Reverse phase trick for recording!

Hi all

Someone was telling me about a method for recording where you can monitor using speakers(instead of cans) in the live room whilst tracking, then record the backing track in the space afterwards at the same levels etc. you then  phase invert this and play it back with the recorded track + vocals/whatever to cancel out and leave you with whatever it was you were recording.

Sounds like it could be a useful technique but have yet to try it. Does anyone use/know of this technique and have any tips to share?

Cheers

Chris

Never tried but I think you should keep the environment intact (including the singer standing there) when doing the second round. Maybe not necessary, don't know how well this method works.

Yeah, had wondered what would happen if the singer/singers were out of the room when recording just the track, due to the change in acoustics.
Planning on trying this technique in a few days as it would certainly be useful if recording a vocal ensemble etc.

Thanks for the reply

Chris

I don't see this working in any practical sense

however what WAS done and I have done this and it works, up to a point... is to take a pair of speakers like NS-10's or whatever and put them up on stands at singer's head height - make an equilateral triangle roughly 4' on a side with the two speakers and the mic at the vertices - mic pointing away perpendicularly from the base of the triangle opposite...  you can play with the pattern on the mic...  you'd think a super cardioid or hard cardioid mic would be better than a sift cardioid or omni and it should but side lobing on some of the more directional mics is not always even and symmetrical, so it can get tricky - feed a MONO cue to speakers and have speakers be out of phase from each other...  speakers should match reasonably well in response and gain...  have a couple wide dead gobos up behind the singer.  If you have things set up reasonably well you should be able to crank up the speakers with cue and foldback and have enough of a null at the mic so feedback is not instantaneous...  wiggle things around and play with the setup and you can get a decent enough cue level for a single to sing to without cans...  and generally the singer won't need so much of themselves in the cue mix as with headphones, unless they have Moreme's disease.  At some point you get the gains up too high and the movement of the singer's face gets to be a problem but back off a bit distance-wise and that gets better...  this does not work with people who can't sing at volume and have crappy up close mic technique - especially if they have unsymmetrical faces...  (joke that last bit) anyway...  this works in smaller rooms that are pretty dead and larger rooms where you can get some distance from reflections...  it's a gimmick...  but if you have someone come in with a Bouffant and no in-ear's then this may do the trick...

another trick - this is the Grateful Dead Wall of Sound Mic Trick is to have two well matched omni's taped to a spacer like a block of wood (for temp playing - the dead used metal project boxes and had the mics combined in the boxes) with the capsules about 3" apart...  take the two mics and sum them with phase on one flipped...  set gain to null out noisy back ground...  put singer in room with band...  give him/her a side fill if he wants no headphone...  singer singers close up on one mic (typically the top one) and you may need to tweak gains to re-null with face present. 

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You'll get a washy Europe '72 sounding vocal and the singer can sing with band in same room - this is also a gimmick but kinda fun once


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    Have a Day

I've done something similar to Klett's method above a few times.

Mono the mix, then flip the phase / polarity of one of your monitors, so there should be a mid point where cancellation is best. Then with headphones find the best null point with your vocal mic - hypercardioids are easiest to position.

You can go quite loud without a great deal of spill.

I'm not sure about the track-then-cancel approach. You certainly couldn't compress whilst tracking.

Hi,



  I have successfully done this with vocals. You HAVE to leave the monitor level at exactly the same. It only works up to a point, and the vocal can seem a tad hard afterwards. I used it to cancel the backing track from a pa whilst recording a 300 strong school. getting a "control" pass with no singing was the problem I tried one pass with the school present, and one with the children out of the way. The pass with the kids cancelled much better(same room acoustics), but you obviously double the bumshuffling!



    Kindest regards,



    ANdyP


     

Adding to what Klett said, it was common to use two matched omni's in Nashville 40 years ago, one above the vocal mic, above the singers head.  Flip polarity, lose a lot of the band.  I imagine the volume levels are pretty low in that case. 

I have used speakers to re-create bleed for punch repairs.  Acoustic and vocal kept as final, replace a word, pipe the guitar back through a speaker and play with voice mic position until you get a match, and don't notice the guitar sound changing at the punched word or phrase. 

Thanks guys for all the input, some interesting ideas there. The main problem recently has been with tuning when doing group vocals  using cans and hence trying to find alternatives and make people more comfortable. This hasn't been a problem tonight though and we've had a great session. I'm still interested in trying out some of the techniques suggested at some point in the future though. Thanks again.

Chris

I've done it lots of times. It works... partly.

Last time I did it was 12 months ago, when I had to record about 8 passes of a small kids 'choir' and mail the files on.

I used speakers and a pair of U87's. NOTHING was moved, and we stacked passes. Then the kids stood there, while I recorded a silent pass.

I then sent them the 8 passes plus the silent pass, then I sent them one 'bounce' version with 4 passes in + polarity, and 4 passes in - polarity, summed equally.

I've played with lots of cancellation options using this method, and -if you're stacking things like strings or kids, it certainly reduces bleed.

'Eliminates'? -No. -Not even CLOSE. but reduces? -Yes. Very much so.

HOWEVER:

The cancellation is limited to longer wavelengths, since air convection patterns in a moderately large room, TEMPERATURE DIFFERENCES between passes... (-yes... they DO matter!) and various other factors all conspire to reduce the effective cancelation.

-On the subject of temperature; you can easily demonstrate it by quickly recording two short passes back to back in an empty room, then adjusting the temperature one way or the other, and recording two more passes... then allowing it to return to the original temperature and doing two more.

...Then try canceling the first pair. -It'll cancel fairly well. So will the second, and so will the third. But the first and the second wont cancel, nor will the second and the third, but the first and the third should cancel quite well. Also, if the room is significantly ventilated, air currents will affect cancellation.

