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#1
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
If I read the Dolby Surround/Pro Logic white papers at www.dolby.com
correctly, a Dolby Surround encoded surround signal is simply a monaural track with the following modifications: One side is phase shifted "+90 deg." and the other, "-90 deg." The DPL decoder theoretically has horse-blinders on it that see signals of only this type and route it to the surround speakers. I would assume this is not the same effect as reversing the leads to one speaker/channel(which mosf of us here have heard and experienced), is it? If so, then how could this "+90 / -90 deg" business be achieved at home? And how could the phase of something be shifted plus 90 degrees anyway? Minus-90 I could see. -ChrisCoaster |
#2
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
"ChrisCoaster" wrote in message
... If I read the Dolby Surround/Pro Logic white papers at www.dolby.com correctly, a Dolby Surround encoded surround signal is simply a monaural track with the following modifications: One side is phase shifted "+90 deg." and the other, "-90 deg." The DPL decoder theoretically has horse-blinders on it that see signals of only this type and route it to the surround speakers. I would assume this is not the same effect as reversing the leads to one speaker/channel(which mosf of us here have heard and experienced), is it? If so, then how could this "+90 / -90 deg" business be achieved at home? And how could the phase of something be shifted plus 90 degrees anyway? Minus-90 I could see. -ChrisCoaster My limited understanding of the analog surround is that the delta between the two channels is sent to the rear. By phase shifting you are creating that delta. |
#3
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
On Sep 26, 8:00*am, "jamesgangnc" wrote:
"ChrisCoaster" wrote in message ... If I read the Dolby Surround/Pro Logic white papers atwww.dolby.com correctly, a Dolby Surround encoded surround signal is simply a monaural track with the following modifications: One side is phase shifted "+90 deg." and the other, "-90 deg." *The DPL decoder theoretically has horse-blinders on it that see signals of only this type and route it to the surround speakers. I would assume this is not the same effect as reversing the leads to one speaker/channel(which mosf of us here have heard and experienced), is it? If so, then how could this "+90 / -90 deg" business be achieved at home? *And how could the phase of something be shifted plus 90 degrees anyway? *Minus-90 I could see. -ChrisCoaster My limited understanding of the analog surround is that the delta between the two channels is sent to the rear. *By phase shifting you are creating that delta.- Hide quoted text - - Show quoted text - ____________ Understood. I'm just curious as to what a positive-90-degree phase shift means and how it is accomplished. Obviously both channels producing this modified sound will not be in phase with either of the front speakers or the center, increasing its chances exponentially of its differential being routed to the surround channel. I'm just curious how it's done, that's all. I'd like to try it at home if the equipment/technology isn't too expensive or demanding knowlege-wise. -CC |
#4
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
On Fri, 26 Sep 2008 18:11:02 -0700 (PDT), ChrisCoaster
wrote: I'm just curious as to what a positive-90-degree phase shift means and how it is accomplished. Obviously both channels producing this modified sound will not be in phase with either of the front speakers or the center, increasing its chances exponentially of its differential being routed to the surround channel. I'm just curious how it's done, that's all. I'd like to try it at home if the equipment/technology isn't too expensive or demanding knowlege-wise. This is something that puzzles everyone when they first study reactive circuit elements. Capacitive and inductive reactances lead and lag phase. That's a concept pretty easy to accept when it would seem that "effects follow causes" but is certainly not obvious (and, of course, not even really possible) when "effects would precede causes". It sometimes helps to say that it's just another way of saying that the signal is differentiated or integrated, but this explanation relies on an implied framework of algebraic modeling. Maybe the easiest way to approach the problem is to keep in mind that "phase" is a total fiction, a convenient construct from the world of mathematics and Hoover Dam, that's been grafted, Frankenstein-like, into the world of audio. It's the easiest (but NOT! the only) way to approach issues like feedback stability, time-variant mixing, etc. in audio, but has grown like a cancer beyond proper bounds. If I were to say to you that it's possible for one signal to lead another in phase, but not in time, would that be enough? First imagine them to be similar, but not causally connected. Then take the next step that lagging phase is *not* the same thing as lagging time. BTW, your intuitive rebellion against the non-causal is perfectly placed. Lord knows we need shirtloads more of that clarity of thinking in this world. Much thanks, Chris Hornbeck |
