5 ms·

This overlooks a key detail, namely interlacing. The C64 wasn't producing 60 frames per second, but rather 60 fields: sets of odd or even scanlines. So on the C

This overlooks a key detail, namely interlacing. The C64 wasn't producing 60 frames per second, but rather 60 fields: sets of odd or even scanlines. So on the CRT TVs of the time, you'd get one colour on the odd scanlines, and another on the even scanlines. Scanline striping, essentially.

mthoms 10y ago
Great point. I wonder how much better the modern version of this effect (as seen in the article) would be if they implemented this.
Lerc 10y ago
I was under the impression that there must have been some additional signaling to make interlacing happen. If for no other reason than the display would need to know which frame was the upper scanline and which was the lower. Apart from the fact that they would have advertised the increased resolution (albeit flickery). On the Amiga, which did do interlaced modes. you could tell on a screen showing a single colour whether it was interlaced or not.
- mikejmoffitt 10y ago
  The difference in signaling is that in an interlaced mode, the final scanline is half-length - Vsync begins halfway through the line, causing the next frame to be offset by half a line vertically. The monitor doesn't "know", in that case. The interlacing is simply a physical phenomenon. Later digital capture devices must recognize this situation and handle it appropriately. Many assume it's always happening, as progressive-scan 252-field NTSC is out of spec anyway.
- phire 10y ago
  It's not an additional signaling. TVs were designed in the 30's, where electronics were extremely primitive and expensive. You wanted the consumer device to be as simple as possible so it could be within the consumer price range. So TVs were little more than a Radio receiver hooked up to a cathode ray tube (CRT). To drive a CRT you need 3 signals: X position, Y position and brightness. The dumbest possible design is to have 3 radio receivers and transmit all 3 signals over the air. But the extra receivers are expensive and besides the X and Y signals are very repetitive, which would be a waste of bandwidth. So two flyback transformers were added to the design of the TV, which generate a saw wave pattern. Starting at 0% they would consistently increase power until 100% before rapidly snapping back to 0. One would run at the vertical refresh rate to drive the CRT's Y signal and the other would run at the horizontal refresh rate to drive the CRT's X signal. The Brightness would come from the radio receiver. With this design, you just need a way of synchronizing the TV studio's cameras and all the TVs in the area to the same horizontal and vertical refresh rates. You might think: Easy, we just use the mains power frequency for vertical and then divide it by 525 to get the horizontal frequency. But a 525 frequency divider way too expensive to put in every TV. Instead, they only put one divider in the studio to calculate the horizontal refresh rate and transmit a synchronization pulse embedded into the brightness signal. A simple circuit in the TV detects the synchronization pulse and nudges the flyback transformer to match. A second, longer synchronization pulse is transmitted between every field for the TV to synchronize the vertical flyback transformer. So a basic Black and White TV is just a radio receiver, two flyback transformers and two synchronization detectors hooked up to a CRT. It doesn't know anything about interlacing or even how many lines there should be in every frame. Back then, a TV studio could theoretically start transmitting at 61 Hz or with a few extra lines per frame and every TV would follow along, right up until the point where the horizontal or vertical refresh rates went out of the spec of the shittest flyback transformers in consumer TVs. Interlacing is a brilliant hack that is 100% done at the studio end. All they do is pick a horizontal and vertical refresh rate that don't divide into each other a whole number of times. 