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?
Julpay: Nope! There are 2 kinds of dts as well. Ac3 "was" or refers to the Lazer disc"type" of dd.That is why a modulator is needed to decode them.(or transfer the signal). The first DTS for lazer discs also works with the audio only cds in DTS. The DTS on dvds is diferent. I had to sent mt DTS processor in for a chip update when I got a DTS dvd player. If you don't have a LD player;no need to concern yourself on the DD/ AC3 issue. As far as dvds go; Most all are in DD. For most all that have both (few) you have a menu choice.There is the "Hell Freezes Over" in DTS/with no DD option,and probably others.
Yes. AC-3 Refers specifically to the "Perceptual Encoding" system developed by Dolby Labs to contain 1 to 6 discrete channels of compressed audio within a digital bitstream. The system was called AC-3 long before DVDs, when available only on Laserdisc. "Dolby Digital" was a more descriptive, marketable term, created about 2 years later. If you look at an early DD Laserdisc, like "True Lies" or an early-generation DD decoder like the Pioneer Elite SP-99D (I had one for several years), You will only find "AC-3" as the name for the digital surround system called Dolby Digital today.
Thanks for the information. but one of you say yes, and the other says no... I guess the most pressing question is will my harman kardon reciever (which can decode AC-3) decode a DVD move encoded with a Dolby Digital signal
Julpay- unless I am wrong the only practical difference is in the interconnects. AC-3 usually is output as an RF signal, I think, while DD is usually output as either toslink or digital co-ax. Could be wrong on this, tho.
I know the receiver has an optical input....would this send the signal over?
A bit more light to help confuse.Did you know the AC-3 from the LD transport can not be carried by a Illuminatins D-60? It requires a video cable.I also had the same Pioneer AC-3processor. I sold mine to get Lazer DTS capability,before DVD came out.Kthresh said nothing that contradicted anything I said. You may have percieved it that way ,I didn't.On my present processor setup,I have the DD/DTS, input on one coax with signal recognition. Then I have the AC3 from the LD as a seprate coax input. Now the point I missed, is the question I don't know the ans to. As in, will your piece do DD,from a dvd player? Well, If it doesn't MSB,or others can do a chip upgrade cheap,I would guess.
Yes, Dolby Digital and AC-3 are the same "technology". All the comments about how it is hooked up have nothing to do with the Dolby circuitry.
The followinig site is a another valuable resource for any questions concerning audio and video questions, and will provide links to the Dolby site as well as others:

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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.
I have always been under the impression that AC-3 is the term for the general encoding scheme, and that DD is a specific type of encoding under that scheme. Actually, with all the different "flavors" of DD, it's several different specific types of encoding under the general scheme. In any case, Yes, your receiver will decode the output of your DVD player.
I thought that DD used data compresion and DTS rearanged the data to fit like a puzzle. Eliminating the need for compresion which degrades the signal.
As it was explained to me DD uses about 11/1 and DTS uses about 4/1 -compressions.