#5
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
"ChrisCoaster" wrote in message
... On Sep 26, 8:00 am, "jamesgangnc" wrote: "ChrisCoaster" wrote in message ... If I read the Dolby Surround/Pro Logic white papers atwww.dolby.com correctly, a Dolby Surround encoded surround signal is simply a monaural track with the following modifications: One side is phase shifted "+90 deg." and the other, "-90 deg." The DPL decoder theoretically has horse-blinders on it that see signals of only this type and route it to the surround speakers. I would assume this is not the same effect as reversing the leads to one speaker/channel(which mosf of us here have heard and experienced), is it? If so, then how could this "+90 / -90 deg" business be achieved at home? And how could the phase of something be shifted plus 90 degrees anyway? Minus-90 I could see. -ChrisCoaster My limited understanding of the analog surround is that the delta between the two channels is sent to the rear. By phase shifting you are creating that delta.- Hide quoted text - - Show quoted text - ____________ Understood. I'm just curious as to what a positive-90-degree phase shift means and how it is accomplished. Obviously both channels producing this modified sound will not be in phase with either of the front speakers or the center, increasing its chances exponentially of its differential being routed to the surround channel. I'm just curious how it's done, that's all. I'd like to try it at home if the equipment/technology isn't too expensive or demanding knowlege-wise. -CC You're wondering how you output a signal before it comes in. Your logic is correct. Shift everything else 90 degrees back. What is left is 90+. Do you have the source in individual pieces such that you have a part to send to the rear? Are you doing your own recording? I suspect there are a number of sound editing pc software choices that allow you to assemble surround tracks without getting into the weeds on phase shifting. |
#6
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
On Sep 27, 9:45*am, "jamesgangnc" wrote:
"ChrisCoaster" wrote in message ... On Sep 26, 8:00 am, "jamesgangnc" wrote: "ChrisCoaster" wrote in message .... If I read the Dolby Surround/Pro Logic white papers atwww.dolby.com correctly, a Dolby Surround encoded surround signal is simply a monaural track with the following modifications: One side is phase shifted "+90 deg." and the other, "-90 deg." The DPL decoder theoretically has horse-blinders on it that see signals of only this type and route it to the surround speakers. I would assume this is not the same effect as reversing the leads to one speaker/channel(which mosf of us here have heard and experienced), is it? If so, then how could this "+90 / -90 deg" business be achieved at home? And how could the phase of something be shifted plus 90 degrees anyway? Minus-90 I could see. -ChrisCoaster My limited understanding of the analog surround is that the delta between the two channels is sent to the rear. By phase shifting you are creating that delta.- Hide quoted text - - Show quoted text - ____________ Understood. *I'm just curious as to what a positive-90-degree phase shift means and how it is accomplished. *Obviously both channels producing this modified sound will not be in phase with either of the front speakers or the center, increasing its chances exponentially of its differential being routed to the surround channel. I'm just curious how it's done, that's all. *I'd like to try it at home if the equipment/technology isn't too expensive or demanding knowlege-wise. -CC You're wondering how you output a signal before it comes in. *Your logic is correct. *Shift everything else 90 degrees back. *What is left is 90+.. *Do you have the source in individual pieces such that you have a part to send to the rear? *Are you doing your own recording? *I suspect there are a number of sound editing pc software choices that allow you to assemble surround tracks without getting into the weeds on phase shifting.