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Recording choir with backing track playing out loud?

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Recording choir with backing track playing out loud?

Postby Ian Back » Sat Feb 02, 2019 3:36 pm

I am due to record a choir in a few weeks time over a backing track which has already been recorded. I obviously want to minimise the spill of the backing track into the recorded choir. I have ruled out the possibility of recording with multiple headphones for practical reasons. Several threads I have read talk about a technique of making a recording of the backing track with the (silent) choir still present, and later inverting the phase to cancel out the backing track.

The music is pop/ballad style, and the choir approx 40 people. I will only have one session to make the recording, so there is no opportunity for me to trial different methods.
From reading around the subject, I am intending to use several spaced mics as close to the choir as possible (using cardioid SDCs), and play the backing track on a monitor speaker behind the conductor i.e. trying to use the quietest point of the cardioid pattern mic pick up.

But I have been thinking whether it would help to "simplify" the sound signal which I am trying to cancel out by -
    - playing the backing track through a single speaker in mono
    - altering the mix balance to emphasise drums, bass and synth/pad, and turning down or off any other instruments which are not needed for the choir to stay in time and in tune
    - removing any added reverb
    - using EQ to cutting off the highest (and maybe also the lowest) frequencies

Does anyone have experience of this technique overall, and do you think these additional measures will make any difference at all?
Thanks.
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Re: Recording choir with backing track playing out loud?

Postby Hugh Robjohns » Sat Feb 02, 2019 3:48 pm

Yes to single speaker replay behind the conductor. Yes to ditching the backing track reverb. And yes, the technique of a second playback recording with mute choir in place, mixed in opposite polarity can work extremely well.

H
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Re: Recording choir with backing track playing out loud?

Postby The Elf » Sun Feb 03, 2019 10:18 am

Also using two speakers, set to opposite polarity and equidistant from the mic's, can
help - in addition to the techniques already mentioned.
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Re: Recording choir with backing track playing out loud?

Postby Ian Back » Sun Feb 03, 2019 5:00 pm

Thanks for the replies. I decided that the trick of using 2 speakers with opposite polarity would only work if you use a single coincident pair, as other mics would not be equidistant. I was going to use multiple spaced mics to give better rejection of the backing track by being closer to the voices.

I am still interested to know whether it is worth cutting high frequencies from the backing track. My thinking goes like this - we are trying to reproduce the backing track on the second recording exactly in order to cancel out the original. Higher frequency audio is more likely to be affected by small differences between the recordings e.g. draughts of air, choir member standing 6" further back etc, and therefore may not cancel so well.

This is all theoretical though, and maybe makes no difference in practice?
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Re: Recording choir with backing track playing out loud?

Postby ManFromGlass » Sun Feb 03, 2019 6:05 pm

I’ve never been in this scenario before. So -
Is the reason backing track playback is required is so the choir will be in tune with the track?
Or are there other reasons?
I’m assuming the conductor will deal with accurate timing.

Thanks
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Re: Recording choir with backing track playing out loud?

Postby Wonks » Sun Feb 03, 2019 6:20 pm

Would it be possible for just some key choir members and the conductor to have headphones (open backed) to guarantee pitching and timing with the original recording?
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Re: Recording choir with backing track playing out loud?

Postby The Elf » Sun Feb 03, 2019 6:29 pm

Ian Back wrote:I decided that the trick of using 2 speakers with opposite polarity would only work if you use a single coincident pair
That's correct.

Ian Back wrote:I am still interested to know whether it is worth cutting high frequencies from the backing track.
I do think it is worth doing.
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Re: Recording choir with backing track playing out loud?

Postby Ian Back » Mon Nov 25, 2019 6:56 pm

It's a long time since I posted the initial enquiry, but I thought I would write an update for anyone interested in this topic.
I ended up deciding on a completely different solution - "Silent Disco" headphones. I heard about these from my children - apparently all the rave at university!
For about £150 I hired 70 sets of wireless headphones, together with an FM transmitter. The backing track was played back to the choir in the hall where they rehearse, and recorded with 2 x SDC mics (MC930) in ORTF. Sound quality through the headphones was quite decent, and certainly fine for the choir. No latency (that was noticeable). No backing track bleed was audible on the recording, and great fun was had by all!
Obviously, there's the cost to consider, but I strongly recommend this as a practical solution.
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Re: Recording choir with backing track playing out loud?

Postby Hugh Robjohns » Mon Nov 25, 2019 11:04 pm

:clap: :thumbup: Good thinking. Nice solution, and thanks for completing the story.
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Re: Recording choir with backing track playing out loud?

Postby John Willett » Tue Nov 26, 2019 10:49 am

Ian Back wrote:It's a long time since I posted the initial enquiry, but I thought I would write an update for anyone interested in this topic.
I ended up deciding on a completely different solution - "Silent Disco" headphones. I heard about these from my children - apparently all the rave at university!
For about £150 I hired 70 sets of wireless headphones, together with an FM transmitter. The backing track was played back to the choir in the hall where they rehearse, and recorded with 2 x SDC mics (MC930) in ORTF. Sound quality through the headphones was quite decent, and certainly fine for the choir. No latency (that was noticeable). No backing track bleed was audible on the recording, and great fun was had by all!
Obviously, there's the cost to consider, but I strongly recommend this as a practical solution.

Good idea, but you *have* to have analogue FM headphones - a digital system will have latency.
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Re: Recording choir with backing track playing out loud?

Postby Sam Spoons » Tue Nov 26, 2019 10:56 am

Actually wouldn't latency only matter if the choir were monitoring their voices through the fb cans, which they probably weren't? But, the few 'silent disco' rigs I've come across have all been FM.
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