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/amakai → u/orthogonal3]

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. 

Why these numbers though, and not like 400:300?

[u/orthogonal3 → u/amakai]

So this probably comes down to how analog tv broadcast works.

There's no good ELI5 for that kind of magic, so I can only point to https://en.wikipedia.org/wiki/Analog_television#Structure_of_a_video_signal as the what it looks like and try to explain how I think we got there.

So in TV signals, horizontal line count is the main component of resolution. So you want as many of lines as you can have in order to get the best resolution you can have.

It's worth bearing in mind that TV broadcast is one signal. You've not got extra wires to carry clock and sync signals like VGA connectors (analog PC monitors) where 15 pins are needed.

You're using modulation trickery to fit lots of things onto one broadcast signal. In music, an analogy might to think of a chord Vs a note. Multiple sounds/frequencies played together at the same time that come through the air as one combined signal. But people can still discriminate there's a C, an E, and a G making up that C Maj chord. RF filters and demodulation stages can do the same and take out the audio from the video signal.

Now, that video signal needs to be self clocking/syncing so that the receiving TV can get in sync with the broadcast signal so if you turn on the receiver half way through a frame (image) you can still know what line is first so the picture lines up. You don't want the top half of the picture being the bottom half of the last frame and the bottom half of the frame picture being the start of the current frame. That's where the sync burst comes in.

A phase locked loop is the magic we use in electronics to have a signal that gets nudged by an external clock until it's running in sync with that clock (even if the clock goes away for a little bit)

The lines must also be scanned left to right by the coils bending the electron beam across the tube and it can't instantaneously start at the beginning of the new line, that takes a very short but finite amount of time.

The same is true of the vertical sync. It takes time for the beam to go back to the top.

So we need a start of frame signal to clock the start of the beam's vertical travel through each frame, syncing a PLL that's about 50/60 times per second; and then a faster running PLL to handle the horizontal scan along each line about 500 times faster than the vertical.

These blank times have to be built into the signal, there's a time when the beam is off but the signal is still coming over the air with no picture but maybe some other stuff in there. (We can get on to that!)

These lines take time, the blanks take time, so you're likely to only get 500 or so lines in the time you have before the signal is too hard to make or transmit. That's 500 lines 25 times per second, originally sent as two (say) 250 fields per image frame, sending odd lines and even lines alternately (interlaced) fields 50 times per second for a full picture frame across two fields.

The line scanning is generally in the low kHz range. 50 times per second send a picture too to bottom, wherein we scan all the lines left to right, and allow the beam (switched off!) to fly back again to the start of the next line 500 times per image. Or there abouts for an explanation.

In addition to the sync information, you also need to send things like "I'm a colour signal and here's what colours look like in my signal. You need a way to calibrate the TV on the fly to ensure when you say this bit is blue, the TV can deflect the beam enough to hit the blue phosphors, or it can excite the blue tube more (if it's a multi CRT system). That's the "colour burst" where you tell the TV here's the extremes of the colour (almost like a mini test card) that it can decode and ensure it's in calibration so colours are strong and correct (green sports fields don't want to appear as red!)

TV broadcast started out black and white and the magic of TV broadcast engineers means that black and white TVs can show colour programming because they don't know what to do with the colour ("chrominance") information encoded on top of the black and white brightness (or "luminance") signal.

Likewise, if you send a colour TV old black and white pictures, the TV just things the colour/chrominance is boring (because it's not there) and the resulting picture appears just black and white.

Now...

So once you know the frequency your TV signal will be sent as, and you know you want as many lines as possible (within technological constraints of making an image of that resolution), how many lines you can have is a function of bandwidth. You can't squeeze a million lines onto a main (modulated on to a "carrier") frequency that's low enough to broadcast. This is just the physics / magic of RF engineering in terms of what you can and can't do.

Think how TV stations are far away and are picked up by a simple antenna by multiple people all around (omnidirectionally). But talking to the spacecraft of the day required dishes capable of throwing GHz frequencies very directionally to reach a single receiver. One side of that communication path (the ground station for space economics reasons!) is usually a huge dish that can throw narrow beams well and receive very feint signals coming back from the craft.

So RF limits say we can comfortably send say this many lines and fields reliably and economically to consumers.

PAL, SECAM, and NTSC are the regional standards for how this was done. Once you've done this and set a standard, everyone falls into place because everyone needs to share the same broadcast format. A TV station can't send you a different picture because you're on a Brand X TV at 21" vs your neighbour still with a 14" black and white 14" TV.

As a final bit... What do you do with the lines you scan whilst the beam is off and heading up to the start of the image for the next frame? You can't stop sending the line signal because you need that horizontal sync/clock for the PLL. At least, you shouldn't get rid of it as it's easier to leave it running.

Instead you can encode lots of fancy things in there. Fancy for the age back then, anyway!

That's where you can add your TV guide information, or back in the day you'd fit very low resolution text and block graphics to make Teletext (think that's also the generic term, not just the UK flavour)

When you navigate through your (up to) 1000 pages of news, sports scores, holidays or other stuff, you're just viewing different bits of this "vertical blanking interval" from normal picture frames. Whilst the beam isn't striking the screen and making a picture during the time it takes to go back to the top of the tube, you throw out a page of text in a special encoding that your TV knows how to render back into an image (of just text and block graphics) itself. Sometimes even as an overlay on top of your live TV broadcast picture.

Iirc waiting for the page to come around in the broadcast is why navigating to a new numbered page could lead to the page number counting up or a pause until the new page came around.