Composite/Svideo input stream used as WebCam?

[SiRocket]

Hey all, i do a ustream broadcast every Friday with a group of dj's - http://www.thevideomixshow.com and have a simple question hopefully.

I need my imac (machine used to broadcast) to be able to have a video input (composite or svideo) device recognized as a webcam input. So lets say i have a music video playing into my composite video capture device on my imac, i need to be able to select it as a webcam input during my broadcast.

Are there any usb devices out there like - http://www.blackmagic-design.com/products/videorecorder/ that will do this? or will this black magic product be recognized as a webcam/video device through ustream, ichat, or any other "video broadcast program"? just like a usb webcam would?

Appreciated everyone.

 

[SiRocket]

going to go ahead and also throw this out there for either option 2 or for a future option.

Is there such device that instead of being composite/svideo... that takes vga or dvi as the input (not output) into the mac via usb or firewire?