Modern display technology is pretty amazing. It’s come such a long way since its early days of black & white. And since shifting from the Cathode Ray Tube (CRT) TVs of the 90s to flat panel Plasma, LCD and OLED technology, they’ve come even further. But how do they actually draw that image on the screen and make it look like things are moving across the screen?
Obviously, pixels themselves do not move. It’s all an illusion. Still images played back rapidly, and our brain’s persistence of vision takes care of the rest. But you don’t really see exactly what’s going on until it’s filmed at over 380,000 frames per second and slowed down. Which is exactly what Gavin and Dan at the Slow Mo Guys have done.
Seeing how the different technologies work at such a slow speed makes them seem even more incredible. Just the speed at which they draw these images on screen. Especially with decades-old CRT technology.
Most of us who were around then know that such screens would draw each frame line by line. Much of it was interlaced, which is a whole other topic, but for the rest of this post, we’re going to assume a progressive video signal for the sake of simple explanation. That’s what many later generation games consoles and DVD players put out, so it will suffice.
Standard definition NTSC televisions had a pixel resolution of 720×486. That’s 349,920 pixels. At 30 frames per second, that’s a pretty insane 10.5 million pixels per second being pumped out. PAL was 720×576, for 414,720 pixels, but being played back at 25fps, that meant a slightly lower 10.4 million pixels per second. And each pixel is only lit for a brief moment. This is why, even to the naked eye, we’d sometimes notice a flicker in certain things, especially on larger screens.
But it typically happened so fast that it was imperceptible. When you film it at 380,117 frames per second, though, you quickly see just how each frame is drawn. Line by line. Pixel by pixel.
Even with such a slow down, it all happens far too quickly to be able to really see each pixel lighting up individually. It’s just a fast blur going from left to right. What I didn’t realise before is that each pixel wasn’t persistent. I thought they stayed lit the whole time until they needed to change.
This is the opposite to how LCD TVs work. They are persistent and have a solid light behind each pixel. In the past, these were CCFL (cold compact fluorescent) lights. These days, they’re typically LED. This allows for thinner, lighter displays that require less power to run. But they still refresh the image in pretty much the exact same way. One line at a time.
What’s really interesting, though, is when Gavin pulls out a macro lens to get a really close up view of how these TVs create the illusion of colour. Yes, this is an illusion, too. As we know, pixels are made up of red, green and blue elements. These combine at different brightness levels. When viewed up close, it’s obvious. But in TV screens, they’re so small that our eyes can’t distinguish the detail, and they blur into a single combined colour. But that’s not what’s interesting. What’s interesting is that this hasn’t really changed from the days of CRT until now.
They’re flipped so that they go BGR instead of RGB, they’re aligned in a cleaner array instead of interlaced, and they’re “always on”, but the principle is still exactly the same.
It’s gotten to the point where TVs and monitors have such high resolution and refresh rates, that one really has to wonder how much better they can get. Of course, as photographers and filmmakers, we know that they can still improve somewhat on colour accuracy and gamut, but for regular consumers, they’re easily as good as many will ever need.
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