How does a TV work?

An LED-LCD TV is a sandwich. At the back is a backlight: rows of white LEDs, a white reflector behind them, and a milky diffuser and prism sheets in front that spread the light into one even glow.

In front of the lamp sits the LCD panel. Two polarizer films, two sheets of glass and, between them, a layer of liquid crystal thinner than a hair. The back glass carries millions of tiny transistors, one switch for every dot. The front glass carries a colour filter of red, green and blue stripes.

The liquid crystal works like millions of tiny shutters. It lets more or less of the backlight through each coloured stripe, and together they paint the picture. Behind it all, a main board (a small computer with HDMI and Wi-Fi) and a power board run the show, and small speakers fire down from the bottom.

An OLED TV skips the backlight completely: every dot makes its own light, so the panel can be thinner than a coin.

Look closely at any screen and the picture breaks into a grid of tiny squares called pixels. A 4K TV has 3840 across and 2160 down: 8.29 million pixels. Each one is really three subpixels, a red, a green and a blue, so that's about 25 million little lights or shutters.

Your eye can't see dots that small from the sofa, so their light blends. That's additive colour: red and green light together look yellow, all three at full make white, and none makes black. Three dimmers per pixel, each with 256 or more steps, give over 16 million colours.

How many pixels do you need? A sharp eye can just tell apart two dots 1 arcminute apart (a sixtieth of a degree). On a 55-inch TV the pixels of a 1080p picture blend together beyond about 2.2 m, and 4K pixels beyond about 1.1 m. Sit further back than 2.2 m and 4K can't look any sharper than 1080p, though it may still have better colour and brightness.

Light is a wave, and a polarizer only lets through waves shaking in one direction, like a letter through a letterbox. Put two polarizers at 90° to each other and nothing gets through: the first passes only up-and-down waves, the second only side-to-side ones.

Now fill the gap with liquid crystal. Its rod-shaped molecules line up with fine grooves on each glass. The grooves are at 90°, so the molecules form a gentle twist, like a spiral staircase. The light's shaking follows the twist, turns through 90°, and slips out through the second polarizer. Bright!

Put a voltage across the glass and the molecules stand up in the electric field. The staircase is gone, the light isn't turned, and the front polarizer blocks it. Dark! A voltage in between gives a shade of grey. A transistor on the glass sets that voltage for each subpixel, many times a second.

Most TVs use VA or IPS cells, which work the other way round or turn the molecules sideways, but the idea is the same. And no shutter is perfect: a little light always leaks, so an LCD's black is really a very dark grey.

An LCD's backlight is always on, even behind the black parts of the picture. The shutters leak a little, so in a dark room a night sky looks grey. Better LCDs split the backlight into local dimming zones and turn down the dark ones, but a bright star lights its whole zone and makes a faint halo called blooming.

An OLED has no backlight. Each subpixel is a sandwich of thin organic (carbon-based) layers between two electrodes. Electrons flow in from one side and "holes" from the other. When they meet in the middle layer they give off a flash of light. No current, no light: black is simply off, so the contrast is practically infinite.

The catch: organic materials slowly wear out, faster when driven hard. A logo left on screen for thousands of hours can leave a faint ghost called burn-in. TVs fight it by shifting pixels slightly and refreshing panels while you sleep.

QLED TVs are LCDs with a clever backlight. Blue LEDs shine on a film of quantum dots: crystals only a few nanometres across. A dot soaks up blue light and glows a pure colour set only by its size: about 2.5 nm across glows green, about 5 nm glows red.

A TV never shows real motion. It shows a quick series of still pictures, called frames, and your brain joins them up. Cinema films use 24 frames a second. TV in India and Europe grew up with 25 and 50, and in the Americas and Japan with 30 and 60, matching the mains electricity (50 or 60 Hz). Games and sports now run at 120.

An LCD or OLED holds each frame until the next one arrives. When your eyes follow a moving ball they glide smoothly, but the ball on screen stands still, then jumps. So each frame smears across the back of your eye: sample-and-hold blur. The faster the ball and the fewer the frames, the wider the smear.

Old CRT TVs flashed each frame for an instant, which looked sharper but flickered. Old broadcasts were also interlaced: every 1/50 of a second they sent only the odd lines, then only the even ones, saving half the signal. Modern video is progressive: every line, every frame.

A 4K picture changing 60 times a second, with 10 bits for each of red, green and blue, is 3840 × 2160 × 60 × 30 ≈ 15 billion bits a second (15 Gbit/s). An HDMI 2.1 cable can carry up to 48 Gbit/s over a couple of metres, but no broadcast or home internet comes close.

So the picture is compressed by a codec such as HEVC or AV1, down to about 15–25 Mbit/s: a thousand times smaller. Two tricks do most of it. First, the picture is cut into small blocks, and fine detail your eye barely notices is rounded away. Second, most of a video doesn't change from one frame to the next. Now and then a whole keyframe is sent; in between, only the changes are sent, with motion vectors saying "this block moved a little to the right".

Squeeze too hard and you see the blocks: smeared faces, and blocky skies. Your TV's chip decodes it all again, fast enough to keep up.

The data reaches you by radio from a tower, from a satellite dish (in India, DD Free Dish brings free channels to millions of homes with a one-time dish), by cable, or by internet streaming.