You no longer need to have a substrate that's at least 1 1/4 inches thick and that makes your tile floor too high where it meets up with a different type of floor (carpet, hardwood, etc.)  Now there's a great product out called "PER-MAT" by BLANKE, a German company.  This product is only 1/8 thick and you can put it down over your current 3/4 plywood floor and tile right over it.  Doing this will allow you to have the structural integrity of a 1 1/4 inch substrate while maintaining a lower profile.  No more tripping into the next room!