mynm wrote: ↑28 Jul 2021, 13:08
But I don't understand well how interlaces resolution works, is it using 25 half fps to do 50fps at 50hz or 50 half fps at 50hz ?, so are maybe 25fps at 50hz inerlaced like 50fps at 50hz progessibe?
It’s easier to understand interlaced if you think of the fields-vs-frames methodology.
A TV field is a half-vertical-resolution frame (either the even pixel rows or odd pixel rows)
A TV frame is two fields (two images).
You can still send 60 frames per second in fields, and the eye sees all of them temporally 1/60sec apart.
Doing 60fps 60Hz interlaced to a TV is just:
TV Field #1 = even pixel rows of GPU frame #1 (TV frame #1)
TV Field #2 = .odd pixel rows of GPU frame #2 (TV frame #1)
TV Field #3 = even pixel rows of GPU frame #3 (TV frame #2)
TV Field #4 = .odd pixel rows of GPU frame #4 (TV frame #2)
TV Field #5 = even pixel rows of GPU frame #5 (TV frame #3)
TV Field #6 = .odd pixel rows of GPU frame #6 (TV frame #3)
TV Field #7 = even pixel rows of GPU frame #7 (TV frame #4)
TV Field #8 = .odd pixel rows of GPU frame #8 (TV frame #4)
Etc.
So resist your mental terminology confusion by pretending “field” is a “frame” and ignore “TV frame” and you will understand your eyes are still seeing 60 frames per second despite the TV literature saying “60 Hz interlaced, 30 frames per second”
Re-interpret the terminology confusion (TV fields, TV frames, and GPU frames)
Then the 60 TV refresh cycles is much easier to understand as:
TV refresh cycle #1 = GPU frame #1 (just only its even pixel rows)
TV refresh cycle #2 = GPU frame #2 (just only its odd pixel rows)
TV refresh cycle #3 = GPU frame #3 (just only its even pixel rows)
TV refresh cycle #4 = GPU frame #4 (just only its odd pixel rows)
TV refresh cycle #5 = GPU frame #5 (just only its even pixel rows)
TV refresh cycle #6 = GPU frame #6 (just only its odd pixel rows)
TV refresh cycle #7 = GPU frame #7 (just only its even pixel rows)
TV refresh cycle #8 = GPU frame #8 (just only its odd pixel rows)
Interlacing is metaphorically “temporal antialasing” only in the vertical dimension to turn 240p into 480i. Essentially an analog yesteryear equivalent of temporal antialiasing to increase vertical resolution without using extra bandwidth.
Many retro consoles disabled this behavior by doing 240p instead of 480i by displaying the pixel rows (scan lines) on top of each other instead of between each other next refresh cycle, via minor changes to the analog signal timing. An even number of scanlines per pair of refresh cycles (e.g. 524 scanlines) created 240p in analog CRT circuits. An odd number of scanlines per pair of refresh cycle (e.g. 525 scan lines) created 480i in analog CRT circuits. The CRT didn’t know what resolution it was, it was simply clever analog signal timing sheninigians to allow the same TV to do 240p or 480i (262p or 525i including VBI), to trick the defacto vertical temporal antialiasing of doubling vertical resolution.
Notice To CRT oldtimers from the 1970s: I humbly apologize for bastardizing my terminology in an attempt to explain to modern 2020s LCD users how interlacing works… We never invented the phrase “vertical temporal antialiasing” to refer to how CRTs turned 240 to 480, but it’s essentially what it is,
in modern terminology.