Errm, Rob, you're thinking of SCMS (serial copy management system) that's been around since something like 1987 with the first consumer DAT decks that shipped in the US. This was mandated by the AHRA.

I don't believe SDMI have ever published a completed DRM specification or a watermarking specification, although this is what their aim was, due to a total lack of agreement between manufacturers and the music industry.

Hugo