Originally Posted by sworth
Does anyone know the technical aspects of Dolby Stereo surround?
Dolby Stereo Surround is what is decoded with a Dolby Pro Logic (DPL) decoder (not to be confused with DPL II or any later DSP options). In the encoding process, one starts with 4 channels: Right, Left, Center, and Surround. These are mixed together in a special way down to 2 channels (called "Lt" and "Rt" in the quote that follows):
The L and R inputs go straight to the Lt and Rt outputs without modification. The C input is divided equally to Lt and Rt with a 3 dB level reduction (to maintain constant acoustic power in the mix). The S input is also divided equally between Lt and Rt, but it first undergoes three additional processing steps:https://web.archive.org/web/20120328...ic_Decoder.pdf
• Frequency bandlimiting from 100 Hz to 7 kHz.
• Encoding with a modified form of Dolby B-type noise reduction.
• Plus and minus 90-degree phase shifts are applied to create a 180 degree phase
differential between the signal components feeding Lt and Rt.
The DPL decoder reverses this process. That is, whatever is in phase in the right and left is sent to the center, and the surround is derived from taking whatever is out of phase, applying a delay, a filter for removing anything above 7kHz, and decoding the noise reduction (both surround speakers get the same signal; the original source was 4 channels). Everything else stays in the right or left channel. The decoding process isn't perfect, so one does not have the same channel separation one would have with discrete channels (not to mention the frequency response for the surround channel). But it is a very clever way of getting 4 channels when your playback device can only have 2 discrete channels, and it works reasonably well.
Now, for your other questions:
Originally Posted by sworth
I'm reading online about Dolby matrixed mixes that lose their rear channel information when they are remastered. How does that work? I understand that the center channel is mono material and the rears are out of phase, but how would out of phase material be put back into phase in the mastering process?
If someone remastered a Dolby Stereo Surround encoded 2 channel recording without first decoding it, they would likely mess up the relationships of the encoded channels, making it impossible to properly decode it. In order to preserve the surround encoding, one would want to decode it, remaster the 4 channels separately, and then re-encode it. (Or, really, it would be better to go back to the original 4 channel recording, remaster it, and encode that remastered 4 channel version.)
Basically, if you muck about with the encoded material before you decode it, you will mess up the decoding process, as it relies on the signal being as it is, not changed via remastering.
Of course, you could still apply a DPL decoder to whatever you end up with, just like you can apply it to any 2 channel source. However, doing that makes it just a DSP mode, not a decoder. (One can only decode
something that has been encoded
. This, by the way, is why I referred to DPL II above as a "DSP option" rather than a decoder; since the material that is mixed in for the surround channel is the same for both surround speakers, any change in that is not decoding what is encoded, and is altering the signal. The same idea applies to having frequencies outside of the range of 100Hz to 7KHz, since there is no sound outside that range that is encoded into the surround channel, any such frequencies that are sent there must be either added frequencies [distortion] or come from the front channels [misdirected, since those frequencies are from the front channels], neither of which is decoding
.) You will not get what was originally intended that way.