seeking opinions on out of phase audio signals

[JMackenzie]

Hi everyone. This is a bit of a weird one, probably simple to explain, but I am missing something in understanding. Several times a year our students take part in competitions or shows which normally involves most of them singing to backing tracks on CD or ipods. I do my best to persuade them to give me high quality copy of these tracks. Almost always they purchase Karaoke tracks , or buy similar things from the internet.Sometimes, however people will turn up with a CD of the track they want to sing, but it has also got the vocals on it. Then they ask "can you take the vocals off for us?. I say no, and they are disappointed. Serious use of EQ sometimes has an effect in reducing the vocal level.

Quite by accident one day , I was setting up a laptop to play sound through my PA, having bought a 3.5mm jack to XLR to plug into the stage box. I stuck on a CD to test it and it sounded a bit odd.

I knew the tracks well, and realised that the main vocal had gone from the mix but backing vocals were still audible. I realised that the stereo signals were out of phase and I was getting cancellation.

Last night , just before a competition started I was presented with the same problem- girls wanting to sing to a CD , which contained the original vocals. I set up the laptop and plugged my cable into the stage box-and it worked!! The main vocal had gone from the mix, just like before. They were so happy, and everyone else did not believe this was possible. What I dont understand is WHY the vocal vanishes but the rest of the mix seems to be there. It does lose some content , I admit, but it is a remarkable thing to be able to do so simply.

The cable is wired from stereo 3.5mm jack to screen on pin1 XLR, and the Left and Right signal to pins 2 and 3. This goes into a normal balanced input on the stage box and appears at the mixer as a single channel input.

Why does the whole signal not vanish?

I am sure I have heard of similar effecs with speakers out of phase, but nothing as dramatic as this.

I would value any opinons on this. As I say, I am missing something in understanding what is going on .

[Doug Siddons]

Its due to centre cancellation, the vocals are in the centre of the stereo mix so when the left and right are put out of phase it cancels the centre and the vocs disappear (ish!)

[Kevin Ross]

It is quite easy once you think about it.

 

By flipping the polarity of one of the stereo channels and then re combining them anything that was equal and in phase on BOTH channels will disappear leaving only the difference between both channels. As the main vocal tends to be panned down the middle it is one of the things that will disappear.

 

 

 

Not a pretty solution but one that works sometimes

[Peter F]

Its due to centre cancellation, the vocals are in the centre of the stereo mix so when the left and right are put out of phase it cancels the centre and the vocs disappear (ish!)

 

This picture (fingers crossed for my linking and pointing ability)

http://upload.wikimedia.org/wikipedia/commons/8/8a/Interference_of_two_waves.png

 

shows the cancellation effect.

Right hand side diagram is showing 2 signals 180 degrees out of phase with each other. Although most likely what you are hearing is a signal with polarity reversed on 1 side. This isn't the same thing, but in this case it's the same result. I am sure someone will be along shortly to give a far more detailed explanation or possibly point out a previous topic along the same lines.

 

Cheers,

 

Peter

[paulears]

A well know 'feature' - the only snag is that it is so unpredictable. try it on early Beatles or Beachboy tracks and even early big band music - very interesting results. The other snag is that many tracks have treated main vocals. So a centre image from the lead signer has stereo reverb applied. This has subtle differences between left and right channels, so the centre vocal is removed, but leaving the phantom reverb from a non-existent singer. Elton John tracks are often unusable because of this. Another problem is tracks where they have been double tracled to thicken vocals. In this case, a second, or sometimes second and third almost identical lead vocal is placed almost in the same place, but often slightly slipped in time - this is the only time when "out of phase" would be technically correct. Instead of the singer copying the first track as closely as they can, the studio just use the same audio file and delay it a tiny bit. This is then truly out of phase, and although different from the real identical(ish) track, the end result is similar. The cancellation effect here can only work with reversed polarity addition - people have called it phase for such a long time, that it's stuck - like the label on mixers that says phase, which isn't.

 

There are some useful plug-ins to help the cancellation around nowadays - most use some kind of eq to remove the lows and highs that might get in the way of successful cancellation, but none can remove vocals as a matter of course. Bringing in the tracks on a normal stereo mixer on two faders, both panned centrally, but with one channel with the polarity(phase) pressed will give you a little more control, as the cancellation can be maximised by adjustment of the two faders against each other.

 

When it works well, it can be very good - but you can never guarantee it - and this, most folk can't understand and the engineer gets the blame when it's just something the original mixer enabled, by accident.

[MarkPAman]

I'd just like to point out that this depends entirely on how the track was mixed in the studio. It sometimes works but more often doesn't.

 

You may end up with vocal effects but no vocal. Other parts may also be in the middle and disappear too. The low end can end up sounding strange.

