How to remove backing tracks from choir recording - phase reverse?

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How to remove backing tracks from choir recording - phase reverse?

Post by dickiefunk »

Hi,

I have a session coming up where I’m recording a choir with backing tracks. The tracks will be played through monitors at low level and I’ve heard about removing the backing track from the recording using phase reversal?

Could someone advise me how to do this please?

Thanks
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by Wonks »

It's going to be hard as I assume you'll be using more than one mic and more than one monitor?

You'll get different phase relationships of the sound picked up by each mic from multiple monitors, so you will probably have to treat each mic's recording separately and probably have to have a polarity reversed track mixed in for every monitor used. So for three monitors you'd need three instances of the backing track with slightly different time delays. The fewer monitors used, the better.

The monitors will also add in their own phase movement around the crossover points (unless DSP ones with a linear crossover) and add their own EQ signature on top of the direct track's sound, which will result in less than perfect phase cancellation. Plus you have the reverb sound to contend with

If you are going to add the choir back in over the backing track in the mix, and the monitor sound is very low compared to the choir sound, it may be worth trying the mix without trying to remove the monitors' sound first.
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by Hugh Robjohns »

You'll never remove the backing track completely with phase reverses, and you probably don't need to anyway. The spill just needs to be low enough not to cause problems in the mix.

I'd try to use just one monitor and place it either behind the choir (so their bodies soak up most of the sound before it gets to the choir mics), or at the base of your stereo mic array, keep the level as low as you can, and it might help to roll off the lows and highs if possible.
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by Wonks »

Not possible to have a conductor wearing closed-back headphones being fed the backing track and choir mic mix?
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by frankmahler »

Removing backing tracks from a choir recording can be a difficult and complex process, and using phase reverse alone may not be enough to achieve the desired result. However, here are the basic steps involved in attempting to remove backing tracks using phase reverse:

Import the choir recording into a digital audio workstation (DAW) software.

Locate the section of the recording that contains the backing tracks you want to remove.

Isolate the backing tracks using EQ or filtering techniques, so that only the frequencies of the backing tracks are audible.

Create a duplicate of the backing tracks channel in your DAW, and invert the phase of the duplicated track.

Play the recording with both the original and inverted tracks playing simultaneously.

If the backing tracks were recorded in phase with the choir, the two channels will cancel each other out, and the backing tracks will be removed from the recording.

If there are still remnants of the backing tracks audible after applying the phase reverse, you may need to try additional techniques, such as filtering or EQ, to further isolate and remove the backing tracks.
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by Wonks »

If you do try to do a phase reversal, then record a known section of the backing track on its own with the choir mics so you can balance the track levels correctly. Don't change the backing track playback level without recording another section of the backing track. It will be a lot easier to get the levels matching in the DAW without the choir singing.
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by Hugh Robjohns »

Yes, that's what I've done in the past with surprisingly good effect.

It requires a patient choirmaster and choir, though, because it takes two recording passes for each take: once with choir and backing track, and secondly with backing track only. The choir needs to stay in place and not move throughout the solo backing replay.

In post, you can then align the choir+backing track and the backing-only track, flip the polarity of one, and hope the reduction in backing track is sufficient for your needs. It speeds the post process up if the backing track has a sync mark (a click or tone blip) a few seconds in advance of the music start (the gap needs to be longer than the recording room's natural reverb!)
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by The Elf »

If you're using one mic/co-incident pair, then flipping the polariy of one speaker and placing the speakers equidistant from the mic(s) will reduce part of the spill. Then also using the null-pass trick described by Hugh above will reduce it further. By this time I find that the spill is at least as low (and in some cases better) than a single singer using a decent pair of closed-back cans.
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by Hugh Robjohns »

The Elf wrote: Fri Mar 31, 2023 4:16 pm If you're using one mic/co-incident pair, then flipping the polariy of one speaker and placing the speakers equidistant from the mic(s) will reduce part of the spill.

This idea works well in a dead studio control room where you only have the direct sound from the speakers to worry about. But in a reverberant space the reversed polarity concept goes to pot due to all the reflections with radically different path lengths. I've never made a two-speaker foldback arrangement work successfully in a reverberant space.
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Re: How to remove backing tracks from choir recording - phase reverse?

Post by The Elf »

Fair point. I've only used that technique in a studio environment.
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