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Old March 9th, 2009, 11:11 AM   #1
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Dual graphics cards in Mac Pro with Final Cut Pro?

My wife's early 2008 Mac Pro has two 256MB ATI 2600XTs so she can use three monitors, a 30" HP LP3065 & two 24" HP 2465 monitors. We bought the system a few months ago & it has performed flawlessly until now. She has started using FCP & encounters a problem when trying to use some of the Render Effects e.g. Swirly or Two Color Ray which produces the error "Hardware cannot render at the requested size and depth".

I have been trying to run down the cause of this problem but there is conflicting advice in various forums that I have consulted. Some say that it is because the graphics card isn't up to snuff & there is an old Apple article referring to getting this error message with old graphics cards with less than 64MB memory. The other school of thought is that FCP can't handle two graphics cards & there does seem to be some evidence for this. If I disconnect all but one 24" monitor then the error does not appear but I am wondering if this is because I am using only one graphics card or only one 24" monitor. Just using the 30" monitor we do get the error so I am prepared to believe that it may be because the graphics card does not have sufficient RAM as the display is 2560x1600 pixels. Simply powering off the excess monitors is not sufficient they actually need to be unplugged from the graphics card.

Is there anyone using more than two monitors & thus two or more graphics cards with FCP? I can understand that the current graphics cards need upgrading but it seems amazing to me that it wouldn't work properly with two particularly as there are no warnings in any of the Apple descriptions of the hardware or FCP. I am on the point of buying two ATI 3870 graphics cards but am reluctant to do so without confirmation that this will fix this problem.


Nigel Barker
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Old March 9th, 2009, 11:50 AM   #2
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There are 2 possible issues:

One is a little-known bug in FCP/FXPlug that is specific to using (2) identical GPU's (video cards). This issue is simple but one that Apple hasn't issued a fix for yet. FCP sends the rendering code and instructions to the GPU directly; when it finds more than one card with an identical rendering engine it gets confused and can't figure out which one to address, so it either hangs, freezes or crashes.

There's only one way to fix this problem; you need 2 completely different cards, either a different model or different brand altogether. Then FCP can easily choose which card to send it's render commands to based on whichever has the most resources available at that moment.

The other potential issue is that you may in fact be running out of GPU processing power with a card that only has 256MB of VRAM. I'd opt for replacing them with cards that have 512MB and (2) Dual-link DVI ports.

If neither of these fixes solves the problem then it's time to take it to the Genius Bar and have them sort out the memory addressing issues between the cards.
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Old March 10th, 2009, 07:59 AM   #3
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FCP can't fully utilize 2 graphics cards

I have the same problem with two (different) NVIDIA graphics cards. I considered getting two similar cards to solve the "can't render" problem, but found on the Apple site an article stating the Final Cut Pro cannot recognize two graphics cards. Also read elsewhere that Color doesn't recognize two cards either. My FXPlugs don't work and I can't render a huge chunk of video (>10 minutes).

If I find the Apple written article, I'll send a link to it.
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Old March 11th, 2009, 07:35 PM   #4
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I'd give credence to any of these theories about FCP bugs with identical cards. But, there's no question that video card memory is a possibility. Sometimes these errors have multiple ways of triggering them.

Memory is used for a number of things in the graphics world and that includes storing patterns and drawing instructions and telling the card to execute them. If there's not enough room left over for a given operation after taking away the memory needed for the screen's resolution, then it follows that some complex rendering operations that need more memory cannot be done.

Have you tried running the 30" at a lower resolution which *might* use less memory and thus not throw the error? It's not conclusive if it fails but it would be conclusive if it works.
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