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Author Topic: Is there a trick to balancing channel volume?  (Read 2580 times)

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Offline Gutbucket

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Re: Is there a trick to balancing channel volume?
« Reply #15 on: April 25, 2017, 10:16:04 AM »
You can also check visualy if the channels have flipped phase. This procedure does not apply to patterns with negative lobe, eg.  fig 8 or hypers. Find a short, independent clap, which is noticeable on the right and left channels. If the signal rises in the right and left channels, record should be in phase. If the signal in one channel rises and in the second goes down, the recording is out of phase.

With crossed figure-8's (Blumlein) you can identify what quadrant the sound arrives from using that method.  The transients will first swing positive in both channels for sounds arriving from the front quadrant, negative in both channels for sounds arriving from the rear, positive in the left and negative on the left channel for sounds from the left quadrant, etc.  Blumlein has equal sensitivity to sounds arriving from all horizontal directions.  Only level and the polarity relationship between channels changes with direction.

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I hope I use the correct terminology. Phase shift is a time delay of one channel over the other. It has an effect on the stereo image. Out of phase is flipped phase (flipped polarity) but no delay.

That terminology is popularly accepted.  Even though technically incorrect, it's common to refer to polarity inversion as phase-inversion or flipped-phase.  I always try to use the term polarity when referring to a positive/negative inversion (no time component, same inversion across all frequencies), and phase when referring to waveform alignment which tends to involve time delays and vary with frequency.  And it bugs me to see polarity inversion switches on gear labeled phase instead of polarity, due simply to widespread popular acceptance of the technically incorrect term.  Not too hard to print the correct word on label.  People will figure it out and that avoids potential confusion.

Note that in some special cases a true phase shift can have the same degree of shift across all frequencies and/or no time shift involved, but those are not the common phase-shifts we deal with ordinarily.  I won't go further into that unless someone is interested.
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Offline Gutbucket

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Re: Is there a trick to balancing channel volume?
« Reply #16 on: April 25, 2017, 10:30:37 AM »


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It's interesting to play the mono signal in the stereo speakers with flipped phase in one channel.

Yes, and what you'll hear depends somewhat on the microphone technique.  The primary common denominator is less perceived low bass when polarity is inverted in one channel.  Above the low bass region, you'll hear more effect from a polarity flip with near-spaced and coincident mic'd recordings than with spaced omnis.  That's because with spaced techniques the phase relationship between channels becomes randomized above a certain frequency based on the spacing distance, yet even rather widely spaced omnis produce signals which remain in-phase at very low frequencies.

Some folks are more sensitive to polarity/phase differences than others.  For some an out-of-polarity or significantly phase-shifted channel will sound "uncomfortable", for others it may sound "phasy", while other still may not notice anything different except for the effects on the low bass.

All that is based on the phase of the two signals (from two speakers) combining at the entrance to the ears.  The phase cancellations are far greater, and will be far more apparent if the signals are combined electronically before being reproduced.  The mono signal of the combined channels with the polarity of one channel flipped is the difference signal.  It is equivalent to the side-channel of M/S stereo.  Its also basically what is routed to the surround speakers from any surround matrix decoder when a two channel signal is played through it.  In fact, the old Halfner surround technique of wiring up an extra speaker or two by connecting it (them) across the Left and Right positive  speaker outputs of the amp and placing them in the back of the room is a simple difference signal extraction technique.  It can be highly effective on live music recordings that aren't primarily monophonic.  If you have a speaker or two and some wire laying around, give it a try.
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Offline JonG

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Re: Is there a trick to balancing channel volume?
« Reply #17 on: April 25, 2017, 03:25:26 PM »
Thanks for all the tips and info guys, I'll be messing around with things as time allows!

Thanks to anr for giving it a go and uploading your result, that's something for me to go off of, too.

Cheers.  :cheers:

Offline kuba e

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Re: Is there a trick to balancing channel volume?
« Reply #18 on: April 26, 2017, 03:01:49 PM »
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Note that in some special cases a true phase shift can have the same degree of shift across all frequencies and/or no time shift involved, but those are not the common phase-shifts we deal with ordinarily.  I won't go further into that unless someone is interested.
Nothing comes to my mind. Maybe electronic filter? I would be very happy to learn more.

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In fact, the old Halfner surround technique of wiring up an extra speaker or two by connecting it (them) across the Left and Right positive  speaker outputs of the amp and placing them in the back of the room is a simple difference signal extraction technique.  It can be highly effective on live music recordings that aren't primarily monophonic.  If you have a speaker or two and some wire laying around, give it a try.
Thank you for a good tip. This is a great cheap method to try surround sound. I was looking for it on the internet, a good search term is the Halfner hook. I'll check if my amplifier can handle this configuration and i will try it on the audience and onstage recordings.

