How does a 50 Hertz image get displayed?

Hi, everyone. I'm trying to visualize this. Let's say that I have a 320x256 screen. There are 50 "frames" per second, and a vertical blank interrupt for each one.

Only, a normal TV set doesn't display 50 frames per second. It displays 50 fields per second, where each field is half of a frame.

So did the Amiga display the entire 256 lines 50 times per second? If that were the case, then there's a problem. A PAL TV has 576 distinct lines, and expects 288 of them every 50th of a second. But every second field is moved down half a line - producing a kind of wobbliness.

So what's the deal?

Richard

[Toni Wilen]

If mode is not interlaced, Amiga creates odd or even fields only (depending on LOF bit state in VPOSW). I guess today it would be named "312p" screen mode.

Most (all?) old 1980s home computers used same trick to create stable non-interlaced mode.

Technical explanation: timing of sync lines during first few scanlines after vblank tell the TV if field is odd or even.

[Retro-Nerd]

Also known as LDTV, non-standard PAL.

http://en.wikipedia.org/wiki/Low-definition_television

[Amiga1992]

I suppose this is why my shitty capture card doesn't take the input of the Amiga, as it probably expects an interlaced signal and not progressive.

You haven't answered my question. If all the lines get displayed every 50th of a second, then every second set of lines must be displayed half a line below the first, right?

[robinsonb5]

Originally Posted by Richardcavell

You haven't answered my question. If all the lines get displayed every 50th of a second, then every second set of lines must be displayed half a line below the first, right?

No, as Toni says, the pattern of sync pulses at the beginning of each field tell the receiving TV set or monitor whether this field is odd or even, and thus which set of scanlines should be used. In non-interlaced video, the Amiga generates only even, or only odd fields, so every 50Hz "frame" touches the same set of scanlines, and leaves the interleaving scanlines untouched.

[Toni Wilen]

(I was just about to explain it better..)

btw, many LCD displays don't understand this "progressive" mode and show same field twice, skipping every other field. Result is jerky 25fps motion.

I suppose this is why my shitty capture card doesn't take the input of the Amiga, as it probably expects an interlaced signal and not progressive.

Probably. It is confirmed if it works when mode is interlaced.

[Amiga1992]

Originally Posted by Toni Wilen

Probably. It is confirmed if it works when mode is interlaced.

And that's a great test, I didn't think of it until you have explained this. I will try it soon.

[mark_k]

If you have trouble recording/capturing the output from your Amiga, for system-legal programs you can force interlace mode on with Lacer, supplied with Workbench 2.0x and later. Do Lacer 1 to enable, Lacer 0 to disable. The Lacer program itself probably works under Kickstart 1.2/1.3 too. There are also some programs which perform the same function on Aminet.

Maybe a similar option should be suggested to the developers of WHDLoad installers. It should be very easy to implement.

[Amiga1992]

Maybe that could work, yeah. Worth a test.
I Just tried the capture card with an interlace mode and, yes, it worked perfectly. So the piece of garbage just doesn't like the Amiga's progressive output.
I don't understand why the C64 doesn't work, though, does that display progressively too?

[Toni Wilen]

Originally Posted by Akira

I don't understand why the C64 doesn't work, though, does that display progressively too?

Yes. It was the only way to have rock solid display (does not include other usual analog artifacts) on PAL/NTSC TVs. Real interlaced mode would have "wobbliness" that OP mentioned.

[Amiga1992]

I love the fact that they are all progressive. I gotta get a proper video capture unit...

[pandy71]

Not sure but DScaler probably support capture for 288/286p video sources.

However DScaler is very old (ok i see some 2012 compilation) and it have support only for Bt848/878 chips (and some SAA7134 too).

[mark_k]

DScaler can also be used with USB video capture devices or anything which has a DirectShow driver. Most capture devices, if they work with 240p/288p sources at all, cause the computer to think the captured video is interlaced (when in fact it isn't).

DScaler has some special handling for that case, presenting output in 480p/60Hz for 240p input for example. I've done some experiments with the DScaler code and got it to have a nice scanline effect which is great for old consoles and the Amiga.

Whether a particular capture device works with 240p/288p input probably depends on both the hardware and the Windows driver for it.

[Photon]

Originally Posted by Richardcavell

Only, a normal TV set doesn't display 50 frames per second. It displays 50 fields per second, where each field is half of a frame.

It's simpler than that, it shows 50 frames per second.

The confusion is about PAL input signal vs PAL "format" (for content). A DV disc that says "PAL" almost always outputs interlaced video. This still results in a PAL signal where display contents update at 50 fps - interlaced from the material that was filmed at 25 fps. (Even today, high-budget movies are filmed at such slow framerates.)

Originally Posted by Richardcavell

So did the Amiga display the entire 256 lines 50 times per second?

Yes (and still does), both in interlaced and non-interlaced mode. This is easily testable.

Demos and games look and play markedly better in 50fps than 25 fps. This is why displaying Amiga on a newer display not capable of exactly 50fps feels like a downgrade, and it's also why so much effort is spent by coders to make render loops run at full framerate.