most TVs have a tendency to display whites with too much blue because buyers think whites look brighter with a bit of blue added, however, blue-only mode is not useful to detect this kind of problem. Blue-only mode detects only whether there are equal amounts of blue in the white, blue, magenta, and cyan. There could be 120% of blue in all of them and the blue-only mode would be perfectly happy.
Q - My HDMI TV has a blue-only mode, is that useful to adjust my colors?
A - Yes, the blue-only mode is useful if the white point of the monitor is neutral grey. Blue-only mode helps verify that the white, magenta, cyan, and blue have the same amount of blue in them. Since, in general, such monitors will generate the same intensity of blue, green, and red when generating white, magenta, and cyan; making sure there is an equal amount of blue in the white, magenta, and cyan guarantees that these colors are right.
Q - What if my monitor or TV does not have a blue-only mode?
A - Blue-only mode can be useful but it is not required. Matrox has developed a technique for checking and adjusting the colors on your monitor or TV without the need for a blue-only mode. The Matrox HDMI Calibration Utility generates a pattern that is very sensitive to error in both hue and intensity for all primary and secondary colors. The Matrox HDMI Calibration Utility wizard lets you adjust the actual color of the primary and secondary colors to make sure they have the right amount and hue of red, green, and blue.
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