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Four channel recording with two figure 8 mics ???
#31
13th November 2011
Old 13th November 2011
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Quote:
Originally Posted by John Willett View Post
Agreed - the Blumlein set-up is XY figure 8s.



This was told to me in a lecture a whie ago and we were viewing the original films of Alan Dower Blumlein doing the stereo "walking and talking" tests.

But I said that he was using two fig.8s in MS which matrix to XY fig.8s.

In fact I would be surprised if he didn't prefer this method, as you have a mic. pointing directly at the centre of the action and this matrixes to perfect "Blumlein" - you also have the added advantage that if you alter the side level the pattern widens or narrows, but without any change in the shape of the figure 8 pattern.
Guys, OK, help me out here. I am working with a Pearl DS 60. Four cards which can be used as the Blumlein MS by flipping one channel for the fore and aft and one channel for the beam to beam (LR). Fine. It works, I have tried it. Now what I read says this decodes as a true Blumlein array. And if what I have read here is correct, I can widen or narrow the sound field without changing the shape of the figure 8's I have created out of the four cardioid channels. I am assuming I do this by varying the gain in the fore and aft simultaneously or in the beam to beam simultaneously. Is this assumption correct?

This would allow me to almost crowd the soundstage with the Blumlein MS and then pump up the gain on the beam to beam (LR) channels to effectively widen the recorded soundstage, or am I all at sea here? Reducing the LR signal would then, conversely, reduce the effective recorded soundstage, yes??

I can also dink around with it in the Schoeps DMS plugin. I just have not quite gotten my head around how to run that as four channel and have it come out as Blumlein stereo, two channel. Further poking around and maybe a re-read of the tech literature. RTFM!

Any light you boffins/propeller-heads can shed on this would really help.


Cheers
#32
13th November 2011
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When I configured the DS 60 on my Sonosax mixer in the above fashion the image skewed to the LH a little.
It seems that unless the side cards are summed and then split and phased they are not an absolute fig 8.
#33
13th November 2011
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Althought I don't have a dS 60, if the cardiod groups act like fig 8s, then what you're proposing seems to make sense.

Side question, how is the low end extension on the ds60? Specs look like its tonally like an omni.

Tom
#34
13th November 2011
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Quote:
Originally Posted by Tommy-boy View Post
Althought I don't have a dS 60, if the cardiod groups act like fig 8s, then what you're proposing seems to make sense.

Side question, how is the low end extension on the ds60? Specs look like its tonally like an omni.

Tom
Tom, so far the Pearl has not stumbled. The low end has been good. I have not tried it on any really bass heavy stuff yet. I may this coming Thursday. There is a member on the board who ran it as an orchestral main mic for some time. Rolo 46 has used one extensively and so has a third member of the board any of whom could be more helpful. In short, a great mic.
#35
13th November 2011
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Quote:
Originally Posted by Rolo 46 View Post
When I configured the DS 60 on my Sonosax mixer in the above fashion the image skewed to the LH a little.
It seems that unless the side cards are summed and then split and phased they are not an absolute fig 8.

Roger, I am not quite sure how you would do this. Could you elaborate? And am I right, I can effectively widen the soundstage without changing the shape of the figure 8's? That would seem to mean that they "get larger" but it could mean the included angle in the center gets wider. Which is it or is there another case I have not deduced?

I would like to run this on Thursday at the stage lip in Blumlein MS so I would like to know what the theory is behind this.

Thanks.
#36
13th November 2011
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Bottom end is excellent and an extended and smooth top comes from the rectangular capsules.
Recording to separate channels and then processing in a DAW seems the best production advice.
#37
13th November 2011
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Two cards of opposite polarity do not make a true 8

The cards must be summed ,then split, one + ,panned LH, the other - ,RH.
This is not easily done in a mixer with no MS decode.
So better in a DAW, in post.
All manner of config is then available.
#38
14th November 2011
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#39
14th November 2011
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"And am I right, I can effectively widen the soundstage without changing the shape of the figure 8's?"

I think it's widened simply by changing (augmenting) the ratio of side-fig 8 to mid fig-8 volume (in Blumlien ms). It's easier to control though, by having the mid and side recorded un-matrixed and then matrixing and adjusting the stereo image with a software ms stereo imager.

