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There's something about the VDP and NTSC timing that has confused me for a long time.

My understanding is that NTSC is an interlaced format with a total of 525 lines. There are 60 interlaced frames per second where only half of the total scanlines (~262) are drawn, resulting in 30 full frames per second. Is this correct?

Now, the TMS9918A data sheet says, in section 1.1:

"The TMS9918A/9928A have a 525-line format for U.S. televisions."

Then, in section 3.6.2 it says:

"The TMS9918A/9929A operates at 262 lines per frame and approximately 60 frames per second in a noninterlaced mode of operation."

It seems to me that in the second quote, "noninterlaced" should really be "interlaced".

So, what does the VDP really do?

Thanks.

--
Eric Quinn

Quote

> "The TMS9918A/9929A operates at 262 lines per frame and approximately 60 frames per second in a noninterlaced mode of operation."

> It seems to me that in the second quote, "noninterlaced" should really be "interlaced".

No, "noninterlaced" sounds correct to me.

If you're not going to used interlaced mode to get a higher vertical resolution, then you can just treat the display as a 262 line display (minus nonviewable area) that's updated at 60fps. The interlaced mode 'fakes' (in an industry standard manner) a higher resolution display by splitting it up into two actual frames. I know you know this, I'm just explaining my point that they wouldn't call it a 60fps 262 "interlace" line mode.

As I understand it, in interlace modes, the odd frames are actually drawn half a scanline* lower than even frames - but the output device is responsible for generating a signal that is offset by that half-scanline (I'm not sure how, don't know a lot about TV signals really).

Which console has the TMS chips you're talking about... I recall the Genesis having support for an interlaced mode (which is how the split-screen 2player part of sonic 2 works), but I don't know of any such feature of the SMS's VDP.

(*in this context I mean half of one of the 262 physical scanlines)

Thanks for the help. Apparently, I have long been suffering under the delusion that the VDP output matched a NTSC broadcast signal (precisely). This doesn't seem to be the case. The VDP generates a very specific signal that is compatible with an NTSC television, but not identical to a broadcast signal (which uses an interlaced mode).

Quote

> Which console has the TMS chips you're talking about... I
> recall the Genesis having support for an interlaced mode
> (which is how the split-screen 2player part of sonic 2 works),
> but I don't know of any such feature of the SMS's VDP.

I was mostly referring to the TMS9918A which the SMS VDP is based off of. I figured understanding the TMS9918A would give me a better understanding of the SMS VDP.

Thanks again.

--
Eric Quinn

Quote

> As I understand it, in interlace modes, the odd frames are actually drawn half a scanline* lower than even frames - but the output device is responsible for generating a signal that is offset by that half-scanline (I'm not sure how, don't know a lot about TV signals really).

> Which console has the TMS chips you're talking about... I recall the Genesis having support for an interlaced mode (which is how the split-screen 2player part of sonic 2 works), but I don't know of any such feature of the SMS's VDP.

> (*in this context I mean half of one of the 262 physical scanlines)

Does this mean you could generate a 256x384 or even 256x448 image by flicking between two images (even image lines on even frames, and odd image lines on odd frames), even without hardware support?

Anyone tried it?
Does any game use it (should we emulate this effect)?
Does any other game on any other console use this effect (apart from Sonic 2 and the Censor demo)?

(OT - just tried Shadow of the Beast, and it has a very clever-looking parallex scrolling effect underground... I like it ;)

Quote

> Does this mean you could generate a 256x384 or even 256x448 image by flicking between two images (even image lines on even frames, and odd image lines on odd frames), even without hardware support?

I don't know, I don't think it would look quite right if the beam isn't repositioned (if I still had my amiga hooked up I could try comparing a probler 640x480 interlace mode with one approximated by just flipping buffers every frame). I'm not sure thought if sonic 2 for genesis uses any real interlace mode or just flips scanlines though... I suspect the latter, I first played Sonic 2 when I was a long time amiga user and was used to interlace flicker, and I thought it looked 'weird' on the Genesis.

Quote

> Does any game use it (should we emulate this effect)?
> Does any other game on any other console use this effect (apart from Sonic 2 and the Censor demo)?

Aside from consoles with commonly used interlaced modes (psx, dreamcast, and above), I don't know of any.

Quote

> Does this mean you could generate a 256x384 or even 256x448 image by flicking between two images (even image lines on even frames, and odd image lines on odd frames), even without hardware support?

Nope. The whole key to the effect is having the next frame offset vertically, so it fills in the empty spaces between the previous frame's scanlines.
Otherwise you have a blurry, flickering mess that doesn't look like much of anything.

Quote

> Anyone tried it?

Yeah, I did. I wrote a Genesis program that allows the user to change the window settings in interlace mode 2, where you have 8x16 tiles. I then modified it to work in interlace mode 1 (where you have 8x8 tiles), but changed the pattern data on the fly, on each frame, and the end result was 8x16 tiles, with a drain on CPU power. After all that, I tried the same thing (pattern switching every frame) in the regular non-interlaced mode, and it didn't look anything like it should have, compared to the previous two tests.

Quote

> Does any other game on any other console use this effect (apart from Sonic 2 and the Censor demo)?

Funcar Rally and Combat Cars use interlace mode 2.
On the SNES, one of the Ranma games (maybe the first one) uses an interlaced display mode for it's title screen.

