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Author Topic: how to do a digital 90 degree phase shift for all frequencies?  (Read 7154 times)

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Offline Gutbucket

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how to do a digital 90 degree phase shift for all frequencies?
« on: December 16, 2008, 12:12:10 PM »
Looking for ideas and advice for taking a mono sound file and performing a 90 degree phase rotation across all frequencies.  Are functions for doing that commonly implemented in DAW software (such as Samplitude) or via an available plugin?

I'm very slowly coming to terms with Samplitude but my experience there is still meek.

[edit to clarify title]
« Last Edit: December 21, 2008, 01:55:21 PM by Gutbucket »
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Offline Charlie Miller

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Re: how to do a digital 90 degree phase rotation?
« Reply #1 on: December 16, 2008, 12:28:23 PM »
Looking for ideas and advice for taking a mono sound file and performing a 90 degree phase rotation across all frequencies.  Are functions for doing that commonly implemented in DAW software (such as Samplitude) or via an available plugin?

I'm very slowly coming to terms with Samplitude but my experience there is still meek.

George, I can talk you through it if you call me. It's easier than typing.

Do you still have my number?

edit:
actually, I only know how to do 180. Never tried to do 90.
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Re: how to do a digital 90 degree phase rotation?
« Reply #2 on: December 16, 2008, 12:50:52 PM »
If I understand you correctly.... (I'm thinking phase shift rather than rotation maybe) imagine you had a constant 1000hz sine wave.  Moving it left 90* would require chopping .00025 seconds off the front.  Doing it to some other frequency would require a similar feat, chopping 1/(4*freq) seconds off the front.  Doing it on a complex wave would mean breaking it down into a variety of frequencies via FFT or equivalent, and then shifting them all by their individual 1/4 waves, and recombining them.  Yuk.  I'm not aware of anything that does this.  Is this what you are talking about?

If you are asking "how do I take a MID signal and turn it into a SIDE signal", that's another story, and I'm not sure how to do that either.
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Offline JasonSobel

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Re: how to do a digital 90 degree phase rotation?
« Reply #3 on: December 16, 2008, 01:12:37 PM »
there are many VST plugins that will shift all frequencies by a specified (arbitrary phase).
just google "phase shift vst" and many come up.  some free, some not free.  I haven't used any, so I can't recommend a specific one.

however, all of the Voxengo that I have tried are great.
http://www.voxengo.com/doc/pha979/
but their phase shift plugin is $40.

this link lists several free phase shift plugins:
http://www.audiomastermind.com/browse-phase_shifters-5887084-1.html
if you find a good one that you like, please post back here and let us know.

Offline Gutbucket

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Re: how to do a digital 90 degree phase rotation?
« Reply #4 on: December 16, 2008, 03:16:14 PM »
there are many VST plugins that will shift all frequencies by a specified (arbitrary phase).
^^^
This is what I'm trying to do.  Thanks for the links Jason, I'll look into them.

Joe, your are correct that a simple time shifting technique will create a varying phase shift that is frequency dependant instead of a constant shift across all frequencies.

Charlie, thanks for the offer but I think you have me confused with someone else.  I'm Lee.

-------------------

If anyone is curious, I want to try this for decorrelating a single mono signal to create two signals.  If any of you are aware of good alternate techniques for decorrelating a single mono signal to two signals please let me know.  Maybe some 'pseudo stereo' techniques could work?

I've been doing a bunch of 4 channel surround recording by adding a second stereo pair consisting of a center and rear pickup to a standard left/right stereo spaced or baffled omni pair. Playing the files back on a typical 5 channel playback system, I currently split the mono rear feed to the two (or more) rear surround speakers.  The primary thing I want to do is decorrelate the single rear surround signal to create discrete left surround and right surround signals in an attempt to defuse the ambient surround information around the back half of the room.  I'm not concerned with imaging back there, instead looking for an open ambient 'feel' of the space.  Sometimes it's a issue of trying to make the sound of the occasional loud clapper that was right behind my rig, more spread out and less localized right at the rear speaker, so as not to draw attention to the speaker.

