Eli5: What is the purpose of the different colored bars on the tv error screen? Why not just have a black screen that says ‘error’?
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The general concept is called a [Test card](https://en.wikipedia.org/wiki/Test_card), which originated as a pattern broadcast during the night hours in the early eras of TV before 24-7 programming. It was a known pattern that could be used to adjust a cathode-ray tube TV’s calibration. When TV stations went to 24-hour programming, test patterns stuck around as an established convention for “Our signal is still broadcasting but our content is off the air”—just used during technical difficulties instead of nightly.
[The specific pattern of colored bars](https://en.wikipedia.org/wiki/SMPTE_color_bars) was developed for testing the accuracy of color information being broadcast. Engineers know what the test pattern *should* look like, and by measuring the colors on the receiving end, they can figure out what adjustments to make on the broadcast end to ensure a good picture for viewers.
If you’re referring to the color bars, that’s a [SMPTE test pattern](https://en.m.wikipedia.org/wiki/SMPTE_color_bars)
Coloured bars are very useful for adjusting the TV’s colour. If you take a transparent dark blue filter and put it between you and the TV, every other vertical bar will be blue.
Without the filter, you’ll notice a small part of the coloured bars has a few of the colours in reverse order from the rest of the coloured bars. But with the filter, the colours all look blue.
Well, almost. You’ll notice some spots are more blue than others. That’s where you adjust the TV’s colours.
Call up the TV’s colour controls and find the “Tint” and “Color” sliders. Move them around and see how it affects the bars through the blue filter. Your goal is to make the big and small bars appear to be exactly the same brightness.
Once that’s set, your TV’s colour is adjusted properly!
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