Audio stream decoding

[e21959]

Hi. i have an *.Xvid movie that has an AC-3 2.0 Dolby Digital 16 bit, 2 channel,192 Kbits/s,48,0 Khz as audio stream.when i select A/52 over S/Pdif it is decoded as AC-3 Dolby Pro logic in my home theatre system and not as Dolby Digital 2.0 ( i ve tried with Windows Media Player and another player too and they decode the right format).any suggest or is it k?
tvm in advance for any explanation.
Enrico /italy