Why no Dolby on Proceed PAV

I've inherited a used PAV but no sound output exists on THX or Dolby Surround modes. Stereo, mono surround, mono all work fine, as do all speakers (center, side mid/high range, rear). Tried multiple DVDs and DVD players including tweaking the sound options on the players themselves. But the dolby modes are dead.

Any thoughts?
the PAV only does pro logic, you need the PDSD to decode DD.
Thanks, Kennyt. But this shows my ignorance re: dolby. OK - here's my brilliant 101 observation - there's no such thing as standard dolby. Kinda like Windows - sure it's an operating system, but you've got to know if you're workiong with 98, NT, 2000, ME, XT, XP, XYZ, etc.

So help me with the evolutionary path here. I understand the PAV is an older unit. So is it not "compatible" with the newer dolby versions?

Thanks in advance for the lesson. I'm off to see how much an engine to this new car goes for.

the pdsd is the dolby digital decoder for this pre, which came out prior to DVD players.
Maybe your DVD can do the decoding.
The PAV is an analog preamp. How are you connecting it to the dvd player? It should be connected to the PAV's audio inputs ie, VCR1 VCR2, left and right channels. If you are doing this already and still you can only play stereo then your unit has an eprom that will make it play with the PDSD unit only. Does the sub outputs work? If not then probably thats the reason why.