Comment by m132

Good post. For anyone wondering "why do we have these particular resolutions, sampling and frame rates, which seem quite random", allow me to expand and add some color to your post (pun intended). Similar to how modern railroad track widths can be traced back to the wheel widths of roman chariots, modern digital video standards still reverberate with echoes from 1930s black-and-white television standards.

BT.601 is from 1982 and was the first widely adopted analog component video standard (sampling analog video into 3 color components (YUV) at 13.5 MHz). Prior to BT.601, the main standard for video was SMPTE 244M created by the Society of Motion Picture and Television Engineers, a composite video standard which sampled analog video at 14.32 MHz. Of course, a higher sampling rate is, all things equal, generally better. The reason for BT.601 being lower (13.5 MHz) was a compromise - equal parts technical and political.

Analog television was created in the 1930s as a black-and-white composite standard and in 1953 color was added by a very clever hack which kept all broadcasts backward compatible with existing B&W TVs. Politicians mandated this because they feared nerfing all the B&W TVs owned by voters. But that hack came with some significant technical compromises which complicated and degraded analog video for over 50 years. The composite and component sampling rates (14.32 MHz and 13.5 MHz) are both based on being 4x a specific existing color carrier sampling rate from analog television. And those two frequencies directly dictated all the odd-seeming horizontal pixel resolutions we find in pre-HD digital video (352, 704, 360, 720 and 768) and even the original PC display resolutions (CGA, VGA, XGA, etc). To be clear, analog television signals were never pixels. Each horizontal scanline was only ever an oscillating electrical voltage from the moment photons struck an analog tube in a TV camera to the home viewer's cathode ray tube (CRT). Early digital video resolutions were simply based on how many samples an analog-to-digital converter would need to fully recreate the original electrical voltage.

For example, 720 is tied to 13.5 Mhz because sampling the active picture area of an analog video scanline at 13.5 MHz generates 1440 samples (double per-Nyquist). Similarly, 768 is tied to 14.32 MHz generating 1536 samples. VGA's horizontal resolution of 640 is simply from adjusting analog video's rectangular aspect ratio to be square (720 * 0.909 = 640). It's kind of fascinating all these modern digital resolutions can be traced back to decisions made in the 1930s based on which affordable analog components were available, which competing commercial interests prevailed (RCA vs Philco) and the political sensitivities present at the time.

While analog video did not have the concept of pixels, it specified the line frequency, the number of visible lines (576 in Europe, composed of 574 full lines and 2 half lines, so some people count them as 575 lines, but the 2 half lines are located in 2 different lines of the image, not on the same line, thus there are 576 distinct lines on the height of the image), the duration of the visible part of a line and the image aspect as being 3:4.

From these 4 values one can compute the video sampling frequency that corresponds to square pixels. For the European TV standard, an image with square pixels would have been of 576 x 768 pixels, obtained at a video sampling frequency close to 15 MHz.

However, in order to allow more TV channels in the available bands, the maximum video frequency was reduced to a lower frequency than required for square pixels (which would have been close to 7.5 MHz in Europe) and then to an even lower maximum video frequency after the transition to PAL/SECAM, i.e. to lower than 5.5 MHz, typically about 5 MHz. (Before the transition to color, Eastern Europe had used sharper black&white signals, with a lower than 6.5 MHz maximum video frequency, typically around 6 MHz. The 5.5/6.5 MHz limits are caused by the location of the audio signal. France had used an even higher-definition B&W system, but that had completely different parameters than the subsequent SECAM, being an 819-line system, while the East-European system differed only in the higher video bandwidth.)

So sampling to a frequency high enough for square pixels would have been pointless as the TV signal had been already reduced to a lower resolution by the earlier analog processing. Thus the 13.5 MHz sampling frequency chosen for digital TV, corresponding to pixels wider than their height, was still high enough to preserve the information contained in the sampled signal.

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.

Yeah. I recently stumbled across this in an interesting way. Went down a rabbit hole. I was recreating an old game for my education[1]. Scummvm supports Eye of the Beholder and I used it to take screenshots to compare against my own work. I was doing the intro scenes and noticed that the title screens are 320x200. My monitor is 1920x1200 and so the ratios are the same. It displays properly when I full screen my game and all it good. However, on scummvm, it looked vertically elongated. I did some digging and found this about old monitors and how they displayed. Scummvm has a setting called "aspect ratio correction" which stretches the pixels vertically produces pillarboxing to give you the "original nostalgic feel".

Notes: 1. https://eye-of-the-gopher.github.io/

Here's some captures from my Comcast system here in Silicon Valley.

https://www.w6rz.net/528x480.ts

https://www.w6rz.net/528x480sp.ts