 

Programs such as Audition have a centre channel extractor which uses this effect, but in a more sophisticated way, which may sometimes get you closer to what you're after.

 

I find that most of the time you don't get good results. If you get lucky that's good, but don't promise anything until you've had a listen!

 

 

 

E2A .........I must type faster :(

[JMackenzie]

Thanks guys, I do understand a bit better now. What about this then as a supplemental question?

If I take the same XLR to 3.5mm jack lead and put a XLR to mono jack adaptor on it, does the same thing happen, or does it have to travel through the balanced cable in the mullticore for the cancellation to happen? Remember its going into my mixer XLR input. Normally I convert the left and rights from a device from either mini jack to two XLR's or Phono's to two XLR's, and use convertors on the tails to put these into line inputs on the mixer. This other cable , being wired as a stereo XLR is something I had not seen before, and I cant think why such a thing exists. What could you plug it into to have any use with normal gear?

John

[MarkPAman]

Thanks guys, I do understand a bit better now. What about this then as a supplemental question?

If I take the same XLR to 3.5mm jack lead and put a XLR to mono jack adaptor on it, does the same thing happen

 

No

, or does it have to travel through the balanced cable in the mullticore for the cancellation to happen?

 

Have a look at the Blue Room Wiki on balanced signal. It's the way that the desk uses the signal that is causing this cancellation.

 

Remember its going into my mixer XLR input. Normally I convert the left and rights from a device from either mini jack to two XLR's or Phono's to two XLR's, and use converters on the tails to put these into line inputs on the mixer. This other cable , being wired as a stereo XLR is something I had not seen before, and I cant think why such a thing exists. What could you plug it into to have any use with normal gear?

John

 

"Stereo XLR" is not a normal thing, though I do have one or two in my "adaptors box" which I use with a breakout on the other end to two mono plugs of various sorts. This gives me quite a lot of options for unbalenced stereo adaptors & at a push lets me use a mic lead to make them longer. For a run of any length though, a pair of DI boxes is a much better solution.

[Kevin Ross]

They are not of common but you do see them from time to time. They carry an unbalanced stereo signal over normal 3 core cable with an XLR on each end. The problem comes when you try to plug it into an input that is expecting a balanced signal. It will take one of the sides to be the “cold” side of the balanced signal and flip its polarity and try to take it away from the hot side. You could make a short XLRF to 2x XLRM that would split the stereo signal back out into two XLRs to allow you to use it without the mixer trying to take one side from the other

 

Beaten to it by Mark, I make that 1-1, next one wins it!

[JMackenzie]

They are not of common but you do see them from time to time. They carry an unbalanced stereo signal over normal 3 core cable with an XLR on each end. The problem comes when you try to plug it into an input that is expecting a balanced signal. It will take one of the sides to be the "cold" side of the balanced signal and flip its polarity and try to take it away from the hot side. You could make a short XLRF to 2x XLRM that would split the stereo signal back out into two XLRs to allow you to use it without the mixer trying to take one side from the other

 

Beaten to it by Mark, I make that 1-1, next one wins it!

 

Well now, that explains it all perfectly. I did suspect that the mixer was playing a big part in this, and the Polarity "flipping" explanation makes it all the clearer. Thanks to you all for your comments.

 

Cheers,

John

[Shez]

One thing I might add to this is that it generally doesn't work on MP3s (or CDs created from MP3s). So much data has already been thrown away that if you try to process it further, it sounds exceptionally horrible.

 

I discovered this whilst trying to remove the vocal from a track for someone; I've done it that way for years with (in some cases) a good degree of success. (Extra tip - apply a HPF to one side of the signal - you won't loose all the bass then). I tried it with this one CD and twigged that it must have once been an MP3 because it sounded very strange, in that data compressed audio kind of way. I used the analogy of a jenga tower to explain it to her - the original tower is like a proper CD / wav - you can do a lot with it without any problems but the mp3 is like the version with lots of holes in it - as soon as you try to tweak it further, it all collapses!

[Chris Adam]

when I were but a lad, I did a bit of "tea boy" work in a studio, and did plenty of watching over shoulders. Many of the engineers used to record the vocal line phase reversed from the rest of the mix. When asked why, one engineer offered "it just makes the vocal stand out from the mix".

I haven't done studio work for years so the other reasons escape me, but thought I'd drop it into this topic. If I remember right, I might not even have got a decent answer. I certainly haven't ever used this technique live - only a bit of phase shift if it's needed with many mics in place.

[Killyp]

I can't really see why that would be an advantage TBH. Arguably, it could cause the spill from the vocalist's headphones to be cancelled in the mix and in theory, cleaning up the mix a little; but the sound from the headphones reaching the mic would have a timing difference anyway so this would be largely useless.

 

Sounds like a case of expectations overriding sense to me...