I tried a different cable, one that's DVI at one end and HDMI at the other. It works plugged into my Dell U2415, which is a monitor that I'm pretty sure has no analogue capability.
On the contrary… the Dell
is HDMI analogue
I tried plugging the Powerbook into a DVI to VGA adapter, then that into an active VGA to HDMI adapter, and then that into the capture card. The Powerbook consistently sees the adapter as a monitor, but I only get black output. This VGA to HDMI adapter works on my retro PC, though that's directly from VGA out on the video card.
This scheme does have a chance of working, but only a slim one. What's probably happening here is the PowerBook is outputting a Mac resolution that the card can't follow… What???
Here's the what:
The DVI-to-VGA adapter is adapting the connection, but is
not converting the Mac resolution to actual VGA. SO…
The active VGA-to-HDMI unit may be failing to handle the signal properly, and so there's nothing for the card to "see". OR…
The VGA-to-HDMI unit
is passing the signal at the Mac resolution (very slim chance), but the card can't read that resolution anyway.
You're trying to drag and old piece of tech kicking and screaming into the modern video era, and it's hard, to say the least. The biggest problem is that nobody feels it's necessary anymore to provide detailed specs when it comes to all things HDMI. According to them, you're not supposed to use anything pre-HDMI anymore. You're supposed to just buy all new shit and then everything is just plug and play.
That is of course, except for interconnecting Macintosh and PC stuff. Apple has always made that as hard as possible to keep you in the Mac ecosystem and lock out all of the tons and tons of PC equipment and it's driver requirements etc.
That "just point a damn camera at the screen" idea starts to look better and better, doesn't it?