Comment by kmeisthax
4 hours ago
CRTs do not have pixels. At all. The shapes you might see on the screen if you look really closely at it are solely different bands of color phosphors. CRTs are capable of drawing arbitrary beam shapes on them in one color[0]; but you need regularly spaced filters and phosphor patterns in order to get multiple colors. If these """pixels""" were bigger, you'd see a perfectly normal red part of the image, next to a perfectly normal green part of the image, next to a perfectly normal blue part of the image.
What a CRT actually draws, though, are lines. Analog television is a machine that chops up a 2D plane into a stack of lines, which are captured, broadcasted, and drawn to the screen with varying intensity. Digital television - and, for that matter, any sort of computer display - absolutely does need that line to be divided into timesteps, which become our pixels. But when that gets displayed back on a CRT, the "pixels" stop mattering.
In the domain of analog television, the only property of the video that's actually structural to the signal are the vertical and horizontal blanking frequencies - how many frames and lines are sent per second. The display's shape is implicit[1], you just have to send 480 lines, and then those lines get stretched to fit the width[2] of the screen. A digital signal being converted to analog can be anything horizontally. A 400x480 and a 720x480 picture will both be 4:3 when you display it on a 4:3 CRT.
Pixel aspect ratio (PAR) is how the digital world accounts for the gap between pixels and lines. The more pixels you send per line, the thinner the pixels get. If you send exactly as many horizontal pixels as the line count times the display's aspect ratio, you get square pixels. For a 4:[3] monitor, that's 640 pixels, or 640x480. Note that that's neither the DVD nor the SD cable standard - so both had non-square pixels.
Note that there is a limit to how many dots you can send. But this is a maximum - a limitation of the quality of the analog electronics and the amount of bandwidth available to the system. DVD and SD cable are different sizes from each other, but they both will display just fine even on an incredibly low-TVL[4] blurry mess of a 60s CRT.
[0] There were some specialty tubes that could do "penetrative color", i.e. increasing the amplitude of the electron gun beyond a certain voltage value would change to a different color. This did not catch on.
[1] As well as how many lines get discarded during vertical blanking, how big the overscan is, etc.
[2] Nothing physical would stop you from making a CRT that scans the other way, but AFAIK no such thing exists. Even arcade cabinets with portrait (tate) monitors were still scanning by the long side of the display.
[3] There's a standard for analog video transmission from 16:9 security cameras that have 1:1 pixel aspect ratio - i.e. more pixels per line. It's called 960H, because it sends... 960 horizontal pixels per line.
https://videos.cctvcamerapros.com/surveillance-systems/what-...
[4] Television lines - i.e. how many horizontal lines can the CRT display correctly? Yes, this terminology is VERY CONFUSING and I don't like it. Also, it's measured differently from horizontal pixels.
Yes I know all of that, but I think it still doesn't answer my question.
I know you are arguing semantics and I hoped people would see past the "pixels aren't pixels" debate and focus on what I was actually asking, which is how physical dot/pixel/phosphor/mask/whatever patterns have anything to do with frame sizes of a digital video format, and I still assert that they don't, inherently... short of some other explanation I am not aware of.
All I was trying to say was that I thought OP was conflating physical "pixel" geometry with aspect ratios. Perhaps my question was too simple and people were taking it to mean more than I did, or thought I was misunderstanding something else.
I think how one interprets the usage of “square pixels” here is highly dependent on what that person is most familiar with. When I saw the title, even without reading the article, I immediately knew it’s talking about PAR, since I’m already familiar with video encoding standards. It didn’t even occur to me that it could be talking about the shape of physical elements on various display technologies.
> which is how physical dot/pixel/phosphor/mask/whatever patterns have anything to do with frame sizes of a digital video format,
In that context, the answer is: They don't really have any relationship at all.
Plenty of TVs of the past (from any continent) also had no physical dots/pixels/patterns at all. These were monochrome, aka black and white. :)
They had an inflexible number of lines that could be displayed per field, but there was no inherent delineation within each line as to what a pixel meant. It was just a very analog electron beam that energized a consistently-coated phosphorescent screen with a continuously-variable intensity as it scanned across for each raster line.
Pixels didn't really happen until digital sources also happened. A video game system (like an Atari or NES, say) definitely seeks to deliver pixels at its video output, and so does a digital format like DVD.
But the TV doesn't know the difference. It doesn't know that it's displaying something that represents pixels instead of a closed-circuit feed from a completely-analog pixel-free tube-based camera.
The "non-square pixel" part is just a practical description: When we have a digital framebuffer (as we do with a DVD player), that framebuffer has defined horizontal and vertical boundaries -- and a grid of pixels within those boundaries -- because that's just how digital things be.
When we smoosh the pixels of a DVD player's 720x480 framebuffer into an analog display with an aspect ratio of 4x3, we wind up with a combined system, with pixels, and those pixels aren't square.
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And that's perfectly OK, though it does lead to weirdness.
For example: To produce a display of a perfect square that is 100 lines high using a 4x3 NTSC DVD is actually impossible. It'd have to be 100 pixels high and 112.5 pixels wide, which we can't accomplish since the format doesn't have that kind of precision. It's impossible with an LCD, and it's impossible with an analog set from 1955 just the same.
It's a DVD limitation, not a display limit. There's no great trick to producing a perfect square with analog gear, where pixels aren't a thing -- but we can't get there with the DVD's non-square pixels.
That weirdness doesn't have anything at all to do with how the display is constructed, though: Again, the TV has no concept of what a pixel even is (and a shadow mask doesn't necessarily exist at all).