How do you get 7 chaos emeralds in sonic heroes

In the second act of each zone, you need to obtain a key. These keys are scattered throughout the level, but you only need one when you reach the goal. If you have the key, you will enter a special stage where you must catch up to the Emerald. You can use the balloons and other gimmicks to speed up and help yourself in chasing it down. There is one of these per zone, with a total of seven zones. Happy hunting.