I'm watching Alien on blu-ray and noticed an audio that I've never heard of before. Dolby 4.1. From what I understand, this is supposed to be front left, front right, back left, back right, and subwoofer. But there seems to be a center channel as well. Why is there a center channel ? The 4.1 sounds fine, but I'm just wondering why there appears to be a center channel on a 4.1 mix.