One simple way to decorrelate is to duplicate the mono rear signal and flip the polarity of one of them, but that means one side is 'in phase' with the front 3 channels and the other 'out of phase' - less than ideal.  Rotating or shifting the rear signal 90 degrees before splitting them and flipping polarity makes them out of polarity with each other, but symmetrically 90 degrees out of phase with the 3 main front speakers. Delay and eq could be used to decorrelate the mono signal but seem like less than ideal solutions that might just make for comb filtering phasy weirdness. 

Part of the reason I'm looking to do it this way is that as I understand it, this constant 90 degree phase shift over the whole spectrum is a technique used by the matrixed Dolby surround versions to decorrelate a mono surround signal to two speakers.  Secondarily, even though my primary goal is a discrete 5 channel output, I'd also like to play around with DIY matrix encoding to stereo for playback on a typical surround receiver. I'm imagining folding the mono center and mono phase shifted surround signal into the right/left signal to create a Lt/Rt matrixed stereo output that might decode reasonably well on a DPLIIx, Circle surround, Logic 7, or DTS NEO6 receiver.  I'd guess there are VST surround tool plugins for doing matrix encoding but that phase is further down the road and I haven't yet looked into it.
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Offline Charlie Miller

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Re: how to do a digital 90 degree phase rotation?
« Reply #5 on: December 16, 2008, 09:17:18 PM »
Charlie, thanks for the offer but I think you have me confused with someone else.

Whoops, I do that a lot.
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Offline boojum

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Offline Gutbucket

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Re: how to do a digital 90 degree phase rotation?
« Reply #7 on: December 20, 2008, 02:05:42 PM »
GB - Have you tried this one?  It seems to do what you want.
http://www.tritonedigital.com/product_info.php?cPath=25&products_id=33&osCsid=507390e1aa02f32bb335ed5f0cf9a33e
^^^
That one seems to be frequency dependant and so wouldn't shift all frequencies by the same angle.  In that way it seems similar to standard time based phase tools and wouldn't do what I'm looking for.

The Voxengo PHA-979 plug that Jason linked to looks like just the ticket and should do exactly what I'm trying to do.  I need to get ahold of a multi-channel sound card over the holidays before I can try the demo version.  The question then is if doing so really does what I want.  The free BetabugsAudio PhaseBug I found at his other link might also work.

..If you are asking "how do I take a MID signal and turn it into a SIDE signal", that's another story, and I'm not sure how to do that either.

Not my original question, but actually I've considered something like that for a similar application.  I have two of my 4 channel recordings where the center and back stereo pair is fine, but the primary left and right pair is not (one had a preamp battery run low and distort, the other was operator error, left 'hold' on in rec/pause).  So I have a couple center/back only recordings I'd like to wrangle into L/R stereo.  I've thought about doing the 90 degree shift on the back channel to feed the side signal input of a M/S decoder. The back channel subjectively sounds a lot like a 'side' signal.  No idea how well it would work though until I try it.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline Gutbucket

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Re: how to do a digital 90 degree phase rotation?
« Reply #8 on: December 20, 2008, 04:38:46 PM »
Posted the questions on the Voxengo PHA-979 forum to see what Aleksey others users think.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline Gutbucket

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Re: how to do a digital 90 degree phase shift for all frequencies?
« Reply #9 on: December 21, 2008, 02:06:05 PM »
Aleksey (the Voxengo developer) suggested that my approach seems sound and his PHA-979 should work, but also suggested trying a symmetric 50 degree phase rotation on each surround channel split (positive to one side, negative to the other) instead of 90 degrees and perhaps try adding some shifted front channel to each surround as well.

I also asked about converting the surround signal into synthetic side signal to M/S my center+back orphaned recordings.  No real advice there, just try it & see.

Links to his answers on his forum-
http://www.voxengo.com/forum/pha979/1985/
http://www.voxengo.com/forum/pha979/1986/
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline boojum

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Re: how to do a digital 90 degree phase shift for all frequencies?
« Reply #10 on: December 23, 2008, 05:42:03 PM »
GB, always the bleeding edge of recording experimentation!  Hats off to you.    8)
Nov schmoz kapop.

 

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