Dolby Digital comes in a few flavors: 2.0/5.0/5.1/EX6.1. AC-3 is the older term for the Dolby Digital format and you normally won't hear that term used today. Any DVD that indicates Dolby Digital will be supported by your receiver. DTS is a digital 5.1/ES6.1 format but not Dolby Digital. Most likely your receiver only supports Dolby Digital 5.1 or less. DTS DVDs normally have a Dolby Digital option that allows your receiver to play DTS DVDs.
Are dolby digital and AC-3 the same?
I'm a bit confused by these 2 terms. I've heard about a difference in bit rates, but that's about all. I currently own a Harman Kardon AVR55 reciever, that does AC-3 decoding, and I would like to use it in my home theater setup. I would like to use the money saved to go out and get 2 additional amps for my system, since the receiver has pre-outs. Also, do most DVD movies come with DD and DTS, or just one of the 2?