Topic: Phasing issues ... help from the knowledgeable ?

[cbd]

Ok folks, I have a conundrum I haven't faced before.

I am running 2 sets of B&K compact mics: 4022's and 4027's with each set feeding into a Sonosax and then into my 744. I generally run the 4027's ortf and the 4022's xy both sets in the same DPA mount

recently I noticed that when I mixed the 2 sources in Soundforge that the levels for the "matrixed" wav file went substantially down by 6-12 db and the resulting wav file lost almost all of its bass response. I friend suggested the 2 sets of mics might be out of phase and to try inverting one of the sources when I mixed them. SF has a button under Mix Special that allows you to do this and sure enough it fixed it right up.

My question is what could be causing this and is there a way to fix it?

? It seems odd that it would be occurring with 2 sets of the same brand of mics, 2 of the same pre-amp both feeding into the 744 and I have checked the left/right on all the cables. Now you could fill a thimble -- and a small one at that -- with my technical know how on such things so I thought I would come to the knowledgeable among us.  If it matters, one of the Sonosaxes lost a channel a few months ago and a friend of mine who does audio repair work got the schematics and some spare parts from Sonosax and fixed it for me..... Not sure if that could somehow be the culprit or not

[capnhook]

Sounds like phase reversal or comb-filtering.

[Gutbucket]

That's reverese polarity on one pair.  The big level difference with mics positioned that close together is the key indicator.  Phase is technically different, but is a commonly misussed term when refering to polarity.  Could be due to a swapped wiring issue somewhere, possibly in a connector, or the repair on the preamp, but I'm guessing.  Swap wires mics and gear around between channels to try and trace it down.  Correct polarity should show the start of a sharp transient waveform going up first when zoomed in on the editor display.  With reversed polarity it would go down first.

BTW, if the two pairs are positioned close to on another, you can get actual phasing issues leading to comb filtering when mixing them. That resuts from differences of arrival times to each mic pair which corresponds to the lenght of specific wavelengths canceling and reinforcing.  But that probably isn't the issue here with such a dramatic correction when inverting the polarity of one pair.  However best practice would be to move the two pairs far enough apart from each other to avoid that if you want to matrix them.  I know people matrix two pairs on the same stand all the time, and you might get away with it, but best to have them farther from each other.  One pair being coincident helps with this somewhat, in that you have 3 different but relatively close mic positions interacting with each other instead of 4.

[cbd]

I hate to say it but I find what you said very intelligent and no doubt correct but much of it is over my head and my feeble well of knowledge to draw from. If I wanted the guy who works on stuff for me to look at the Sonosaxes or mics and I told him I suspected "reverse polarity" on something and to check "swapped wiring" would he know what to look for? Also, the 4022's and 4027's don't allow the cables to be swapped as they are hardwired into the mics themselves. Unless you mean just switch between channels on the pre's and I could obviously do that and then check the wav files.



as far as repositioning that[s not really an option for me or a style change I'm willing to make. The majority of my tapes are made fob and I load both pair into the DPA CXO4000 Compact XY/ORTF mount:


no doubt there's better ways to do it I'm sure but for the functionality I need and to keep the peace on the home front I gotta keep it simple and fast

[page]

Unless you mean just switch between channels on the pre's and I could obviously do that and then check the wav files.

Since your dpas are hardwired, and they even specify the positive polarity pin on the specs, I'd be really surprised if the DPAs are out of polarity to each other. I'd see if there is a cable between the sax and the 744 that has the pins accidently flipped. You can test the entire circuit by picking one sax, hook up each set of mics and clap a few times (and then repeat with the other set). If the wave forms for the 4 test tracks all go up at the initial transient, they are in polarity relative to each other. Then keep the same interconnect cables and just switch sax units and repeat with just one set of mics. If the wav forms are different, it's the sax, if they are the same, switch the interconnects and try again.

If after all of that nothing comes back as the opposite polarity, it's either a placement issue in the field or when you did your combining of wavs, maybe shuffle the XY set "sooner" in time by say 30 samples or so using a delay VST. But I agree with the others in that shouldn't account for an almost 12db drop, maybe 4-6db though.