Offline Gutbucket

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Re: Is there a trick to balancing channel volume?
« Reply #19 on: April 26, 2017, 07:06:30 PM »
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Note that in some special cases a true phase shift can have the same degree of shift across all frequencies and/or no time shift involved, but those are not the common phase-shifts we deal with ordinarily.  I won't go further into that unless someone is interested.
Nothing comes to my mind. Maybe electronic filter? I would be very happy to learn more.

Yes, a linear-phase "constant group delay" filter.  I didn't phrase it correctly above, it's not that there is no time shift involved, but that the time shift varies in a linear way with frequency so that the amount of phase-shift remains constant across all frequencies.  By contrast, a simple constant delay across all frequencies produces increasing degrees of phase-shift as the frequency rises.

https://www.allaboutcircuits.com/technical-articles/understanding-linear-phase-filters/

Here's a Voxengo plugin which can do that kind of phase manipulation- http://www.voxengo.com/product/pha979/

One common example in audio are matrix surround encoders, where surround channel information is processed with a 90-degree phase shift before mixing with the main front left & right channels to produce a matrix encoded 2-channel output.  +90 degrees for surround information mixed into the Left surround channel and -90 degrees for the same mixed into the right channel.  The surround decoder detects the phase shift and routes the phase-shifted stuff to the surround outputs.  If played back in mono, the left and right channels are summed, and the +90 and -90 phase rotated stuff cancels out leaving only the left, right and center info represented in the single channel output.


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In fact, the old Halfner surround technique of wiring up an extra speaker or two by connecting it (them) across the Left and Right positive  speaker outputs of the amp and placing them in the back of the room is a simple difference signal extraction technique.  It can be highly effective on live music recordings that aren't primarily monophonic.  If you have a speaker or two and some wire laying around, give it a try.
Thank you for a good tip. This is a great cheap method to try surround sound. I was looking for it on the internet, a good search term is the Halfner hook. I'll check if my amplifier can handle this configuration and i will try it on the audience and onstage recordings.

I wouldn't try it with a tube amp, but it works with basically any solid-state amp.  It's a lot of fun.  I used to put the  difference signal "ambience" speakers in the back of the room and orient them so as to reflect off the back wall or off the ceiling, which adds a bit of delay, increases diffuseness and lowers the level a bit so those speakers didn't stand out as obvious point sources but rather just filled the room with surround ambience.  Its mostly room reverb and applause that ends up back there, anything panned away from direct center will excite them somewhat, more so as the sound is panned fully left or right, maximally for the stuff that occurs in both channels but with opposite polarity.


JonG, my apologies for all this OT material in your thread.

« Last Edit: April 26, 2017, 07:09:51 PM by Gutbucket »
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Offline kuba e

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Re: Is there a trick to balancing channel volume?
« Reply #20 on: April 28, 2017, 04:44:47 AM »
I am sending an apology to JonG too.

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linear-phase "constant group delay" filter.

I've noticed similar things recently. I spoke with Moke about his recording of the Poway Symphony Orchestra. He made a records with cards and omni. When he mixed cards and omni together, it did not sound good. We tried to mix only LPF part of omni and HPF part of cards.
The result mix must have flat frequency transition between LPF and HPF when mixing together. First idea was : 1.Omni - make LPF 2.Cards - get HPF by subtracting LPF (HPF cards=cards - LPF cards). 3. mix it together. On this occasion I noticed that LPF and HPF cause a phase shift for individual frequencies (I'm not sure if this is a linear phase shift, I have to look more on theory). So it is not possible to get HPF by subtracting LPF from original signal. The conclusion was that we can use only "true" LPF and HPF with the same cutoff frequency and 6db slope. Filters with these settings creates a flat frequency transition and the phase shift is the same for both parts of LPF and HPF.

But if there is filter that can regenerate the LPF and HPF phase shift, then we could use the first idea with subtraction (HPF cards=cards - LPF cards) and we would not be limited by 6db slope of HPF and LPF.
Here is the original post:
http://taperssection.com/index.php?action=post;msg=2222325;topic=181444.0

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https://www.allaboutcircuits.com/technical-articles/understanding-linear-phase-filters/

Here's a Voxengo plugin which can do that kind of phase manipulation- http://www.voxengo.com/product/pha979/
Thank you for links.
« Last Edit: April 28, 2017, 04:54:27 AM by kuba e »

Offline JonG

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Re: Is there a trick to balancing channel volume?
« Reply #21 on: May 10, 2017, 03:56:31 PM »
No problem, guys! All the details are beyond me at the moment, but it definitely adds to the wealth of information that TS has, and I'm sure others will appreciate them.

After some more experience with a more recent show I'm working on, I want to thank you guys for turning me on to WaveLab. It's way more user-friendly (and thus powerful) for someone like me than Sound Forge, and I'm having way more success with it! I'll post some comparisons after everything's done with what I'm working on.

If anyone comes across this thread and is looking for tips on editing programs, WaveLab Elements gets a hearty recommendation from me.  :guitarist:

 

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