Stereo micing phase issues

No replies
hogiewan
User offline. Last seen 3 years 33 weeks ago. Offline
Joined: 2007-05-02
Posts:

In the world of DAWs, does it matter if two mics are out-of-phase? Since it is easy to look at the waveforms and slide the audio into phase, shouldn't we worry more about getting the right sound from each mic?

philicorda
User offline. Last seen 1 year 46 weeks ago. Offline
Joined: 2006-05-16
Posts:

This can work when recording a single source, like two close mics on a guitar amp, but when ambient recording a collection of instruments like a drum kit there can be problems.

This is because you have to pick one of the drums to judge the phase by, say the snare.
When you align with the snare you change the phase of the other drums and cymbals that are different distances from each mic. So by aligning for one drum you tend to compromise the others.

Also, any ambient reverb is also phase shifted, which can do odd things (more comb filtering etc). This is why delaying or advancing a track is not the same as changing the mic position in the room.

Still, if it sounds good, do it. There is no such thing as true stereo without some phase differences (otherwise it's panned mono). I think that moving the mics leads to less weirdness in general though.

hogiewan
User offline. Last seen 3 years 33 weeks ago. Offline
Joined: 2007-05-02
Posts:

I was talking about single source micing. I guess shifting the reverberations may cause weirdness, but I don't think it would be that big of a problem.

Obviously you can't do it with multiple sources like a drum kit.

roaldz
User offline. Last seen 1 year 39 weeks ago. Offline
Joined: 2008-11-12
Posts:

Can't the rotate-phase button on top of the mixer track help you out? I heard something about a Hilbert Transformer which rotates phase by 90 degrees. Can that be helpful to you?

John E
User offline. Last seen 15 weeks 4 days ago. Offline
Joined: 2006-10-27
Posts:

Posted by Hogiewan:
" In the world of DAWs, does it matter if two mics are out-of-phase? Since it is easy to look at the waveforms and slide the audio into phase "

It depends how "out of phase" they are. If they're exactly 180 degrees out of phase the most likely explanation is a wiring discrepancy. This kind of discrepancy can usually be corrected electronically with great success. Sliding however would worsen this kind of error, not make it better so if your problem is a wiring discrepancy the answer is "no" you can't correct it by sliding.

OTOH if the phase discrepancy isn't exactly 180 degrees, the most likely explanation is a time displacement between the two microphones. You might think that you could correct this by sliding one of the signals but you can never correct it properly. The reason is that as well as the wanted signal, microphones also pick up unwanted signals from extraneous sources (reflections are the best example). It cannot be guaranteed that these reflections will hit each mic with exactly the same time difference. It depends on what direction they're coming from. Reflection 'X' might arrive at mic A first and mic B a little later, whereas reflection 'Y' might arrive at mic B first. Temporal displacements like these cannot be corrected by simple sliding which will always introduce artefacts. You might be able to compensate enough to produce a subjective improvement in the sound but you can't compensate enough to produce a stereo recording. If you're looking for a true stereo recording (rather than what is called "pseudo stereo") you can only produce true stereo using a coincident pair and (most importantly) the pair must be coincident for all captured signals.

In other words, getting the mic placement right in the first place is the key to success. Getting the placement wrong and then trying to correct it later by sliding might produce an improvement (if you're lucky) but in most cases it will make the sound worse.

thorgal
User offline. Last seen 1 year 21 weeks ago. Offline
Joined: 2007-08-03
Posts:

much easier to work with mono sources but I guess, it depends on what you want to record and produce in the end ;)