Your mic has better capabilities than most stereo mics, because, like a Soundfield mic, you can also create in post a backwards pointing polar pattern to create an optimal reverberation in a good space, and then blend it with the forward facing result.

For me to do that with my AKG426, I attach a Senn 800 (multipattern mic) to it pointing backwards. An advantage to this method is that I can monitor both forward and backward facing mid-side pairs together. Another advantage is that the Senn 800 has a treble eq boost switch. A backwards facing mid-side works well with a treble boost.
#40
14th November 2011
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Yes, I do it all in post. I matrix the four cards. I invert the left and the rear channels. I am also assuming that reducing the mid to side ratio will widen the field/soundstage but the magic of doing it without changing the figure 8 pattern is a mystery to me. I am not clear on this. It is just another entry on a long list. LOL

I have found how to monitor the mic in while it is recording. I can invert channels in my recorder. How sweet. I checked it; it works! Yippee!
#41
14th November 2011
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The Beauty of the DS60 is that it has four separate outputs. This allows you to matrix it in many, many ways.

Rolo had some problems with his experience of it as he was initially given some misinformation as to how this is achieved.

Of the other posts a question that has been raised a couple of times as to the polar pattern of the centre mic, this doesn't change with the variation of the side mic, however, with equal balance, the response of a cardioid with a fig 8 mic is technically given as the same as a crossed pair of hypercardioids. I don't personally agree with this as listening doesn't (IMHO) yield the same results.

To achieve a surround image (4/0 or even 5/0) you need at least 3 capsules. Using the MKH800 dual output and a fig 8 side mic it is possible. As John points out, you can't get height information without at least 4 capsules. The Tetrahedral array used in the Soundfield allows for this and I think you can also achieve this with 2 fig 8's and a back to back cardioid with separate output's such as the MKH8000 dual, though the matixing gets very complicated.

A good friend of mine has experimented extensively with Periphony in two types of configuration (one with speakers placed in effectively eight corners of the room and the other with a quad formation with another quad formation halfway down the room in a vertical "ring"). He found that 8 speakers each set in the corners worked the best, however, with practical considerations in mind it becomes next to impossible to implement successfully in either a domestic or commercial environment.
#42
14th November 2011
Old 14th November 2011
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Quote:
Originally Posted by Rolo 46 View Post
Two cards of opposite polarity do not make a true 8

The cards must be summed ,then split, one + ,panned LH, the other - ,RH.
This is not easily done in a mixer with no MS decode.
Rolo, I don't quite understand your "summed then split". Can you please provide more details?

If a back-to-back pair of cardioid capsules is summed, it yields in principle an omni. But an omni pattern would result from summing a back-to-back pair of _any_ directional pattern capsules. The only thing changing along the series of subcard > card >super- > hyper- would be a decrease in intensity of the resultant omni (with Fig-8's giving the trivial result of zero intensity omni).

So once you have your omni from the summing, how does that omni know who its real parents were? Was it born from two card signals. or two weaker-signal subcards, or two stronger-signal supercards.etc?. Entropy won't allow you to unbake this omni cake and get back unambigously to the input ingredients.

And in any case, how do you actually manage to "split" it...by some kind of matrix manipulation of the omni - but together with what other signal channel?.

Of course one wants to minimize the disparities in a cardioid Front-Back pair, but could it not suffice just do some tests to determine the relative gains needed for best nulling of Front minus Back (with capsules presented with an identical signal source)? This simple gain tweaking wouldn't address any major polar response disparities, but would likely be 'close enough for jazz' :-)
#43
14th November 2011
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Quote:
Originally Posted by Tom McC View Post
Rolo, I don't quite understand your "summed then split". Can you please provide more details?

If a back-to-back pair of cardioid capsules is summed, it yields in principle an omni. But an omni pattern would result from summing a back-to-back pair of _any_ directional pattern capsules. The only thing changing along the series of subcard > card >super- > hyper- would be a decrease in intensity of the resultant omni (with Fig-8's giving the trivial result of zero intensity omni).

