That's a different thing - the PC1 PAC is reading the CD[-ROM] part of the disk (LD digital audio is literally CD audio's RF put through some filters), kind of like a CD rip (especially if it can pull audio) ld-decode works on the analog video and audio portions.
The graphics card is used as a GPGPU (vector processor) that can run things like FFT's and other parallel operations much faster than a CPU can.