Mid Side Technique & Phase inversion ?


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Jeremy R
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In the MICROPHONE section, it's explained that phase inversion can be a problem.

And in the CHOIR section, the Mid Side technique uses phase inversion.

How come it doens't cause any problem?

Isn't phase cancellation an issue if the stereo signal is turned to mono? Or does the blend with other sources (center and ambiance mics) solve the problem?

I assume I must be wrong somewhere but I'm confused!

miguel.roma
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M-S technique basics

M-S technique does not produce an stereo signal by itself (it is not like X-Y or other "pure-stereo" techniques in which one mic points to le left and the other one to the right). You cannot just pan M mic to one side and S mic to the other.

To get a stereo signal (left-right) from the M-S (Mid-Side) you have to combine them through a special decoder matrix:
L=a*M+(1-a)*S
R=a*M-(1-a)*S

where a is a parameter between 0 and 1 that defines, electronically, the equivalent angle separation between left and right channels (a=0 is equivalent to 180 degrees between mics, and a=1 is equivalent to 0 degrees. Values between 0 and 1 are equivalent to angles between 180 and 0 degrees).

The reason lies on the behaviour of the bidirectional characteristic of the S microphone, as it generates inverse polarity if the same audio source is on the left or on the right of the capsule.

When you have not the decoder matrix, you can perform a similar function with your mixer. You can add M and S channel to get the Left signal, but you cannot subtract them, as the mixer only permits to add channels.

That is the reason why you duplicate S channel, shift its polarity and add it to the M channel. This is likely to subtract two signals with an "addition machine" like the mixer.

This a very special case in which you need polarity inversion (some times named, not rightly, as phase reverse). This has nothing to do with the problems of phase cancellation you can have with polarity errors in cables, different distances between two mics used with one source, or even with stereo techniques in which left and right capsules are not in the same place (distant or quasi-coincident techniques).

Apologize for the little maths.

Hope this can help.

whoami
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I think that it is because of

I think that it is because of the miss adjustment or the distance of the mic that varies . Basically when recording in the Choir . From , my personal experience , the main reason behind that would most probably be the mic distance coverage that makes the whole difference in the recording process .

viki
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Well, I have been thinking of

Well, I have been thinking of the same thing! From what I have learned, it seems that the Phase inversion can really be detrimental to the microphones. But if that is the case, then why not there be a problem with the Choir section as the Choir section’s midsize technique, they very well use this phase inversion. It is highly paradoxical to me and makes no sense, to be honest! Please give a rational explanation!

gr8aires (not verified)
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M-S technique does not

M-S technique does not produce an stereo signal by itself (it is not like X-Y or other "pure-stereo" techniques in which one mic points to le left and the other one to the right). You cannot just pan M mic to one side and S mic to the other.

gr8aires (not verified)
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I weigh that it is being of

I weigh that it is being of the escape adjustment or the length of the mic that varies . Essentially whereas recording in the Choir . From , my private adventure , the leading logic rump that would most presumably be the mic range insurance that assembles the total discrepancy in the recording order.

abrahamybb
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Can't understand:

Can't understand:
Isn't phase cancellation an issue if the stereo signal is turned to mono? Or does the blend with other sources (center and ambiance mics) solve the problem?

AudioWorks
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MS Stereo

You have a recording of 3 vocalists on 2 mics (tracks). One faces a directional mic, the other two using one bi-directional mic facing left and right of the directional mic.

The way I understand the MS Stereo idea is by considering the side mic, which is bi-directional. When recording with a bi-directional mic, the sound from the two sides that are common on both sides will be out of phase and cancel each other out. That makes the side of the mic the null point. So if you take the one signal from the one bi-directional mic and patch it into another channel on your mixer, pan one left, one right and invert the phase on one of the channels, that would now make both sides in phase (remembering that bi-directional mics naturally have an out-of-phase element). The result of this created phase shifted condition means the sound of the vocalist on the left side of the mic that “bleeds” into the right side of the bi-directional mic will now not cancel out. The “M” mic and the naturally out-of phase element of bi-directional mics takes care of the canceling part. Think about this, when using both sides of a bi-directional mic, the right side of the mic has a “+ and a –“ factor. The left side of the mic has a”- and a +” factor using the null point of the mic as the divide point. If you invert the phase on say the right side, you change the “+ and a –“ to a “- and a +” but you still have the natural phase differences that come with recording with a bi-directional mic. You now have your “side” of the MS image but you are missing the phase effect of the “mid” factor.

Now here comes the “M” mic, another input on your mixer but up to this point, fader down. You are listening to the ONE bi-directional mic patched into TWO inputs on your mixer with the phase reversed on one input. One paned hard left. The other paned hard right. As you bring up the fader on the forward facing mic, you hear the vocalist on the center mics level increasing as the phase cancellation starts revealing itself by creating the illusion of “stereo”. It makes the other 2 vocals move hard left or right (the way they are panned) as the cancellation reaches 100%. Many say it is not “true stereo” but sure sounds like stereo. A big advantage to MS stereo is the fact you have 100% mono compatibility.

I think what makes MS stereo hard to wrap your mind around is the fact that bi-directional mics have a natural in phase and out of phase element to them. When you flip the phase you make what was in phase out of phase and what was out of phase in phase. AND, since we have two inputs we are dealing with we have both in and out of phase all at the same time. But, you do not hear the effects of phase cancellation until the “M” mic enters the picture or all 3 inputs are summed to mono.

If all this makes your head spin, it could be an out of phase condition and can be corrected by watching a dog chase it’s tail.

bruce23 (not verified)
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New Issue for me

it's one of new issue for me.. I'll visit this post very often to know the great answer

gorge
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MIC DISTANCE CAN BE A PROBLEM.

I am not use to with Choir recordings so far but in my experience too I think this may be the problem of mic distance in the recording studio. Mic distance plays a very vital role in depth of the sound produced during recordings. In my country many Nepali movies songs do not think about the mic distance and this really brings a lot of decrease of quality in the songs.

ticketshub
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Great article! Again the open

Great article! Again the open forum provides education, solutions and often more questions for any toic you dare to imagine. As an avid fan of all things related to music I found this article to be particularly useful. My friends and I have a plan to provide tickets or large than average business cards at our next gig. We plan to identify websites that provide a great source of information for our research for school assignments. Should you not realize, we are students undertaking degree in music industry. Well done and thanks!

Jon
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darren
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You can make and see changes

You can make and see changes by following this guide as it is. L=a*M+(1-a)*S
R=a*M-(1-a)*S. This is also the one use.
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angel1975
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Thanks Darren. I just tried

Thanks Darren. I just tried it and it really worked.

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