ok, I thought that all audio was converted to dolby digital? Also, if two RCA cable were inputted into an amp I thought it could output 5.1 sound if it had the relevant capability.
If you plug two rca leads into an one of the new onkyo amps for example it can output 5.1? I always thought amps could do this, I may be wrong as people on here know a lot more than i do.
Thanks.
Nope, fraid not. Dolby Digital means that the sound is encoded (and compressed) in a digital format. Dolby Digital may be 5.1 channels of sound, but you can also have Dolby Digital with two sound channels, or even Dolby Digital mono.
Basically, all sound on a DVD (or HD-DVD) has to be in a digital form, because that's all that a DVD carries.. digital data. If you've got an optical or coax SPDIF connector (both are equivalent.. think 'optical'), you can send the actual bits directly to your receiver for processing.
If all you have is stereo RCA cables, you're actually sending analog sound to your receiver. The receiver doesn't know whether that analog sound came from a DVD player or a tape recorder or what, so it doesn't label the sound as 'Dolby Digital'.
Now, Dolby Pro Logic is a method of taking stereo analog input and doing some analytic processing of it, to try and redirect the sound from your two 'main' speakers to your center, or to your back speakers (together). Let's say that both the left and right stereo cables are playing the exact same sound at the same time, in the same phase. The receiver can make note of this and decide to put it out your center channel speaker since there's no real stereo separation in the sound, anyway.
Likewise, a Dolby Pro Logic receiver can choose to send some sound to the back speakers, depending on the phase relationship between the audio coming in the two stereo cables.
But these are basically processing tricks to move the sound around. They won't give you anything like the quality you can get from a pure digital connection, in which the high res digital audio from the source encodes information separately for each speaker you have plugged in.
