Comment by drmpeg
6 hours ago
> Videos with non-square pixels are pretty rare...
Before HD, almost all video was non-square pixels. DVD is 720x480. SD channels on cable TV systems are 528x480.
6 hours ago
> Videos with non-square pixels are pretty rare...
Before HD, almost all video was non-square pixels. DVD is 720x480. SD channels on cable TV systems are 528x480.
>Before HD, almost all video was non-square pixels
Correct. This came from the ITU-R BT.601 standard, one of the first digital video standards authors of which chose to define digital video as a sampled analog signal. Analog video never had a concept of pixels and operated on lines instead. The rate at which you could sample it could be arbitrary, and affected only the horizontal resolution. The rate chosen by BT.601 was 13.5 MHz, which resulted in a 10/11 pixel aspect ratio for 4:3 NTSC video and 59/54 for 4:3 PAL.
>SD channels on cable TV systems are 528x480
I'm not actually sure about America, but here in Europe most digital cable and satellite SDTV is delivered as 720x576i 4:2:0 MPEG-2 Part 2. There are some outliers that use 544x576i, however.
Here's some captures from my Comcast system here in Silicon Valley.
https://www.w6rz.net/528x480.ts
https://www.w6rz.net/528x480sp.ts
Cool!
Doing my part and sending you some samples of UPC cable from the Czech Republic :)
720x576i 16:9: https://0x0.st/P-QU.ts
720x576i 4:3: https://0x0.st/P-Q0.ts
That one weird 544x576i channel I found: https://0x0.st/P-QG.ts
I also have a few decrypted samples from the Hot Bird 13E, public DVB-T and T2 transmitters and Vectra DVB-C from Poland, but for that I'd have to dig through my backups.
My DVCAM equipment definitely outputs 720x576i, although whether that's supposed to render to 768x576, or 1024x576 for 16:9 stuff.
It still looks surprisingly good, considering.
DVD also supports 352x480. These pixels are very non square.
Why would you want this? VHS. NTSC has 480-ish visible scanlines, but VHS only has bandwidth for 350 pixels.
Displaying content from a DVD on a panel with square pixels (LCD, plasma, etc.) required stretching or omitting some pixels. For widescreen content you'd need to stretch that 720x480 to 848x480, and for 4:3 content you'd need to stretch it to 720x540, or shrink it to 640x480, depending on the resolution of the panel.
CRTs of course had no fixed horizontal resolution.
Edit: I just realized I forgot about PAL DVDs which were 720x576. But the same principle applies.
Even with modern digital codecs and streaming, there's usually chroma subsampling[1], so the color channels may have non-square "pixels" even if overall pixels are nominally square. I most often see 4:2:0 subsampling, which still has square pixels, but at half resolution in each dimension. However 4:2:2 is also fairly common, and it has half resolution in only one dimension, so the pixels are 2:1. You'd have trouble getting a video decoding library to mess this up though.
[1]: https://en.wikipedia.org/wiki/Chroma_subsampling
Just look at Japanese television… most channels get broadcast at 1440x1080i for 16:9 content instead the full 1920x1080i (to save bandwidth for other things, I assume), so it's still very common with HD too.
It may also be due to legacy reasons. Japan was a pioneer in adopting HD TV years before the rest of the world, but early HD cameras and video formats like HDCAM and HDV only recorded 1080i at 1440x1080. If their whole video processing chain is set up for 1440x1080, they’d likely have to replace a lot of equipment to switch over to full 1920x1080i.
I'm confused... what does DVD, SD or any arbitrary frame size have to do with the shape of pixels themselves? Is that not only relevant to the display itself and not the file format/container/codec?
My understanding is that televisions would mostly have square/rectangular pixels, while computer monitors often had circular pixels.
Or are you perhaps referring to pixel aspect ratios instead?
I'm not 100% sure I understand your question, but in order to display a DVD correctly, you need to either display the pixels stored in the video stream wider than they are tall (for widescreen), or narrower than they are tall (for 4:3). Displaying those pixels 1:1 on a display with square pixels would never be correct for DVD video.
It comes about from digitizing analog video signals. The early standards for sampling (digitizing) analog video signals resulted in the digital pixel horizontal sample size (often) being wider than the line spacing of the displayed analog video. With the result that digitized video of analog signals usually has a "pixel size" (analog video has no concept of discrete horizontal pixels) that is wider than it is tall.
CRTs didn't have pixels at all. They had shadow masks (or aperture grilles) and phosphors, which could be a triad of rectangles, lines spanning basically the entire screen height, or dots. They did not line up with the signal, so it doesn't make sense to call them pixels.
A square pixel has a 1:1 aspect ratio (width is the same as the height). Any other rectangular pixel with widths different than their heights would be considered "non-square".
F.ex. in case of a "4:3 720x480" frame… a quick test: 720/4=180 and 480/3=160… 180 vs. 160… different results… which means the pixels for this frame are not square, just rectangular. Alternatively 720/480 vs. 4/3 works too, of course.
Again I think you're talking about pixel aspect ratios instead, and not physically non-square pixels, which would be display-dependent. OP only said "square pixels" but then only talked about aspect ratios, hence my confusion.
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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.
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