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Reversed Blumlein Channel
Old 1 week ago
  #1
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Thread Starter
Reversed Blumlein Channel

Recently, I recorded a piano quintet (piano and string quartet) with a Blumlein pair of ribbons. (Mic setup "C" in this thread: piano quintet )

I discovered today that I erred in my setup. I had the right channel ribbon rotated 180 degrees from the correct angular position. That is, the right ribbon, had the backside (instead of the front side) facing 45 degrees to the right.

So, the "back side" of the ribbon was facing toward (rotated 45 degrees of course) the players.

Should I invert the right channel?

I will try it tonight when I have the time. Just wondering if it is a correct idea.

Thank you.

DG
Old 1 week ago
  #2
Yes, you must invert that channel. Otherwise the sound is out of phase.
Old 1 week ago
  #3
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Thread Starter
Here are two snippets from the recording session.

The first is simply a re-post of option "C" from the original thread on the recording session, referenced in the first post. The second has the right channel inverted.

What differences do you hear?

To me it sounds like the corrected version is sort of collapsed. Almost the opposite of the first, which is much more "open", to my perception.

DG

P.S. I didn't do anything to the level. Simply inverted the right channel, and excerpted the exact same segment. Examining both tracks (original and 2nd one with rt channel inverted) in their entirety shows both still normalized to -1 DB.

SEE POST #7 . THANK YOU FOR POINTING OUT ERROR. PROPER STEREO FILE NOW POSTED,
Attached Files

C.mp3 (6.27 MB, 511 views)

C-InvRt.mp3 (6.27 MB, 491 views)

C-InvRt-STEREO.mp3 (6.27 MB, 88 views)


Last edited by dgpretzel; 3 days ago at 08:16 AM.. Reason: ADDED CORRECTED STEREO FILE
Old 1 week ago
  #4
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The “more open” sound you are getting is because you are getting a wider stereo image
because the sounds common to both channels are being cancelled out. Think L-R = “Side”
The corrected version has a much fuller sound
and esp. in the lower frequencies which are not
cancelling each other out. This gives more of a centered image. Think more of L+R=“MID”
Old 1 week ago
  #5
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The inverted recording sounds more natural/normal as far as balance and the room acoustic is concerned while the piano comes across too loud with the strings subdued when not inverted.
Old 1 week ago
  #6
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Effectively what you have in the first recording is a Blumlein pair aimed to the left instead of to the stage.
Old 1 week ago
  #7
1. Your original track (C.mp3) is in phase. No channel inversion needed.
2. Your C-InvRt.mp3 is meaningless, as it is mono. Invert or not means nothing.
Old 1 week ago
  #8
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Thread Starter
Thank you for your analysis.

1) Not sure of how things work, then. I definitely had the front of one ribbon pointing 45 degrees to the left. And the other ribbon, the REAR was pointing 45 degrees to the right. I thought the front and rear of a ribbon would be 180 degrees out of phase. Perhaps, I am mistaken.

2) I must have done (something else) wrong. I thought I split the stereo channels, inverted the right, and recombined to a new stereo track. I will have to revisit my work.

DG
Old 1 week ago
  #9
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jimjazzdad's Avatar
Quote:
Originally Posted by dgpretzel View Post
Not sure of how things work, then. I definitely had the front of one ribbon pointing 45 degrees to the left. And the other ribbon, the REAR was pointing 45 degrees to the right. I thought the front and rear of a ribbon would be 180 degrees out of phase. Perhaps, I am mistaken.
As piper succinctly pointed out above, by having the right hand figure eight reversed in a Blumlein pair, you have effectively just pointed the pair 90 degrees to the left rather at the stage. By simply reversing the polarity of the same right hand mic's signal, you are pointed at the stage again. Using a goniometer plug in in your DAW is very helpful for seeing what is happening.
Old 1 week ago
  #10
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Thread Starter
Ahah. Click.

The light came on.

The array doesn't "know" my intent. With the two mics at 90 degrees with respect to each other, it is always a proper stereo recording (in some direction).

And, to the extent that the two lobes are perfectly symmetric, reversing the polarity of one of the channels does just rotate the "target direction" 90 degrees.

Thank you.

I still need to review the mechanics of what I did when I thought I inverted the right channel.

DG
Old 1 week ago
  #11
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Yes, it’s correct that the array was in effect
“rotated” 90 deg but it is also true that the
effect was (when listened to over speakers)
to reduce the components of the music common to both mics ( in effect less MID) and leaving more of the the components not common to both
mics (in effect the SIDE) which gives a thinner (less bassy) sound with exaggerated stereo width.
Old 1 week ago
  #12
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Thread Starter
"exaggerated stereo width"

Yes, definitely what I perceived.

DG

P.S. Also interesting to me that there was such a present stereo effect. The array was centered on the quartet. For this session, the piano was (unfortunately) a little to the left of the quartet. So, looking at it as the array was actually set up, I was recording a stereo soundstage that would present to me if I stood where the array was, facing the quartet, and then did a left turn. Physicall, all the sound sources would be on my right. But, in the recording, I heard meaningful sound from the left, too. That must all have been from reflections, because the quartet, and some of the piano, would have been in one of the null areas. What an (unintentional) interesting learning experience.

Again, thank you all for illuminating me.

Last edited by dgpretzel; 1 week ago at 07:13 PM.. Reason: Added P.S.
Old 3 days ago
  #13
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Thread Starter
Please refer to Post #7 . Thank you for pointing out error. Proper stereo file now added above in post #3

DG
Old 3 days ago
  #14
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Did you monitor the recording and mono it ?
Old 3 days ago
  #15
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Thread Starter
Actually, I made the error in post. When I exported the corrected clip, I inadvertently had the "mono" option checked. Not sure how that happened. Pure pilot error (on top of the bad positioning of the ribbons in the first place. You can be sure that in the second recording session I did not repeat that error, I will now be forever paranoid about my ribbon setups. I guess that's good, though.

DG
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