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After reading around on the web, including the authoritative site Scanlines Demystified I still am having trouble understanding the difference in signal between 240p and 480i. After reading the wiki on NTSC it appears as though a tv will flicker 60 times per second, alternating between odd fields and even fields. 480i matches that standard, providing 240 lines of data in one draw and then another 240 for the next draw.

My understanding is then that 240p is still a 480i signal, but it only sends half (one field) of data. So one field is full of color, and the next one is black. This results in the recognizable scanlines. Apparently the Framemeister has difficulty with changing between 240p to 480i. Is this just because it has different processing techniques when it detects a 480i signal or a 240p signal?

  • Hi. There are votes to close your questions as off-topic. You might want to edit the question to put it in a more Retrocomputing-specific context. – Nick Westgate Sep 20 '18 at 5:47
  • Just removed gaming references to make it basic to a discussion of video standards. If 240p vs 480i distinction is not a relevant part of retrocomputing - so be it, I just figured this might be the right community to ask considering there is no "retrogaming" SE sub-community. – BobtheMagicMoose Sep 20 '18 at 14:03
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240p means a field is sent out 60 times a second. The scanline "gap" between lines is just a consequence of how the CRT beam sweeps across the display.

It's colored black because the beam isn't illuminating the phosphors on the tube so they remain dark, though as you turn up the brightness the scanned area above and below will bleed into that area and make the dark area smaller.

480i means that two fields are sent out alternating at 30 Hz. The second field is offset in time such that it occupies the gap of the previous field. Due to persistence of vision we see this as a 480-line tall frame that updates at 60 Hz, though technically only the even or odd lines are updating at every 60 Hz interval.

This is why if you play a game running at 480i and pan the camera around you will see every other line lag slightly, because one field shows the even lines with the camera at one position, and the other field shows the odd lines with the camera at a slightly different position.

An uncommon variant is to output a 480i display but send the same field data on both frames. The resolution is effectively 240 lines, but it looks brighter as the second field is occupying the gaps between the lines of the first field. Some consoles can do this.

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    "though as you turn up the brightness the scanned area above and below will bleed into that area and make the dark area smaller." - With real CRT (of that time) you won't even need to do so, as they mix up anyway. It's just modern LCD plus missinterpreting deinterlacers that add dark lines. After all, many of these problems are not so much originated in original hardware setups but rather modern technology working different. – Raffzahn Sep 19 '18 at 16:40
  • Oh I meant on my CRT when I max out brightness/contrast the gap between scanlines is quite smaller as the lines bloom/bleed into the empty space. I guess I was trying to imply that there is some leakage into the empty area so it isn't really empty. :) – raisin-wrangler Sep 19 '18 at 17:07
  • Understood. Still, it depends a lot on your CRT. There are uge differences between 1980s and 190s CRT - as teh later where of much higher quality, often already digital internal – Raffzahn Sep 19 '18 at 17:15
  • "240p means a field is sent out 60 times a second. The scanline "gap" between lines is just a consequence of how the CRT beam sweeps across the display." - 1 question: Wouldn't this cause sequential frames to be offset by 1 line because the first frame would be rendered on the even lines while the next frame would be rendered on odd lines? Would game engines account for that? Do upscalers just double odd frames down and even frames up? – BobtheMagicMoose Sep 19 '18 at 19:18
  • The offset that makes a frame occupy the even or odd lines is controllable on a frame-by frame basis by the video hardware, so for 480i it adds that offset to every other frame, and for 240p there is no offset added. That way even and odd frames in 240p occupy the same physical lines on the screen consistently, which is why the scanline gap also remains consistent across frames. – raisin-wrangler Sep 19 '18 at 19:43
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I guess this builds on what others have said, but here goes anyway:

  • there are two independent circuits controlling beam position, one controlling its horizontal position and one its vertical;
  • they are genuinely independent, neither influences the other;
  • the CRT will lengthen or shorten the period each one takes to sweep the entire display to match the pattern of incoming syncs (up to a point, but if your syncs are within that screen's tolerance of the specification, don't worry about it).

Suppose (0, 0) is the top left of the display, and (1, 1) is the bottom right. In that case the pattern the beam follows on the surface of the screen during a frame might be:

  • the diagonal from (0, 0) to (1, just less than 1/525); then
  • the diagonal from (0, 1/525) to (1, just less than 2/525); then
  • the diagonal from (0, 2/525) to (1, just less than 3/525); etc.

If you want progressive scan, your job is easy — make sure that vertical retrace completes at the same time as horizontal retrace. So each new frame starts at (0, 0).

If you want a two-field interlaced scan, you can ensure your vertical retrace ends when horizontal deflection is halfway across the screen. So your next frame will include:

  • the diagonal from (0.5, 0) to (1, just less than 0.5/525);
  • the diagonal from (0, 0.5/525) to (1, just less than 1.5/525);
  • the diagonal from (0, 1.5/525) to (1, just less than 2.5/525); etc.

Alternate between those two and you've got interlaced video. The two sets of diagonals have the same slope as each other, but one is 0.5 lines above the other.

Two misconceptions tend to arise:

  1. that interlaced video at 60Hz is just a way of describing 30Hz motion in a different order; and
  2. that there must be gaps between the lines in a single field.

On a real, analogue video camera, the brightness captured at location (x, y) is the brightness at that location at the exact moment the raster passed it. So each field is in total a representation of its subset of the image 1/60th of a second later than the previous. Each line is a representation of its subset of the image 1/15750th of a second later than the previous. Etc. It's not an instantaneous snapshot that is serialised, it's a continuous capture.

