you have to indicate which kindof g5.
there are several with different pci slot types - all incompatible.

alchemy tv card only ever compatible with 1st generation g5 ( not pci-e ) - might work in pcix- dont know. - and support abandoned some time ago.

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you might contact active silicon -

http://www.activesilicon.co.uk/products_fg_lfg.htm

they have single channel composite cards that seem to work in my newer machines. not tested in any old g5's