- Hide quoted text - - Show quoted text - ________________ All I know is that the kind of phase-shift employed in a surround signal is not as simply as "reversing the leads to one channel/ speaker". That means one "side" of that signal is still in phase with what the DPL(or DPLII) decoder is supposed to be sending to the front center(and/or mains). A track encoded for surround is typically a mono track split into two channels "equal in amplitude but 180 opposite in phase". I take that to mean that one side is shifted +90 relative to the fronts/center and the other side is shifted -90 relative to the fronts/center. This isolates both halves of that track from information intended for any of the front speakers, and creates the HUGE differential that DPL decoders look for. This prevents the decoder from sending a guitar that is panned 2/3 to one side - but in phase - into the surround speakers. My question remains: how simple is this to accomplish at home? A 6ch Mackie line out to my PC loaded with the appropriate software? That could be dangerous! -CC |
#7
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
In article
, ChrisCoaster wrote: If I read the Dolby Surround/Pro Logic white papers at www.dolby.com correctly, a Dolby Surround encoded surround signal is simply a monaural track with the following modifications: One side is phase shifted "+90 deg." and the other, "-90 deg." The DPL decoder theoretically has horse-blinders on it that see signals of only this type and route it to the surround speakers. I would assume this is not the same effect as reversing the leads to one speaker/channel(which mosf of us here have heard and experienced), is it? If so, then how could this "+90 / -90 deg" business be achieved at home? And how could the phase of something be shifted plus 90 degrees anyway? Minus-90 I could see. -ChrisCoaster If I recall correctly, encoding is dynamic compression, bandpass, and then addition to the left and right with opposite polarity. Decoding is the difference between left and right, bandpass, and dynamic expansion. The processed sound is removed from the left and right through simple addition and subtraction. A similar trick works for the center channel. Dolby Surround was made for movie sound effects. The expansion in the decoder protects some stereo signal but not always enough for music. Running normal music through Dolby Surround is hit and miss. It can sound very bad. -- Google is a pro-spamming service. I will not see your reply if you use Google. |
#8
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
"ChrisCoaster" wrote in message ... If I read the Dolby Surround/Pro Logic white papers at www.dolby.com correctly, a Dolby Surround encoded surround signal is simply a monaural track with the following modifications: One side is phase shifted "+90 deg." and the other, "-90 deg." The DPL decoder theoretically has horse-blinders on it that see signals of only this type and route it to the surround speakers. I would assume this is not the same effect as reversing the leads to one speaker/channel(which mosf of us here have heard and experienced), is it? If so, then how could this "+90 / -90 deg" business be achieved at home? And how could the phase of something be shifted plus 90 degrees anyway? Minus-90 I could see. If you have buffers and delay lines, it is easy to delay the reference channel so that another channel appears to "happen" before the reference channel. So the plus 90 is plus zero in real-time, and the reference is -90 in real-time, and the -90 is -180 in real-time. Of course, with recorded material, the phase lag may be days, months or years. The phase shift of my Sgt. Pepper's Lonely Hearts Club Band album is minus many millions of degrees, some 40 years from the original performance, which itself never occurred in real-time anyway. What I'd like to know is how you get a 90-degree phase shift with wideband audio. A phase-linear delay line that would cause a 90-degree phase shift at a 20Hz would cause much greater phase shift at 20KHz. |
#9
Posted to rec.audio.tech
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Encoding *anything* for Pro-Logic
"Chronic Philharmonic" writes:
[...] What I'd like to know is how you get a 90-degree phase shift with wideband audio. A phase-linear delay line that would cause a 90-degree phase shift at a 20Hz would cause much greater phase shift at 20KHz. Exactly. What you need is a filter with a different time delay at each frequency. When those time delays are 90 degrees, such a filter is called a "Hilbert transformers" (google for details). The key thing about a Hilbert transformer is that the magnitude is one and the phase is 90 degrees (or pi/2 radians) over the entire bandwidth. That's the ideal, analytic, continuous time version. You cannot make a perfect Hilbert transformer in practice (try delaying DC by 90 degress!). -- % Randy Yates % "My Shangri-la has gone away, fading like %% Fuquay-Varina, NC % the Beatles on 'Hey Jude'" %%% 919-577-9882 % %%%% % 'Shangri-La', *A New World Record*, ELO http://www.digitalsignallabs.com |
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