60 hz divided by 15.750 kHz is 262.5 lines. This means that when the TV's vertical flyback transformer reverts to zero (putting the CRT's Y position back to zero), every second frame, the X position of the CRT will be halfway along the screen. One thing you might have noticed is that the Y position is constantly incrementing, it doesn't step down by one line worth of y position at the end of each line. This means that the TV signal is actually rotated slightly, with the end of each line having almost the same Y position as the start of the next line. Which means if the field starts halfway through a line, the start of the first full line on that field (and every line after that) will be half a line lower than it was on the previous field. Other interlacing schemes are theoretically possible, just by picking appropriate horizontal and vertical refresh rates. You could have Triple interlacing or quadruple interlacing (though I doubt either would be pleasing to look at). But most early game consoles and computers pick a horizontal and vertical refresh rate which divide into each other with a whole number of lines, resulting in a progressive display.
  - jacobush 10y ago
    Amiga had the A2024 monitor and I always thought is was kind of quad interlace weird interlace for an exotic monitor with long afterglow. However, the thruth is different but also strange. http://bboah.amiga-resistance.info/cgi-bin/showhardware_en.cgi?HARDID=863 http://bboah.amiga-resistance.info/cgi-bin/showhardware_en.c... Apparently, the monitor samples four frames, puts them in 4 frame buffers, then outputs a complete image to the CRT, each frame representing a quadrant on the screen. Must have been very expensive...
to3m 10y ago
If the TV displays N fields per second, and the C64 produces N fields per second, then you'll see one at a time. The only effect of the interlace is to shift alternate frames down by 0.5 scanlines. TV phosphor decay is too rapid for anything else to happen. By the time the next field starts the previous one is already gone. (I took some short-exposure shots with my camera and it looks like the phosphor decays from white to black in less than 1/6th of a frame.) (Well... it's been a long time since I've seen a C64 plugged into a TV, but I don't see how anything else could happen! You definitely don't get an alternating effect from a BBC Micro.)
- dragonwriter 10y ago
  You are accounting for phosphor decay but not persistence of vision.
  - to3m 10y ago
    Try it and see!
  - vidarh 10y ago
    If you use an Amiga, which has interlaced graphics modes, you see very easily that persistence of vision is too short to make those images seem stable unless the colours on each pair of lines are very close to each other. It works quite well for photos etc., but is awful for e.g. text or graphics with sharp lines at high contrast.
mikejmoffitt 10y ago
That isn't quite correct. The C64 outputs a progressive-scan image, which is a hack on top of NTSC or PAL. In this configuration, there are only even or odd fields, which double-strike the same area. Thusly, it is 60 fields per second, and since only one field forms the full frame, it's 60 frames per second as well. There is no scanline striping; that is strictly an artifact of poor NTSC capture devices (like most HDVTs) that naively assume the incoming signal is a 480i image with odd and even fields.
- TazeTSchnitzel 10y ago
  A progressive-scan image emitted as an interlaced image results in, well, an interlaced image. That the scanline pairs are usually identical merely means that it's wasting vertical resolution. And nonetheless a CRT from the time would show these as different scanlines.
  - ghusbands 10y ago
    No, the signal indicates whether a given field is for even or odd scanlines. I used plenty of computers and CRTs around that time and not one incorrectly displayed progressive as interlaced.
    - deleted 10y ago
      [deleted]
    - TazeTSchnitzel 10y ago
      To clarify, I'm talking about CRT TVs, not computer monitors. The latter aren't (usually†) interlaced. †There's some interesting exceptions!
      - ghusbands 10y ago
        I'm talking about televisions and similar. You are still simply wrong. Nearly all of the 8-bit and 16-bit consoles and computers that output to CRT default to non-interlaced, without the TV alternating fields. Look at other responses in this thread to see how interlacing used to be achieved and the flexibility it had.
rasz_pl 10y ago
are you sure about that? <1/2 vertical resolution machines (240p/288p) used a trick to always generate odd frames producing progressive video, thats what gave that distinctive (and imo ugly) black bar every second pixel row

