unmodded old ps2 (PAL) plays burnt dvd's - why?

[travisacton]

Hi guys, i was wondering why my old ps2 plays some dvd movies that my mate ripped and burnt on his computer. The main reason i ask this is because i just purchased a sharkport online, which is not for a pal system and to return it would be a pain in the arse. When i insert the disk for the sony i get a red screen - please insert ps2 disk. What i wanted to do was rip the ps2 disk and burn it again. Is there any chance i could get this to work without a mod chip, and, are there any other methods you guys could recommend for doing this, or should i just mod my ps2?

Thanks guys,

self confessed noob