Hello, I am a newbie in the surround sound arena and I have a very basic question. I have been reading that in surround sound technologies such a Pro Logic, the decoder can take a two channel Stereo feed and output it as a simulated 5.1 surround environment. I thought this was possible only if the stereo feed had embedded the additional channel info. In other words, I can understand how Prologic (or other similar technologies) can extract from the original stereo feed the additional channel info "if" this information was present and available........if this information was not present on the original stereo feed how would Prologic know what goes to which channel? In other words, if I am hearing a Music CD that is standard 2 channel only, can something like prologic simulate that into 5.1 surround?
-Andy
San Francisco
With Dolby Pro Logic, (hereafter referred to as DPL), they have 4 separate channels in the studio. The front right, front left, front center, and rear (also called "surround", which is why it is "S" in the quote below). These four channels are then mixed together down to two channels in a special way:
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:
• 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.
From:
http://www.dolby.com/assets/pdf/tech_library/208_Dolby_Surround_Pro_Logic_Decoder.pdf
Your DPL decoder at home reverses this process to give you 4 channel sound from a two channel source. This whole scheme was developed for theaters to use, so that there would only need to be two channels of sound on the film. There was not room on the film to easily add more channels, and also, this way, they could use existing projectors with two channel readers, which then feed the signal to a special decoder. But it also is perfect for two channel VCRs and 2 channel sound on analog TV.
Now, of course, they can do sound differently than when DPL was invented, and they can keep the channels totally separate from each other. Dolby Digital and dts (in their 5.1 versions) do just that.
Basically, the matrixed Dolby Digital EX and the matrixed dts-ES mix the rear center channel in the right and left rear in a manner similar to how the front center channel is mixed into the front right and left in DPL.
Now, with ordinary stereo music, there will tend to be some things that are in both the right and left channel, and applying the DPL decoder to this will redirect whatever is the same into the center channel, and whatever is the same but out of phase, it will process as if it were the rear channel, sending that information to the rear speakers. Whether you like the results of this or not, this is technically not decoding when it is used this way, because in order to
decode something, it must first be
encoded. But many people enjoy processing sound in a variety of ways to create new channels, and this is also what is happening when DPL II or DPL IIx is applied to a DPL encoded source.
