TVs: how do they work?

 My HD Smart TV started displaying static. Like, old-fashioned, analogue-TV's-not-tuned-in static.

And I don't know where it came from or why. I've got a computer connected to a sound system which is then connected to the TV, all via HDMI, and the sound kept playing - so the connection between the PC and the sound system was fine. And I switched the source on the sound system to see if that was the issue, and that worked. So it seems like the connection between the sound system and the TV was also fine.

So how and why was there static on my TV? It must be deliberately created fake static, right? Because there's no analogue signal., This TV can't take any form of analogue input.

And the audio didn't drop out, which usually happens for a moment whenever you turn the TV on or off, or unplug/replug the cable, because HDMI does that for whatever reason.

I am bamboozled.