There was. Way back in the day, I even had an mpeg-2 accelerator card that passed through a signal to do its magic. The card used in the P4 was a Geforce 2 GTS,

There was. Way back in the day, I even had an mpeg-2 accelerator card that passed through a signal to do its magic. The card used in the P4 was a Geforce 2 GTS, which did have mpeg acceleration. The onboard graphics of the P3 accelerated certain primitives, like motion compensation, but not to the degree of the P4.