So once you have your omni from the summing, how does that omni know who its real parents were? Was it born from two card signals. or two weaker-signal subcards, or two stronger-signal supercards.etc?. Entropy won't allow you to unbake this omni cake and get back unambigously to the input ingredients.

And in any case, how do you actually manage to "split" it...by some kind of matrix manipulation of the omni - but together with what other signal channel?.

Of course one wants to minimize the disparities in a cardioid Front-Back pair, but could it not suffice just do some tests to determine the relative gains needed for best nulling of Front minus Back (with capsules presented with an identical signal source)? This simple gain tweaking wouldn't address any major polar response disparities, but would likely be 'close enough for jazz' :-)

What Rolo was trying to say (and I do know as we have discussed this in a number of PM's) is that you need to sum the two cardioid capsules together after inverting the phase of one of them.

It was suggested by someone that panning the two cardioids L & R and inverting the phase of one was enough, this is not the case. To do it successfully (I own a DS60 and have done it numerous times) you need to invert the phase of the right capsule, sum the resultant signal, then split that summed signal L & R and then invert the phase of the right channel to produce a "true?" side signal. By keeping the mid front and rear pairs separate, it is possible to produce whatever polar response required from fig 8 right up to omni by changing levels and/or polarity. Taken a step further it is also possible to produce 5 discreet signals for surround, this I alluded to in my earlier post about producing surround from 3 microphone outputs. To achieve this you ideally need a back to back dual microphone with separate outputs. the only one of those I know about is the Sennheiser, I think perl may also make one.
#44
14th November 2011
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I no longer have the DS60 its gone back to Mike Skeet
However
2 cards are never quite one 8
The classic 8 is a pressure gradient single diaphragm.
Adding them together produces an omni of sorts
Splitting and phase reversing the RH gives a fig 8 of sorts
Possibly 'close enough for jazz' and better than not summing them
(This causes a minor image drift to the LH)

Splitting can be transformer based, mixer based with nulling, recorder/DAW based with MS encoding from the menu.

I enjoyed the DS 60,it has a lovely natural sound.
However not being a surround hound I find the maths of a single diaphragm 8 in a MS array much easier to decode and comprehend...
#45
15th November 2011
Old 15th November 2011
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Quote:
Originally Posted by boojum View Post
I am also assuming that reducing the mid to side ratio will widen the field/soundstage but the magic of doing it without changing the figure 8 pattern is a mystery to me. I am not clear on this. It is just another entry on a long list. LOL
Boojum, are you asking for an explanation why a virtual mic from MS matrixing of two Fig-8's retains a constant Fig-8 pattern ["cos(Theta)"], independent of M/S ratio?

1. it's plausible that the virtual mic polar pattern should fall between patterns for the M and S mics.
Thus a cardioid with a Fig-8 results in something narrower than cardioid but wider than Fig-8 i.e supercard

2. the greater the M contribution in the matrix, the more the virtual mic pattern (and direction) resembles that of the M; with similar argument for S.

3. since two matrixed Fig-8's clearly yield a fig-8 in the extreme cases of M-to-S ratio 1:0 and M-to-S ratio 0:1, it seems intuitive that the virtual mic at intermediate M-to-S ratios will have a pattern between Fig-8 and Fig-8..and that is ..well..Fig-8 :-)

But if the above qualitative reasoning leaves you unsatisfied, here's a hard-core trig. version:

Consider a general MS system with a fractional weighting of "Ä" for the M Fig-8 and "1-A" for the S Fig-8.

The polar formula for the M mic is V = A.cos(Theta) , where "Theta" is the angle measured w.r.t the ' straight-ahead' axis.
And for the S mic, V = (1-A).sin(Theta), Note that Theta is still w.r.t. this same axis as for the M mic.

