accapella techniques

hi, first at all i would like to apologize myself for my bad english.

i was with a friend few weeks ago, wondering how we could enchance our fresh beat extract from the mpc. we got the following idea, adding some lyrics to it. but the main problem is neither my friend or me use to sing, ok well except in partys.. anyway, we tried to produce an accapella with somes good hip-hop tracks.
we had the beat with the accapella, and the instrumental version.
so i opened the two tracks in cubase sx2 ( i tried with PT m-powered without succes either), then i invert the phase on the instrumental version an play the both together.
i expect to have just the accapella and some noise, but it didnt work at all, we could heard the beat as usual but with some glitches. we tried on some part of the original track where there is only some beat, it work well ( we dont hear anything at all as we can expect). when we applied the same processus on the whole track it dont work.
did i miss some step, have someone any hints about it ?

not sure what exactly your trying to do....

i think this is the old how can i remove the beat and just have the vocals question. maybe im wrong.

thats kinda what it sounded like. the vocal removers on cool edit have always been crappy

exactly , i want to remove the beat and keep the vocals by using phase invertion. mathematicaly it should work, thats why i am asking if i miss something important

ok i forgot to mention that tracks where in mp3..i retry it with wav version, works much better, maybe is due to bad encoding ( so not exactly yhe same wave forms)

mp3 takes out a lot of data, it really is a lossy format