Similarly though, if you stack in like-polarity, the low end of the speaker foldback sums strongly, the midrange progressively less so, and the high end doesn't sum much at all. You can sum half a dozen passes and observe this cumulatively.

Now, flipping the polarity of alternate passes DOES indeed cancel, but -as the exact converse of that last quoted example, the LF cancels VERY well, the midrange cancels a little less well, and the HF doesn't cancel all that well at all.

Lessons to draw: -When using speaker foldback and polarity-canceling later on, try to keep the speakers slightly 'dull'-sounding. The less HF you can spray into the room, the less will remain.

Because of the multiple-wavelength distances involved, shorter wavelengths are progressively more easily 'shifted' by any one of several factors: reflection (from moving singers/players etc.) convection, air current speed/direction and air temperature... and even pressure I suppose, although I'd only expect to a TINY degree, and insignificant in practical terms.

There are a few great suggestions already in this thread; 'emrr' gives a great idea for making 'patches' less obvious. -But try to maintain a sense of what's reasonable. -There's no 'magic bullet' to make speaker spill completely disappear; but you CAN make it ALMOST disappear at the low end, suppress it a bit in the midrange... and the rest is up to you.

I also did the 'out-of-phase auratones' trick years ago... but be aware that while the PRESSURE component of the wave between the two will cancel, the VELOCITY component is actually REINFORCED by doing this. An omni mic between two out-of-phase speakers will produce a significant dip in level. A Figure-8 mic int he same spot will record an INCREASE in level. -A cardioid probably won't hear much difference in level, (though the tone might change a little).

Nothing's completely simple, nor is anything absolutely foolproof. -But for occasions or locations where headphones are not an option -whether due to the number of singers, or a single insecure vocalist who's scared of headphones- there ARE some tricks. -None of them are perfect, but some of them may help. One of the most educational things to try can be listening to -and metering- the different results from pattern-swapping a multi-pattern microphone, midway between two "out-of-phase" speakers setup facing each other. Set it up and try it sometime.

Keith

Good observations Keith!

Have done it couple of times. In a damped room with little early reflections it worked really well, in a room with more reflections not so well.

Everyone, thank you again for your input. This forum kicks ass........  8)

Chris

You might also consider using a less sensitive microphone that inherently lacks the detail of distant information but sounds good up close to the source. 

I once did a record for a band whose singer was amazing... until she put on headphones, at which point she got totally inhibited, and started 'thinking' about what and how she was singing.... really lost the vibe.  Live (or just in the room with no phones) she was fantastic, however.

So I brought her in the control room, no headphones, just monitoring off of my mid-fields, I put up an SM7 for her, and just let her go.  When I solo'd the track back the only time you could really hear anything other than her vocal was when she wasn't singing, which was easy to edit out.  A LD condenser would have picked up much more bleed, but the dynamic SM7 not only sounded great (I love 'em for chick rock vocals!), it also rejected most of the room sound, so I could have the volume up surprisingly warm to where she felt really comfortable belting along with it.

Lastly, a little bleed doesn't necessarily concern me anyway... in fact, it can really be the 'glue' that gives a production a nice, exciting, live feel.  Just make sure that anything you track using this method is done while monitoring previously recorded elements that will make it to the final mix.  When bleed is minor (like I have described here) it can be almost impossible to identify it in the final mix... unless something has bled in that isn't elsewhere in the mix -- then a tiny smidgen can be surprisingly vivid! 

JC

rascalseven said:

...but the dynamic SM7 not only sounded great (I love 'em for chick rock vocals!), it also rejected most of the room sound, so I could have the volume up surprisingly warm to where she felt really comfortable belting along with it.

Lastly, a little bleed doesn't necessarily concern me anyway...

JC

Click to expand...

well said, jc! i made the same experience.

that is actually how the good old paul mccartney used to record vocals. who wants cans?

also try figure 8 with the speaker in the null point.

The other trick you can do is to use a single speaker - highly directional if you can - and then use a figure of 8 mic with the dead side facing the speaker - works quite well if you keep the volume down and work the mic close.

Mike

madswitcher said:

... use a figure of 8 mic ... work the mic close.

Click to expand...

Good luck with that...

Figure-8 microphones (and multipattern mics switched to fig-8) DON'T like to be worked close, in my experience.

rascalseven said:

Lastly, a little bleed doesn't necessarily concern me anyway... in fact, it can really be the 'glue' that gives a production a nice, exciting, live feel.  Just make sure that anything you track using this method is done while monitoring previously recorded elements that will make it to the final mix.  When bleed is minor (like I have described here) it can be almost impossible to identify it in the final mix... unless something has bled in that isn't elsewhere in the mix -- then a tiny smidgen can be surprisingly vivid! 

JC

Click to expand...

I'll go one further.  I once remixed an indie rock track that the band had tracked themselves which was picked up by a label.  The singer had tracked in front of the nearfields with a 58 and tons of spill, particularly between words when the compressor let go.  Naturally I decided this was "messy" and edited out the spill (I was young).  Spent a day mixing and wondering why my technically much better mix was lacking the excitement of the demo....you can see where this is going.  I deleted my edits and in fact accentuated the compressed room spill and instantly had back the rush and excitement of the demo while keeping my refinements.

I have tracked in front of blazing nearfields with a condenser too but you need a loud singer.  Did this for the Frank and Walters - do you remember them Keith?  They did some work in Parr Street back in the day, loved the place.

I tried the out of phase speakers once ever, I stepped in to check it for the singer and instantly felt sick.  Terrible.

Cheers,
Ruairi

The Frank and Walters.

Yep!

I think it might have been about when I left, but I know they were there.

Keith