QAM is the language spoken on the line. Imagine something like how a stereo interacts with the magnet on a speaker to make sound. DQI is a measurement of errors in the transmission. Like "ums" or "likes" in spoken word. Usually one can compensate for a certain amount of "likes" in a conversation (error correction), but when it gets too overwhelmingly "like...ummmmm...umm...yeah", the subject can me miscommunicated entirely (tiling).
This is CABL.com posting #335532. Tiny Link: cabl.co/mbzrY