The L-angled virtual mic from the M + S matrixing will have the pattern:
A.cos(Theta) + (1-A).sin(Theta)


And this L mic has its main axis (where rate of change of V is zero) pointing at angle "Alpha" to the left of straight-ahead, where:
Alpha = arctan [(1-A/A)]
and it follows that
cos(Alpha) = A/sqrt[ A
^2 + (1-A)^2]
sin(Alpha) = (1-A)/sqrt[ A
^2 + (1-A)^2]

The formula for the L mic - when now restated in terms of the angle w.r.t. the L main axis (rather than w.r.t. straight-ahead) - becomes:
A.cos((Theta + Alpha)) + (1-A).sin(Theta + Alpha)
= A.[cos(Theta).cos(Alpha) - sin(Theta).sin(Alpha)] + (1-A).[sin(Theta).cos(Alpha) - cos(Theta).sin(Alpha)]

After the aforementioned values for cos(Alpha) and sin(Alpha) are plugged in, the L mic formula simplifies to:
sqrt[ A
^2 + (1-A)^2].cos(Theta).

This is clearly a pure Fig-8 pattern (as it only has a cosine term), whose intensity is a function of A.

Apologies to all for the nerdy interlude..back to our normal programming :-)

Last edited by Tom McC; 15th November 2011 at 03:01 AM.. Reason: munged Greek characters
#46
15th November 2011
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OK, pinhead response for Roland:

To do it successfully (I own a DS60 and have done it numerous times) you need to:

1) invert the phase of the right capsule,

2) sum the resultant signal, then

3) split that summed signal L & R and

4) then invert the phase of the right channel to produce a true" side signal.


In my pea brain I take this to mean I must:

1) Invert the phase of Side-right

2) Sum the resultant signal - WTF? You are talking to an History major. Can you say this in LibArtsSpeak?

3) Split the summed signal?? Huh??!! Does this mean run it through a splitter which will give me two mono files, L and R?

4) OK, I can understand this.
#47
15th November 2011
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Quote:
Originally Posted by Tom McC View Post
Boojum, are you asking for an explanation why a virtual mic from MS matrixing of two Fig-8's retains a constant Fig-8 pattern ["cos(Theta)"], independent of M/S ratio?

1. it's plausible that the virtual mic polar pattern should fall between patterns for the M and S mics.
Thus a cardioid with a Fig-8 results in something narrower than cardioid but wider than Fig-8 i.e supercard

2. the greater the M contribution in the matrix, the more the virtual mic pattern (and direction) resembles that of the M; with similar argument for S.

3. since two matrixed Fig-8's clearly yield a fig-8 in the extreme cases of M-to-S ratio 1:0 and M-to-S ratio 0:1, it seems intuitive that the virtual mic at intermediate M-to-S ratios will have a pattern between Fig-8 and Fig-8..and that is ..well..Fig-8 :-)

But if the above qualitative reasoning leaves you unsatisfied, here's a hard-core trig. version:

Consider a general MS system with a fractional weighting of "Ä" for the M Fig-8 and "1-A" for the S Fig-8.

The polar formula for the M mic is V = A.cos(Theta) , where "Theta" is the angle measured w.r.t the ' straight-ahead' axis.
And for the S mic, V = (1-A).sin(Theta), Note that Theta is still w.r.t. this same axis as for the M mic.

The L-angled virtual mic from the M + S matrixing will have the pattern:
A.cos(Theta) + (1-A).sin(Theta)


And this L mic has its main axis (where rate of change of V is zero) pointing at angle "Alpha" to the left of straight-ahead, where:
Alpha = arctan [(1-A/A)]
and it follows that
cos(Alpha) = A/sqrt[ A
^2 + (1-A)^2]
sin(Alpha) = (1-A)/sqrt[ A
^2 + (1-A)^2]

The formula for the L mic - when now restated in terms of the angle w.r.t. the L main axis (rather than w.r.t. straight-ahead) - becomes:
A.cos((Theta + Alpha)) + (1-A).sin(Theta + Alpha)
= A.[cos(Theta).cos(Alpha) - sin(Theta).sin(Alpha)] + (1-A).[sin(Theta).cos(Alpha) - cos(Theta).sin(Alpha)]

After the aforementioned values for cos(Alpha) and sin(Alpha) are plugged in, the L mic formula simplifies to:
sqrt[ A
^2 + (1-A)^2].cos(Theta).

This is clearly a pure Fig-8 pattern (as it only has a cosine term), whose intensity is a function of A.

