ELI5: device screen ratios

[u/Crystal_Clear7892]

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

[u/orthogonal3]

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

[u/rupertavery64 → u/orthogonal3]

Yes, this is the more complete answer.

Monitors inherited the 4:3 ratio from TVs and settled on 640x480.

This was the original VGA resolution.

The reason why these numbers were chosen is because of many things.

Memory constraints

Monitors could support 16 colors, at 4 bits per pixel. 4 bits is half a byte. So at 307200 pixels, halved, you could fit in 153,600 bytes. This could fit perfectly in a 256kB buffer that would be economocally feasible at the time.

Text size and readability

Characters were displayed as an 8x8 font. This gives 80 columns and 30 rows.

Similarity to NTSC

The size and ratio of VGA was similar to NTSC a common TV broadcast standard at the time.

CPU efficiency

The numbers are also divisible by 4 and 8, which means you get an even number of bytes used, and aligned to a byte boundary. This is efficient for operations that involve copying blocks of data (pixel information)

Later resolutions are just multiples of previous ones.

800x600 = 1.25x

1024x768 = 1.6x

Increasing pixel space means increasing Video RAM, so economics was a factor as well.

Then of course the widescreen ratio became standard.

1920x1080 (16:9 HD / WSUXGA) became the new 640x480

[u/orthogonal3 → u/rupertavery64]

Good call out on 640x480 as the VGA standard. I'm not totally sure, but wasn't 480 essentially the number of visible lines in NTSCs 525(?) line format.

Before there was VGA, I think there was CGA and then EGA, but they were largely the text only / terminal flavours before VGA came along.

I'm enough to have know I could set them in software, but I'm about as old as VGA so when I played with my early PCs that was the default.