There's then no actual benefit whatsoever to putting gaps between the lines. There's no original static 535-line picture that the TV is seeking to reproduce. The primary concerns are (i) make the raster too fat and consecutive lines will overlap, causing blur; but (ii) make it too thin and you'll reduce screen brightness, which was a significant problem, especially so after colour introduced a shadow mask.

So you'd normally aim for the raster to be almost large enough for scanlines that are consecutive in time to meet, but not so large that your engineering tolerances (on beam focus and guidance) meant the two might often overlap.

As per Raffzahn's comments to another answer, the idea of thick blank in-between lines is fairly modern and primarily a fiction created by trying to rationalise analogue behaviour into a digital sampling.

  • That's a good point about continuous capture. It's important in making light guns work. – snips-n-snails Sep 19 '18 at 22:03
  • Thank you so much! I think the crucial part that I wasn't thinking about was how the oscillator for the y axis and the oscillator for the x axis are independent and that you just have the cycle offset on the second field. I think I understand now... a way that makes sense for me to think about is where 0,0 is the top left, 1,1 is bottom right (as you described), but to think of the 'frame' as a space going from 0,0 to 2,1. 0,0->1,1 is generally the first field whereas 0,1 -> 2,1 is the second. The vertical oscillator is tuned to go to 0 after hitting ~1.001 and the horizontal is at .5 – BobtheMagicMoose Sep 20 '18 at 14:18
  • Thus moving the second field from ~1,0 - 2,1 to intermesh with the first. I would love to mark your answer as 'the answer' when it comes to interlacing, but I feel the other one still is more of an answer to the original question (re: 240p v 480'). Thanks! – BobtheMagicMoose Sep 20 '18 at 14:20
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480i (480 lines interlaced) is when the even numbered rows are drawn on the screen in 1/30 second, then the odd numbered rows in the next 1/30 second, then the even rows again and so on. The even rows are one "field" and the odd rows are another field, and the two fields combine to form a "frame," so it's 60 fields per second or 30 frames per second.

240p (240 lines progressive) moves the even field down 1/2 row and the odd field up 1/2 row so now a frame is only 240 lines but it draws it 60 times per second. You can see scanlines because now nothing is being drawn between those 240 lines like with 480i.

That's NTSC. PAL is 576i at 25 frames or 50 fields per second, and 288p at 50 frames per second.

  • So does the TV "know" that it is receiving 240p vs 480i? – BobtheMagicMoose Sep 19 '18 at 20:02
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    It would need some smarts to determine that the even and odd scan lines coincide on the screen. Analog TVs don't care, but digital TVs might. – snips-n-snails Sep 19 '18 at 20:08
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On a CRT, the electron beam moves from top to bottom in a smooth continuous motion, then jerks quickly back to the top, about 60 times per second. The beam moves left to right in a smooth continuous motion, then jerks quickly back to the left, about 15,750 times/second. The jerk back to the left will occur if the start of a sync pulse is received roughly 60-70 microseconds after the previous horizontal jerk. The jerk back to the top will occur if a sequence of long sync pulses are detected 16-17 milliseconds after the previous vertical jerk. The electronics driving the CRT don't care about concepts like even and odd frames. They simply performs the vertical and horizontal motions, independently, as requested by the sync pulses. When a horizontal sync pulse is received, the vertical position of the beam will depend upon the amount of time since the last vertical sync pulse.

Because the electronics of a CRT don't care about even and odd frames, it would be possible to output 60 fields (one second) of interlaced video that were 262.5 lines long, then 59 fields (just under one second) of non-interlaced video that was either 262 or 263 lines long, and then 60 more frames that were 262.5 lines long, and a monitor would be perfectly happy. The image would bounce vertically by a fraction of a scan line when switching between interlaced and non-interlaced modes, but would be relatively stable. A device trying to digitize the video as interlaced frames, however, would have a problem since it would expect to interpret pairs of fields as frames, but there would only be 29.5 pairs of non-interlaced fields between the last frame of the first interlaced batch and the first frame of the next. If a device could distinguish between "even field first" and "odd field first" frames, that could resolve the issue, but otherwise it would be impossible to accurately describe the video as a series of 30Hz frames.

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480i is a variation of 240p, in the sense that both write almost 16000 lines per second and almost 60 fields per second. The difference is that 240p writes the same field twice, whereas 480i writes every second field in the space between the lines of the first field. Why repeat the same information twice, right ? This gives approximately 1.5 times vertical resolution, not 2 times. You have to balance the benefits against the "line flicker" artefacts. This does not work well with sharp graphics or text, then you may prefer 240p after all.

The display knows that the signal is interlaced from the phase of the vertical sync signal: for one field it coincides with the horizontal sync, for the other field it lies midway between 2 hor sync pulses. Thus both fields are 525/2 lines long. This causes the vertical deflection of the CRT to nicely interleave the 2 fields, without any special effort, even with pre-WW2 technology.

The cost of a CRT display application depends strongly on the (horizontal) line frequency. Thus 480i (interlaced) is somewhat superior to 240p (progressive), for the same cost. Also for the same cost you could have 480p at 30 frames per second, but that is unwatchable because of the flicker. 480p at 60 frames per second is superior to 480i, but it requires a line frequency of almost 32 kHz which runs very hot. This only became feasible somewhere in the 1970's-1980's. In Europe this was used for 100 Hz interlaced display, rather than 50 Hz progressive.

Then came flat matrix displays which are naturally progressive, like 480p. They require an up-conversion, or de-interlacing, and this is where the troubles began. Affordable de-interlacers create visible artefacts, giving 480i a bad reputation. On a CRT it did not look too bad, though. All modern digital video signals are progressive now: 720p, 768p, 900p, 1080p, 1440p, 2160p (UHD).

Source: I've spent more than half my life in a TV lab.

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