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Guys, the emu I downloaded first time didn't work, it was the 1.0.3 one, but I've replaced it by this one, 1.0.6 beta, and it works perfectly. I've been playing SoulCalibur and Sonic Adventure 2 this eve.
So, this is the link I downloaded from:
Nulldc 1.0.6 beta by Shadow 4300
This one worked because of the ExtraGeom tool, into Options, which was not present in the older version. I could barely play SA2 in nullDC 1.0.3, it gave me lots of graphics errors. On the contrary, nullDC 1.0.6 beta is awesome. Moreover, this one allows me to use my comfortable gamepad instead my seedy keyboard.
By the way, I've been playing Sonic Adventure for PC these days beating Perfect Chaos after some attempts (kinda annoying). I got a version that worked without problems from The Pirate Bay, if someone wanted to know it, cause the first one I downloaded was a downgraded fail almost unplayable, but this one has been fine. I liked Sonic Adventure Director's Cut, but I don't love as much as Sonic Heroes, which is still my favorite 3D Sonic game by far.
I explained how I found some problems at handling with nullDC at the beginning, so I'll just leave here the thread where some BIOS are available along a video tutorial that really helped me out.
Wanna play Sonic Adventure. - Dreamcast-Talk.com
Hope it will be helpful for other Sonic fans like me. This a matter of gratitude, thanks.
PS: Due to Sonic Generation's release lately I'm obsessed with Sonic games, but I'll try Shenmue one of these days. I always wanted.