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We're in a position at work where we're upgrading a couple of G5 systems to Mac Pros. So we'll have these G5s lying around and I was looking for their best use. The first thing that my mind shot to was Video Satellite LE. I thought the G5 would be an easy host for such a task. But Avid specifies a Mac Pro, so no dice on that.
I remembered long ago eyeing VirtualVTR. It looks like much hasn't changed on their page, I don't know if that's because they were revolutionary and way ahead of their time or because they haven't updated in ages. Anyone have experience with it? I have to take a closer look, but I'm assuming that it acts just like any other 9-pin controlled device within ProTools.
With no ability to playback HD concurrently with Pro Tools, HD playback would be really nice (It's a small disappointment to some clients). But perhaps you have a better idea as to how to repurpose an aging, but otherwise perfectly usable computer...
I believe Marti Humphries at The Dub Stage is actually getting away with a G5 running Video Satellite... not supported technically but it seems to work. This was when it first came out, so the current version may not fly. Check the DUC for details.
One of things I use an old quad G5 for is as a transfer station. Think about how many times your main machine is held up while you convert one thing to another, or capture audio from whatever source. I have a machine just dedicated to that function and it saves much time in the mix/edit suites for well - mix and editing. They also make good secondary cutting stations.
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Doesn't address your question regarding video playback, but if I had an extra high-horsepower Mac sitting around I'd consider tasking it as a dedicated audio rendering station. For instance, you've got a ton of 192k library material sitting around that you want to Pitch n' Time down 4 octaves, or want to perform paulstretch on something, etc. etc.