Encoding *anything* for Pro-Logic

[ChrisCoaster]

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

[jamesgangnc]

"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.

[ChrisCoaster]

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

[Chris Hornbeck]

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

[jamesgangnc]

"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.

[ChrisCoaster]

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

[Kevin McMurtrie[_2_]]

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.

[Chronic Philharmonic]

"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.

[Randy Yates]

"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