Are dolby digital and AC-3 the same?

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?