Apologies to all for the nerdy interlude..back to our normal programming :-)
Yikes! My last Trig class was 1957. I was good at it then, but am now a little rusty. Let me read, re-read, and then re-re-read this to see if I can make some sense of it. I have dyslexia, too, so my reading comprehension is a struggle. I will hammer at it until I either get it or do not. What I seem to think is happening is that the virtual fig-8's from the MS getting a larger included angle between them in front and a lesser on the sides. But let me do the reading bit again.

Thanks you for the learned explanation. Truly.
#48
15th November 2011
Old 15th November 2011
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This is from the EMI Archive and shows Alan Blumleins twin ribbon array from
1934.
It was driven by a few car batteries.
Hope link works


http://tinyurl.com/bwshzyl
#49
15th November 2011
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Link did not work! interesting discussion though.

Question for anyone with knowledge about double m/s: do the front and rear cardiods need to be matched SDC's?

Also,the Beyerdynamic mc833 stereo mic internally decodes 3 cardioids to m/s. The wiring diagram is here : http://www.fullcompass.com/common/fi...3_BA_DE_01.pdf . Does anyone know whether this mic uses the recommended method of generating a fig-8 from the two cards? I don't read greek
#50
15th November 2011
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Works now
#51
15th November 2011
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Quote:
Originally Posted by Daniel Stark View Post
Link did not work! interesting discussion though.

Question for anyone with knowledge about double m/s: do the front and rear cardiods need to be matched SDC's?

Also,the Beyerdynamic mc833 stereo mic internally decodes 3 cardioids to m/s. The wiring diagram is here : http://www.fullcompass.com/common/fi...3_BA_DE_01.pdf . Does anyone know whether this mic uses the recommended method of generating a fig-8 from the two cards? I don't read greek
It must do, or it wouldn't work properly and you would end up with the slight image shift to the left that Rolo was describing earlier. If I remember correctly you could also do the same thing with the AKG C422 which came with a pattern control box.
#52
20th November 2011
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Hands Across the Waters - or, those GS folks in GB know a thing or two

Skeptic that I am, I had to try the summing (combining) of the FB and LR channels in the Pearl DS 60 to see if the left drift vanished. I could see no logical reason for it to do so. Left drift?? Why not right drift?? Hmmmppphhh.

So I did the test and damned if Roland and Roger are not onto something. The lopsidedness (left drift) does vanish. I combined ("export - two mono to stereo") in SAM and flipped the phase of one side of each and presto change-o the problem disappears. Obviously Merlin is as active on the left side of the Atlantic as on the right.

Cheers, a tip of the hat, and thanks to you all.

I still do not understand the trig, though. LOL
#53
20th November 2011
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I'll try a non mathematical explanation.

If we take a fig 8 single diaphragm mic used as a side mic to start with.

The signals hitting the left facing (I'm assuming that you have the front of the fig 8 facing left), anything that hits that diaphragm (on axis) is pretty much positive phase, everything hitting the right (on axis) is pretty much inverted phase.

When you split that mic down two channels panned left and right you effectively "add" that to the centre mic. The problem of course is you are adding "positive to positive on the left hand side and positive to negative on the right. Just split you would get an increasing signal on the left, and a decreasing signal on the right, so what you do is to invert the phase of the right channel to make it that you are adding positive to (a now) positive signal. Hey presto, MS, or as it is sometimes called sum and difference.

Now with a dual diaphragm microphone we have to invert that right hand capsule as it is also positive phase, technically both panned left and right you are getting two cardioid's facing left and right, not fig 8.

So you decide to switch the phase of the right facing capsule, the problem then is that you have an out of phase signal on the right, positive left + the mid mic and mid mic + negative right. Signal will be stronger on the left, result the image shifts too the left. This is why we have to invert the phase and sum it together to first obtain our fig 8. At that point we have the same situation as we did with the single diaphragm fig 8 where we have to split it and invert the phase of the right channel to again get a positive add on the right channel, thus keeping the image centred as it should be.

I hope this helps!
#54
21st November 2011
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Yup!
#55
21st November 2011
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Quote:
Originally Posted by Roland View Post
I'll try a non mathematical explanation.

If we take a fig 8 single diaphragm mic used as a side mic to start with.

