Any quick check for stereo phase alignment?

[cphollis]

For those of you who play stereo keys, a warning and and a question ....

 

I bought a four-channel Behringer powered DI to ensure I delivered a nice hot signal wherever I may be.  Nice piece of kit for your rack, except ...

 

As I'd go through the patches of my NS3C, many of them would thin, warbly and hollow.  Different behavior on different patches.  Slowly, it clicked that I had a stereo phase alignment problem, but where?  Ah yes, channel 2 of my new four channel DI had reverse polarity.  Thanks so much, guys!  I was looking for a multi-hour project ...

 

Easy enough to fix, but damn I lost a lot of time on this one.  I realized that random FOH might have this problem as well.   I've created a patch that sounds great in real stereo, but crippled when phase is reversed, which will be my "test" going forward.  Does anyone know of anything better?

 

Thanks!

[Stokely]

No, but I'm interested to see what people say.  I keep trying to go stereo because I can at many gigs, but the ones where I can't I really struggle with some sounds.  Mainly piano, truth be told.   I should just go mono all the time and to be fair I do keep patches pretty simple as far as fx and stay away from "this part to this side, this one to the other" programming.  It's mainly those stereo sampled pianos that have exactly that behavior you describe:  thin and crippled.   I've done gigs where I'm able to monitor in stereo while sending mono to FOH, and a couple times the sound engineer asked me to turn up my piano--in my ears it was loud and proud.

I'm hoping Pianoteq for ios might be better in this regard, not being sampled after all.  While I'd prefer to not have to rely on an ipad, I did it for a while with b-3x and the Modx makes it as simple as can be.

[Delaware Dave]

I just checked out that DI/mixer (assuming it is the DI4000) and see a phase reverse switch on each channel. Interesting, but don't know that I would ever use this feature, I don't recall ever intentionally wanting to reverse the polarity other than on an expression pedal when the keyboard requires a Roland type wiring but the expression pedal is a Yamaha type wiring solution (requiring the Ashby adapter solution to fix it).

[D. Gauss]

easy. send a pure sine wave to both channels. (phone app will do this if keyboard can't). if outta phase, you won't hear anything at all if the volumes/eq of both channels are set the same.

[D. Gauss]

1 minute ago, Delaware Dave said:

  phase reverse switch on each channel. Interesting, but don't know that I would ever use this feature, I don't recall ever intentionally wanting to reverse the polarity 

if you're recording a DI signal plus a Mic-ed amp signal on say a bass, often one is opposite polarity, so the switch let's you correct it.

[Reezekeys]

5 minutes ago, D. Gauss said:

if you're recording a DI signal plus a Mic-ed amp signal on say a bass, often one is opposite polarity, so the switch let's you correct it.

 

Also with drum miking. Engineers like to flip phase on channels when for example they have a mic on the top and bottom head of a snare, to see which setting sounds better.

[Delaware Dave]

7 minutes ago, D. Gauss said:

if you're recording a DI signal plus a Mic-ed amp signal on say a bass, often one is opposite polarity, so the switch let's you correct it.

 

Thanks for the info; this is not a scenario for me.

[stoken6]

Been there, done something similar: https://forums.musicplayer.com/topic/183187-lesson-learned-again/

 

Cheers, Mike

[jpfw]

Wow! its such a great day for audio..Scopes (and synths) used to, and still do cost a fortune but there are so many apps out there that give great functionality for next to nothing including phase inversions (question for  D. Gauss...Know anything about magnetic tape coercivity?)

[Baldwin Funster]

Here's a test. Give your stereo signal to the sound guy and listen back to the floor wedge when he just combines the 2 signals at the monitor send. I did that and had to listen to phased weirdness for the whole set.

[Nathanael_I]

There is absolutely a tool for this.  It's even built into most DAW's. 

 

https://en.wikipedia.org/wiki/Goniometer_(audio)

 

This is what audio engineers use.  You don't even have to hear the sound.  It tells you how mono compatible it is or isnt...

[Mr -G-]

8 hours ago, D. Gauss said:

easy. send a pure sine wave to both channels. (phone app will do this if keyboard can't). if outta phase, you won't hear anything at all if the volumes/eq of both channels are set the same.

In practice with amplified signals that might not be so drastic as to "disappear".  For example (using Audacity) generate a sine (or other shape) wave, copy it to another track and run the "Invert" command. Panning the two tracks in opposite directions should generate a perfect out-of-phase stereo signal. Yet, if the stereo audio is sent out to stereo speakers it can still be heard. The issue becomes critical when mixing to mono and in that case indeed the sound completely disappears, but when using speakers the sound bounces around on walls/furniture and the phase cancellation is not as noticeable (add to this that we have 2 ears!).  If you invert one of the channels back and forth (in and out of phase) there is a change in the sound spatial image, but it is still there.

Here is a link to a  nsmp sample (Nord instruments format) that shows exactly that.  If playing stereo you can hear it, if you press the "Mono" button (which mixes the sound internally) the sound vanishes:

https://www.norduserforum.com/nord-piano-grand-forum-f10/extremely-important-info-for-the-sound-of-piano-samples-t4823-10.html#p31734

[Docbop]

8 hours ago, Reezekeys said:

 

Also with drum miking. Engineers like to flip phase on channels when for example they have a mic on the top and bottom head of a snare, to see which setting sounds better.

 

Multiple mic's on one instrument there is always chance of phase cancelation to deal with and sometimes a bear to locate the culprit.