[Gutbucket]

Appologies for the techno-speak.  Page's post above should help you trace it down.  And to answer your question, Yes- your tech guy who works on the equipment should know what to look for if you suspected the signal wiring might have reversed polarity somewhere.

That DPA holder was actually designed to use one position or the other, but if it works for you to use both simultaneously then no worries. Functionality and keeping the peace at home are far more critical to ultiimate recording sucess.

Just to be clear, potential phase problems arrising from pairs of mics which are positioned near one another only arrise when the signals are mixed together, and even then there is no certainty that the problem would show up in every situation.  Yet if it is a problem, each pair listened to on its own would not have the issue, only the resulting mix of the two.

[morst]

the 744 is  4 clock-locked tracks - look at 'em in a wave editor, if the signal from one pair is upside-down from the other - polarity inversion issue.

Something is flipping your signal, you should definitely try page's troubleshooting procedure and let us know what ya figure out.

[dlh]

Just a suggestion:
Try recording directly on the 744 (without the sax).
It will tell you if the problem is with the sax or the interconnects.
My gut feeling is that it would be connections between the sax and the 744.
(Just something to try that's easy)

Dave

[cbd]

the 744 is  4 clock-locked tracks - look at 'em in a wave editor, if the signal from one pair is upside-down from the other - polarity inversion issue.

Something is flipping your signal, you should definitely try page's troubleshooting procedure and let us know what ya figure out.

that's definitely what's happening. when a friend first suggested what he called phasing as the culprit (hence the thread title) I compared a high peak at the same point on both sources and sure enough one peaked up and one peaked down. I'll try Page's suggestion on Sunday

Dave: since the 744 only has phantom to channels 1-2 I couldn't try it with all 4 at the same time. is your suggestion to plug in the 4022's and record some claps, then plug in the 4027s and do the same and compare?

[page]

is your suggestion to plug in the 4022's and record some claps, then plug in the 4027s and do the same and compare?

Craig; I'd bet you a beer if you plugged your mics directly to the 744 and repeated the test twice (once for each set) they will all have the same polarity. I think there is something between the DPA xlr housing and the 744 that is doing it. I've seen mics that had flipped polarity relative to each other, but it's really uncommon, and I'd expect DPA to have their act together for what basically amounts to measurement gear.

I think it's just tracking down if it's one channel on a sax box, both channels, or a cable out of the sax to your 744 but it would be a good idea to do a baseline test of just the mics>recorder.

[macdaddy]

That's reverese polarity on one pair. Phase is technically different, but is a commonly misussed term when refering to polarity. 

but in a wav editor, doesn't the "invert phase" action reverse the polarity..? (like to fix the polarity error on the ad2k+)

[scb]

If it matters, one of the Sonosaxes lost a channel a few months ago and a friend of mine who does audio repair work got the schematics and some spare parts from Sonosax and fixed it for me..... Not sure if that could somehow be the culprit or not

was the inverted waveform from the pair that went through this "fixed" sonosax?

[Gutbucket]

That's reverese polarity on one pair. Phase is technically different, but is a commonly misussed term when refering to polarity. 

but in a wav editor, doesn't the "invert phase" action reverse the polarity..? (like to fix the polarity error on the ad2k+)

It may well be called that in some wav editors, as it's a very common misuse.  Alot of equipment like channel strips on mixing desks have their polarity inversion switches labled phase, or phase invert though these days the Ф symbol seems more and more common.

Polarity is simple, it's either/or, positive or negative, one of two possible states.  Technically I suppose Polarity is a small, easily defined subset of Phase, which can have much more complex and varied relationships.  Phase isn't just either/or but can have infinite states of variability and often varies with frequency as well- which is why it isn't the best term to use when applied to hardware or software switches that simply invert the polarity of a signal.

[macdaddy]

thanks. this is what I was afraid of. I am using wave editor on a Mac; hopefully the invert phase function simply reverses the polarity...

[Gutbucket]

I think that's a safe to assume.