PowerBook NTSC S-Video Outputs Black Lines

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Date: Monday, February 1st, 1999, 00:00
Category: Archive


Reader Bruce Hamilton writes about an issue he discovered with the NTSC S-Video/Composite output of PowerBooks:

I have come across a problem which I think is probably present in all PowerBooks which use an ATI RAGE graphic controller. I have talked with several friends recently who use PowerBooks and they all have this problem. I recently got a PowerBook G4 and I noticed the problem when using the NTSC output in either S-Video or composite mode. It is important that the settings in the “TV Mirroring” Control Strip first be set to “Best for Video” and “Overscan” (if available) in order to see the problem.

The VGA output or main LCD screen do not exhibit this problem. What happens is that scan lines 10 and 12 (counting from the bottom of the image at 640 x 480 resolution) display only BLACK lines every other frame. This make the image on those lines flicker badly as one frame is displayed correctly while the next has only a black line on lines 10 and 12. I think one reason more people have not noticed it is because on normal TVs it is hidden by the overscan of the monitor (it is easily viewable on any monitor which has an underscan switch).

However, capturing the image into DV is a totally different situation since all scan lines ARE shown. To verify this problem I captured the S-video output of the PowerBook to miniDV and then brought the clip into Final Cut Pro. I exported a second of the video as stills (30 frames) and I confirmed this alternating interlacing frame problem as shown in the two images here. I believe that ATI may have a fix for this problem but has never bothered to get around to implementing it on the PowerBook.

By coincidence, I had purchased a ATI RADEON PCI card for a dual G4 for the Art & Science Lab where I work. It exhibited the EXACT same problem with the NTSC output. However, included with this card are additional pieces of system software. The “ATI Display” has a dialogue box in the “Mac2TV” section which permits a on/off toggle of “Hard Sync to TV.” When it is set ON the black lines are eliminated and all frames now look correct. I do not believe this fix has been implemented for the PowerBook.

I would definitely like to get this problem posted on your site. Perhaps wide exposure of this issue might force Apple or ATI to provide a fix. Your readers could certainly add voices to getting this problem solved.

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