There are many reasons for this little-known, never-advertised fact. First, the width of a DVD player's laser beam is different from a CD player's, and the digital sampling is set by an internal clock that has a different rate. In other words, the way a DVD player reads a CD is, from the very get-go, wrong. You don't hear distortion in the usual sense of that word. But you don't hear things the way they should be heard. More expensive DVD players (close to $1,000 or more) have separate circuitry for DVDs and CDs.