Why does my cat's poop smell REALLY bad?

Ok so I been dealing with this problem for a while now...I know that poop in general smells bad..but this is actually enough to kill someone. Last night the smell was actually so bad I woke up out of a dead sleep and gaged and almost threw up, I even sprayed down the room with glade and it didn't even help. Its REALL REALLY rank and this has been going on for about a year now. I use Tidy Cats Scoopable Litter for Multiple cats, and a covered litter box. I also feed my cat Purina Indoor Formula cat food. Ive tried changing his cat food but it dosen't seem work. I don't give him canned food or treats or table scraps either.