CRTs provide 3 to 4 different mechanisms for calibrating brightness, and apparently many people are not aware of how they differ. All brightness settings are used to manipulate the CRT's triode. If you do not understand basic triode theory, then you won't understand the ideal way to tune brightness. This post outlines the 4 mechanisms for tuning CRT brightness, uses basic triode theory to explain how they are different, and describes how to tune them in order to have proper black levels and the smallest possible cathode ray spot size (for thick blank scanlines and a sharp image). Note that the ideal way to calibrate a CRT is using a colorimeter, but this article will describe how to get a good calibration without a colorimeter, as long as you have access to 240p Test Suite and a very dark room. This post will refer to a single test pattern in 240p Test Suite called "Color Bars" (screenshot below):
Definition of "Brightness"
The term "brightness" refers to the lowest possible light emission levels of a display. This is sometimes called the black levels of the display. When brightness is too low, the darker colors in the 240p Test Suite color bars: black, dark grey, dark blues, dark reds, dark greens, etc, will all "crush" or "clip" into black. When brightness is too high, the darker colors in the color spectrum do not get dark enough. Black looks grey, and dark grey looks like light grey, and similarly for dark green looking more like light green, etc. You can think of "brightness" as controlling the separation between the darker colors in the color bars. However, this is not entirely accurate because brightness increases and decreases the light emission of every color, not just the darker colors. Brightness additively shifts up or down, the light emission of all colors, even the colors on the rightside of the color bars in the test pattern.
Mechanisms for Adjusting CRT Brightness
- "Brightness" setting in the onscreen display (or service menu): If your CRT does not have an OSD, then this is available as pots. This controls the bias voltage of the CRT's cathodes "K". Color CRTs have 3 cathodes: RK, GK, BK. As this brightness setting is decreased, it means the bias voltage of the CRT cathodes is increased. The higher the voltage of the CRT cathodes, the less cathode ray emissions are released from the cathodes because the electrons that form the cathode ray are attracted to the higher positive voltage of the cathodes. As this brightness setting is increased, the bias voltage of the CRT cathodes is decreased, allowing more electrons to be emitted from the cathodes, making the cathode ray have a higher emission. More electrons emitted into the cathode ray means more light emission from the CRT.
- Red, Green, and Blue Cutoff (typically in the service menu): If your CRT does not have an OSD, then this is available as pots. These 3 cutoff settings do the same thing as the "brightness" setting mentioned above, except for a single cathode at a time. That is, "red cutoff" adjusts the bias voltage of the red cathode RK. Similarly for blue and green cutoffs. Due to manufacturing differences and other physical phenomenon, each cathode needs a slightly different bias voltage for proper calibration.
- Screen pot on the flyback: This controls the bias voltage of the CRT's G2 anode, which is used for accelerating the electrons away from the cathodes and towards the screen. Turning the screen pot clockwise increases the bias voltage of the G2 anode, which causes more electrons to be pulled away from the cathodes and shot towards the screen. Turning the screen pot counterclockwise decreases the bias voltage of the G2 anode, which causes less electrons to be pulled away from the cathodes. Less electrons emitted into the cathode ray means less light emission from the CRT.
- Pot for G1 bias voltage: Most CRTs do not have this mechanism, and instead attach G1 to ground. This is typically only found in higher-end CRTs and higher resolution CRTs. CRTs that do have this functionality send an amplified horizontal sync signal to the G1 anode. The G1 anode is a metal tube that surrounds the cathodes and has a small hole at the exiting end of the G1 anode that the cathode ray must pass through, and the G1 anode has a voltage that is negative relative to the cathodes. So the lower the bias voltage for G1, the more electrons emitted from the cathodes into the cathode ray are repelled from the G1 anode and squeezed into a thinner and thinner cathode ray, which is being sucked through the G1 anode's exit hole by the positively charged G2 anode. Since the horizontal sync signal has its lowest voltage during horizontal sync pulse, the cathode ray has the least emission of electrons through the exit hole of the G1 anode during horizontal sync, which lasts for the duration of the horizontal retrace time period (when the cathode ray is being reset to the start of the next line). This thread describes a mod that adds a control for G1 bias voltage.
Calibrating Brightness
While all of these controls allow for changing brightness, they do not have the same result on the image. As explained in this thread, cathode - G1 voltage acts as an electromagnetic iris that the cathode ray must pass through. Increasing cathode - G1 voltage shrinks the diameter of the aperture of the iris, which more tightly packs the electrons into a thinner, more concentrated cathode ray, which creates a smaller illuminated spot size on the screen. However, this also changes the brightness, so G2 - G1 voltage needs to be increased to calibrate brightness. In addition, a higher G2 - G1 voltage also serves to increase the acceleration of the electrons in the cathode ray, which further improves spot size because the electrons have less time to spread out. A much more detailed explanation is in the next spoiler section. This is the core theory of how to calibrate brightness to have proper black levels, as well as the thinnest, most concentrated, sharpest cathode ray possible without modifying the CRT's chassis. In summary, the steps are as follows:
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Repeat the following steps until right before retrace lines become visible:
1. Decrease the brightness setting in the on screen display menus until column 0, the leftmost block that is the darkest shade of the white bar in the 240p Test Suite color bars test pattern becomes so dark that it is the same "true black" as the black background.
2. Increase the flyback screen voltage pot until column 0, the leftmost block that is the darkest shade of the white bar in the 240p Test Suite color bars test pattern becomes visible again and stop increasing right before the background starts to transition from true black to very dark grey.
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Repeat the following steps until right before retrace lines become visible:
1. Decrease the G1 bias voltage until column 0, the leftmost block that is the darkest shade of the white bar in the 240p Test Suite color bars test pattern becomes so dark that it is the same "true black" as the black background.
2. Increase the flyback screen voltage pot until column 0, the leftmost block that is the darkest shade of the white bar in the 240p Test Suite color bars test pattern becomes visible again and stop increasing right before the background starts to transition from true black to very dark grey.
Red, Green, and Blue Cutoffs
Now it is likely that the Red, Green, and Blue color bars in the 240p Test Suite are not aligned. That is, the colors don't transition to black at the same leftmost shade of their respective color bar. One of the color bars should properly transition to black only in the background and the leftmost shade of the color should be barely different than black. For this tutorial, I will assume the color is Blue, but it doesn't matter if your CRT is different. Just substitute your color that properly transitions to black only in the background. Now we need to increase the cutoff value of the other two colors until the leftmost darkest shade of their color bar is barely distinguishable from the black background.
Retrace lines
The higher the cathode - G1 voltage, the blacker the color drawn on the screen. During horizontal retrace, the cathode - G1 voltage must be even higher than the the voltage differential used for the color black, otherwise horizontal retrace lines become visible, as the yoke's electromagnetic field causes the cathode ray to reset to the start of the next line. In the calibration steps above, if you increase cathode bias voltage too high, the chroma signal for the cathode starts to clip and the "blacker than black" cathode - G1 voltage is not possible because the color black is already using the maximum obtainable cathode voltage.
Similarly, if you decrease the G1 bias voltage too far, then the horizontal sync wave form clips and is not able to decrease cathode - G1 voltage so that it is "blacker than black". Repeat after me: horizontal retrace needs a significantly higher cathode - G1 voltage than what is used for the color black. CRTs that allow for controlling G1 bias accomplish this by amplifying the horizontal sync waveform and sending it to the G1 anode. CRTs that attach G1 to ground accomplish "blacker than black" by reserving a voltage range on the chroma signal that is a higher range than the voltage used for black.