My Homepage

Author	Message
Eric R. Quinn Joined: 18 Sep 1999 Posts: 498 Location: Portland, Oregon USA	VDP Timings Posted: Wed Aug 29, 2001 4:56 pm
	There's something about the VDP and NTSC timing that has confused me for a long time. My understanding is that NTSC is an interlaced format with a total of 525 lines. There are 60 interlaced frames per second where only half of the total scanlines (~262) are drawn, resulting in 30 full frames per second. Is this correct? Now, the TMS9918A data sheet says, in section 1.1: "The TMS9918A/9928A have a 525-line format for U.S. televisions." Then, in section 3.6.2 it says: "The TMS9918A/9929A operates at 262 lines per frame and approximately 60 frames per second in a noninterlaced mode of operation." It seems to me that in the second quote, "noninterlaced" should really be "interlaced". So, what does the VDP really do? Thanks. -- Eric Quinn

Heliophobe Site Admin Joined: 25 Oct 1999 Posts: 2029 Location: Monterey, California	Posted: Wed Aug 29, 2001 6:49 pm
	Quote > "The TMS9918A/9929A operates at 262 lines per frame and approximately 60 frames per second in a noninterlaced mode of operation." > It seems to me that in the second quote, "noninterlaced" should really be "interlaced". No, "noninterlaced" sounds correct to me. If you're not going to used interlaced mode to get a higher vertical resolution, then you can just treat the display as a 262 line display (minus nonviewable area) that's updated at 60fps. The interlaced mode 'fakes' (in an industry standard manner) a higher resolution display by splitting it up into two actual frames. I know you know this, I'm just explaining my point that they wouldn't call it a 60fps 262 "interlace" line mode. As I understand it, in interlace modes, the odd frames are actually drawn half a scanline* lower than even frames - but the output device is responsible for generating a signal that is offset by that half-scanline (I'm not sure how, don't know a lot about TV signals really). Which console has the TMS chips you're talking about... I recall the Genesis having support for an interlaced mode (which is how the split-screen 2player part of sonic 2 works), but I don't know of any such feature of the SMS's VDP. (*in this context I mean half of one of the 262 physical scanlines)

Eric R. Quinn Joined: 18 Sep 1999 Posts: 498 Location: Portland, Oregon USA	Posted: Wed Aug 29, 2001 8:52 pm
	Thanks for the help. Apparently, I have long been suffering under the delusion that the VDP output matched a NTSC broadcast signal (precisely). This doesn't seem to be the case. The VDP generates a very specific signal that is compatible with an NTSC television, but not identical to a broadcast signal (which uses an interlaced mode). Quote > Which console has the TMS chips you're talking about... I > recall the Genesis having support for an interlaced mode > (which is how the split-screen 2player part of sonic 2 works), > but I don't know of any such feature of the SMS's VDP. I was mostly referring to the TMS9918A which the SMS VDP is based off of. I figured understanding the TMS9918A would give me a better understanding of the SMS VDP. Thanks again. -- Eric Quinn

Dave Guest	Interlace Posted: Wed Aug 29, 2001 10:59 pm
Dave Guest	Quote > As I understand it, in interlace modes, the odd frames are actually drawn half a scanline* lower than even frames - but the output device is responsible for generating a signal that is offset by that half-scanline (I'm not sure how, don't know a lot about TV signals really). > Which console has the TMS chips you're talking about... I recall the Genesis having support for an interlaced mode (which is how the split-screen 2player part of sonic 2 works), but I don't know of any such feature of the SMS's VDP. > (*in this context I mean half of one of the 262 physical scanlines) Does this mean you could generate a 256x384 or even 256x448 image by flicking between two images (even image lines on even frames, and odd image lines on odd frames), even without hardware support? Anyone tried it? Does any game use it (should we emulate this effect)? Does any other game on any other console use this effect (apart from Sonic 2 and the Censor demo)? (OT - just tried Shadow of the Beast, and it has a very clever-looking parallex scrolling effect underground... I like it ;)

Heliophobe Site Admin Joined: 25 Oct 1999 Posts: 2029 Location: Monterey, California	Re: Interlace Posted: Wed Aug 29, 2001 11:07 pm
	Quote > Does this mean you could generate a 256x384 or even 256x448 image by flicking between two images (even image lines on even frames, and odd image lines on odd frames), even without hardware support? I don't know, I don't think it would look quite right if the beam isn't repositioned (if I still had my amiga hooked up I could try comparing a probler 640x480 interlace mode with one approximated by just flipping buffers every frame). I'm not sure thought if sonic 2 for genesis uses any real interlace mode or just flips scanlines though... I suspect the latter, I first played Sonic 2 when I was a long time amiga user and was used to interlace flicker, and I thought it looked 'weird' on the Genesis. Quote > Does any game use it (should we emulate this effect)? > Does any other game on any other console use this effect (apart from Sonic 2 and the Censor demo)? Aside from consoles with commonly used interlaced modes (psx, dreamcast, and above), I don't know of any.

Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Re: Interlace Posted: Thu Aug 30, 2001 3:25 am
Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Quote > Does this mean you could generate a 256x384 or even 256x448 image by flicking between two images (even image lines on even frames, and odd image lines on odd frames), even without hardware support? Nope. The whole key to the effect is having the next frame offset vertically, so it fills in the empty spaces between the previous frame's scanlines. Otherwise you have a blurry, flickering mess that doesn't look like much of anything. Quote > Anyone tried it? Yeah, I did. I wrote a Genesis program that allows the user to change the window settings in interlace mode 2, where you have 8x16 tiles. I then modified it to work in interlace mode 1 (where you have 8x8 tiles), but changed the pattern data on the fly, on each frame, and the end result was 8x16 tiles, with a drain on CPU power. After all that, I tried the same thing (pattern switching every frame) in the regular non-interlaced mode, and it didn't look anything like it should have, compared to the previous two tests. Quote > Does any other game on any other console use this effect (apart from Sonic 2 and the Censor demo)? Funcar Rally and Combat Cars use interlace mode 2. On the SNES, one of the Ranma games (maybe the first one) uses an interlaced display mode for it's title screen. My Homepage

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