mthoms 10y ago

Great point. I wonder how much better the modern version of this effect (as seen in the article) would be if they implemented this.

Lerc 10y ago

I was under the impression that there must have been some additional signaling to make interlacing happen. If for no other reason than the display would need to know which frame was the upper scanline and which was the lower. Apart from the fact that they would have advertised the increased resolution (albeit flickery). On the Amiga, which did do interlaced modes. you could tell on a screen showing a single colour whether it was interlaced or not.

mikejmoffitt 10y ago
The difference in signaling is that in an interlaced mode, the final scanline is half-length - Vsync begins halfway through the line, causing the next frame to be offset by half a line vertically. The monitor doesn't "know", in that case. The interlacing is simply a physical phenomenon. Later digital capture devices must recognize this situation and handle it appropriately. Many assume it's always happening, as progressive-scan 252-field NTSC is out of spec anyway.
phire 10y ago
It's not an additional signaling. TVs were designed in the 30's, where electronics were extremely primitive and expensive. You wanted the consumer device to be as simple as possible so it could be within the consumer price range. So TVs were little more than a Radio receiver hooked up to a cathode ray tube (CRT). To drive a CRT you need 3 signals: X position, Y position and brightness. The dumbest possible design is to have 3 radio receivers and transmit all 3 signals over the air. But the extra receivers are expensive and besides the X and Y signals are very repetitive, which would be a waste of bandwidth. So two flyback transformers were added to the design of the TV, which generate a saw wave pattern. Starting at 0% they would consistently increase power until 100% before rapidly snapping back to 0. One would run at the vertical refresh rate to drive the CRT's Y signal and the other would run at the horizontal refresh rate to drive the CRT's X signal. The Brightness would come from the radio receiver. With this design, you just need a way of synchronizing the TV studio's cameras and all the TVs in the area to the same horizontal and vertical refresh rates. You might think: Easy, we just use the mains power frequency for vertical and then divide it by 525 to get the horizontal frequency. But a 525 frequency divider way too expensive to put in every TV. Instead, they only put one divider in the studio to calculate the horizontal refresh rate and transmit a synchronization pulse embedded into the brightness signal. A simple circuit in the TV detects the synchronization pulse and nudges the flyback transformer to match. A second, longer synchronization pulse is transmitted between every field for the TV to synchronize the vertical flyback transformer. So a basic Black and White TV is just a radio receiver, two flyback transformers and two synchronization detectors hooked up to a CRT. It doesn't know anything about interlacing or even how many lines there should be in every frame. Back then, a TV studio could theoretically start transmitting at 61 Hz or with a few extra lines per frame and every TV would follow along, right up until the point where the horizontal or vertical refresh rates went out of the spec of the shittest flyback transformers in consumer TVs. Interlacing is a brilliant hack that is 100% done at the studio end. All they do is pick a horizontal and vertical refresh rate that don't divide into each other a whole number of times. 60 hz divided by 15.750 kHz is 262.5 lines. This means that when the TV's vertical flyback transformer reverts to zero (putting the CRT's Y position back to zero), every second frame, the X position of the CRT will be halfway along the screen. One thing you might have noticed is that the Y position is constantly incrementing, it doesn't step down by one line worth of y position at the end of each line. This means that the TV signal is actually rotated slightly, with the end of each line having almost the same Y position as the start of the next line. Which means if the field starts halfway through a line, the start of the first full line on that field (and every line after that) will be half a line lower than it was on the previous field. Other interlacing schemes are theoretically possible, just by picking appropriate horizontal and vertical refresh rates. You could have Triple interlacing or quadruple interlacing (though I doubt either would be pleasing to look at). But most early game consoles and computers pick a horizontal and vertical refresh rate which divide into each other with a whole number of lines, resulting in a progressive display.
- jacobush 10y ago
  Amiga had the A2024 monitor and I always thought is was kind of quad interlace weird interlace for an exotic monitor with long afterglow. However, the thruth is different but also strange. http://bboah.amiga-resistance.info/cgi-bin/showhardware_en.cgi?HARDID=863 http://bboah.amiga-resistance.info/cgi-bin/showhardware_en.c... Apparently, the monitor samples four frames, puts them in 4 frame buffers, then outputs a complete image to the CRT, each frame representing a quadrant on the screen. Must have been very expensive...

to3m 10y ago

If the TV displays N fields per second, and the C64 produces N fields per second, then you'll see one at a time. The only effect of the interlace is to shift alternate frames down by 0.5 scanlines. TV phosphor decay is too rapid for anything else to happen. By the time the next field starts the previous one is already gone. (I took some short-exposure shots with my camera and it looks like the phosphor decays from white to black in less than 1/6th of a frame.) (Well... it's been a long time since I've seen a C64 plugged into a TV, but I don't see how anything else could happen! You definitely don't get an alternating effect from a BBC Micro.)

dragonwriter 10y ago
You are accounting for phosphor decay but not persistence of vision.
- to3m 10y ago
  Try it and see!
- vidarh 10y ago
  If you use an Amiga, which has interlaced graphics modes, you see very easily that persistence of vision is too short to make those images seem stable unless the colours on each pair of lines are very close to each other. It works quite well for photos etc., but is awful for e.g. text or graphics with sharp lines at high contrast.

mikejmoffitt 10y ago

That isn't quite correct. The C64 outputs a progressive-scan image, which is a hack on top of NTSC or PAL. In this configuration, there are only even or odd fields, which double-strike the same area. Thusly, it is 60 fields per second, and since only one field forms the full frame, it's 60 frames per second as well. There is no scanline striping; that is strictly an artifact of poor NTSC capture devices (like most HDVTs) that naively assume the incoming signal is a 480i image with odd and even fields.

TazeTSchnitzel 10y ago
A progressive-scan image emitted as an interlaced image results in, well, an interlaced image. That the scanline pairs are usually identical merely means that it's wasting vertical resolution. And nonetheless a CRT from the time would show these as different scanlines.
- ghusbands 10y ago
  No, the signal indicates whether a given field is for even or odd scanlines. I used plenty of computers and CRTs around that time and not one incorrectly displayed progressive as interlaced.
  - deleted 10y ago
    [deleted]
  - TazeTSchnitzel 10y ago
    To clarify, I'm talking about CRT TVs, not computer monitors. The latter aren't (usually†) interlaced. †There's some interesting exceptions!
    - ghusbands 10y ago
      I'm talking about televisions and similar. You are still simply wrong. Nearly all of the 8-bit and 16-bit consoles and computers that output to CRT default to non-interlaced, without the TV alternating fields. Look at other responses in this thread to see how interlacing used to be achieved and the flexibility it had.

rasz_pl 10y ago

are you sure about that? <1/2 vertical resolution machines (240p/288p) used a trick to always generate odd frames producing progressive video, thats what gave that distinctive (and imo ugly) black bar every second pixel row