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Interlaced VGA output from an ATi Radeon

Started by benryves Jan 16, 2010 at 12:06 PM 1 replies 3.9k views

Original Post

Jan 16, 2010 at 12:06 PM

Does anyone know if it's possible to set an ATi Radeon to output interlaced video from its VGA port, preferably at a different refresh rate and resolution to the primary monitor on the DVI connection? If I plug a VGA monitor into my PC as a secondary display, clone the primary monitor to it and use the Catalyst Control Centre's forced HDTV modes (1080i30/1080i25) I can generate interlaced video, but this is at a very low refresh rate (I'm aiming for i60 but at a lower resolution), in the wrong aspect ratio (1920×1080 looks awful on a 4:3 display) and prevents me from using the primary monitor (my monitor doesn't support such high resolutions or interlaced scanning). I own a copy of PowerStrip, and have attempted to force interlaced video output by setting up custom timing rules. Sadly, even though PowerStrip can configure the timing it can't set the internal "switch" to generate interlaced video, so the result is the top half of the display in both fields instead of alternate scanlines. Edit: An alternative workaround is to connect the TV out to a VGA CRT using a VGA box. I've found that if I set "flicker removal" to its lowest setting and enable "overscan" I get a decent interlaced display, but it's still limited to a maximum of i30 (NTSC). Adjusting the refresh rate in PowerStrip causes the picture to lose colour. [Edited by - benryves on January 16, 2010 12:56:28 PM]

Fiddler

Jan 16, 2010 at 4:45 PM

When you say i60, do you mean 60+60 fields per second or 30+30 (normal NTSC)? I don't think the former is possible through TV out.

Don't newer CCC versions include a method to set custom HDTV modes? There's a slight chance that this could be used to add the mode you wish.

Another idea is to use a DVI->VGA adapter. For some reason, this seems to trip the autodetection mode and allow more resolutions than possible through pure DVI or VGA.

That said, if PowerStrip can't do it... there's only one more flexible approach I am aware of, and that is Linux with the radeonhd driver and custom modelines (not radeon - the former programs the display registers directly, while the latter uses AtomBIOS and AFAIK shares the limitations of the closed-source drivers).

benryves

Jan 16, 2010 at 5:22 PM

Quote:
Original post by Fiddler
When you say i60, do you mean 60+60 fields per second or 30+30 (normal NTSC)? I don't think the former is possible through TV out.

Whether it would be possible or not (unlikely), I don't think my VGA box would handle it. In any case, the only two options I have using TV out are i25 (PAL) and i30 (NTSC). I'd like to go up to i60, based on the fact that I can happily progressively scan a VGA monitor at 640×480 at 120Hz. (This resolution is good enough).

Quote:
Don't newer CCC versions include a method to set custom HDTV modes? There's a slight chance that this could be used to add the mode you wish.

I get a long list of predefined modes, only two of which are interlaced - 1080i25 and 1080i30.

Quote:
Another idea is to use a DVI->VGA adapter. For some reason, this seems to trip the autodetection mode and allow more resolutions than possible through pure DVI or VGA.

Good idea! I can give it a shot. I know that when I had my old PC with a VGA monitor attached to the DVI port I could set it to run at i43.

Quote:
That said, if PowerStrip can't do it... there's only one more flexible approach I am aware of, and that is Linux with the radeonhd driver and custom modelines (not radeon - the former programs the display registers directly, while the latter uses AtomBIOS and AFAIK shares the limitations of the closed-source drivers).

Always an option, if a little extreme. [smile] I had been intending on building a VGA line blanker (if I can find somewhere that sells a suitable video switching IC), so I could simulate an interlaced display by blanking alternate scanlines on every hblank. The ultimate aim is to drive LCD shutter glasses (which I've already built some hardware for anyway), and whilst it works with the HDTV modes I'd really like something a bit less flickery and not distorted by having a 16:9 image displayed on a 4:3 display.

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