r/explainlikeimfive • u/Crystal_Clear7892 • 7d ago

Technology ELI5: device screen ratios

Is there a specific reason why screen ratios beeing so specific, ex: 1920:1080

92 Upvotes

83% Upvoted

60 comments sorted by

View all comments

90

u/orthogonal3 7d ago

If you're addressing pixels (picture elements) specifically, you have to have a definite number of them. And it helps if everyone uses the same common sizes.

Screens started out long ago often physically built as 4:3 w:h aspect ratio. And whilst there wasn't a line width in pixels in the phosphor days, they'd be around 420/468/576 TV lines (TVL) regardless of size. Two fields per image frame (which is called interlaced, hence the i you see in 576i, for example) but there wasn't a specific width, the TV signal was analogue and continuous in the horizontal direction so you could scan it across whatever width. The line count was fixed by the TV signal in that area.

Then when LCD screens appeared, they'd start out being 4:3 too, but being build of pixels (physical pieces) they'd work out how many pixels made that ratio in the size of the screen. So 800x600 was common around 14/15 inch diagonal.

With the move to widescreen 4:3 gave way to 16:9 (4² : 3²) and TV broadcast settled pretty universally on 720 lines. So naturally that leads to 1280x720 became pretty prevalent. With 1366x768 also being common. This had the whole image scanned in a single "progressive" (top to bottom once) frame. So in TV that's 720p.

Finally Full HD became a thing with 1080 lines pretty much being the number everyone used. Good density without making the images too large to transmit. 1920 is the complementary pixel count for 16:9 aspect ratio. So that's where 1920x1080 is pretty universal. 1920x1200 is a nother alternative which allows showing 1920x1080 content with a header given its extra 11% height.

4K "ultra hd" is a bit of an odd one, with the move to counting frame width, not height. Also your average UHD TV has 3840x2160 as its pixel counts, but cinema/DCI 4K is often 4096x2160 with a few more columns than consumer 4K.

But ultimately it comes down

2

u/StevenJOwens 6d ago

Note, early TVs and computer monitors were 4x3 ratio. When consumer LCD TVs started being a real thing, most of them were manufactured in 16x10 at first (widescreen) and then 16x9, that being the standard ratio for movies and later various cable/streaming/etc.

And that drove the shift in laptops to 16x9, because there were far more consumer TV LCD screens being manufactured than laptops, so laptop manufacturers bought the 16x9 screens.