The signals hitting the left facing (I'm assuming that you have the front of the fig 8 facing left), anything that hits that diaphragm (on axis) is pretty much positive phase, everything hitting the right (on axis) is pretty much inverted phase.

When you split that mic down two channels panned left and right you effectively "add" that to the centre mic. The problem of course is you are adding "positive to positive on the left hand side and positive to negative on the right. Just split you would get an increasing signal on the left, and a decreasing signal on the right, so what you do is to invert the phase of the right channel to make it that you are adding positive to (a now) positive signal. Hey presto, MS, or as it is sometimes called sum and difference.

Now with a dual diaphragm microphone we have to invert that right hand capsule as it is also positive phase, technically both panned left and right you are getting two cardioid's facing left and right, not fig 8.

So you decide to switch the phase of the right facing capsule, the problem then is that you have an out of phase signal on the right, positive left + the mid mic and mid mic + negative right. Signal will be stronger on the left, result the image shifts too the left. This is why we have to invert the phase and sum it together to first obtain our fig 8. At that point we have the same situation as we did with the single diaphragm fig 8 where we have to split it and invert the phase of the right channel to again get a positive add on the right channel, thus keeping the image centred as it should be.

I hope this helps!
How many single diaphragm Fig8 mics are out there? I was under the assumption that Fig 8 mics needed to be dual diaphragms to work correctly...
By your description of a single diaphragm is virtually a theoretical "noise canceling" mic, if a signal was equal in both sides it would cancel out, right?
And if that's the case would the rear lobe of a Sennheiser 416 or 816 (or similar) be + or - phase?
#56
21st November 2011
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#57
21st November 2011
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Quote:
Originally Posted by OzGizmo View Post
How many single diaphragm Fig8 mics are out there? I was under the assumption that Fig 8 mics needed to be dual diaphragms to work correctly...
By your description of a single diaphragm is virtually a theoretical "noise canceling" mic, if a signal was equal in both sides it would cancel out, right?
And if that's the case would the rear lobe of a Sennheiser 416 or 816 (or similar) be + or - phase?
Besides most ribbon mics, there are only a few single diaphragm fig 8(Sennheiser, Schoeps, MBHO, AKG ?, Neumann) which come in 3
different designs, single-ended asymetrical (Schoeps ?), single-ended symetrical, and push-pull (Sennheiser ?).

The parts of the signal which are the same on both sides cancel out.
In the back stereo image of a Blumlien or Blumlien mid-side array the
left-right imaging is the reverse of the front.

I believe the rear lobe of a card is - phase. In an ms array, the closer the card is to fig 8 (omni - wide card - card - supercard - fig-8) the more noticable it is that the back stereo image is a left- right reversal of the front. So an omni or wide card mid- side can be placed within an ensemble without reversing the backward facing stereo image.
#58
21st November 2011
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Quote:
Originally Posted by boojum View Post
Google says there is only one, a Senn MKH30:

Sennheiser MKH 30 microphone review | Figure 8 microphones
So would the results vary much using a MKH 30 mic (single diaphragm) versus another Fig 8 mic (dual diaphragm) in a MS or Blumlein setup?

The only fig 8 mics I have access to is the Neumann KM88i and according to the manual it is dual diaphragm.
#59
21st November 2011
Old 21st November 2011
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OG, my guess is that theoretically it should make no difference. The Schoeps is regularly pounced upon as asymmetrical - and to a tiny degree it is - but I have not seen any other fig-8 mics disputed and cannot tell you why. I can only guess. Perhaps the fig-8 guru can share opinions on this?

I use the Schoeps in a CMC68 - CMC64 MS array and the Pearl DS 60 is a four card array which can be a Blumlein and MS array. The Pearl fig-8 pattern looks pretty close to symmetrical to me: Pearl

I have learned it takes a little jiggery-poke to make it a true Blumlein/MS. The FB and LR channels must be combined from two monos into a stereo. I am not 100% aware of why but can tell you it does work, thanks to Messers Roland and Roger who taught me this.
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21st November 2011
Old 21st November 2011
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Quote:
Originally Posted by aracu View Post
The parts of the signal which are the same on both sides cancel out.
In the back stereo image of a Blumlien or Blumlien mid-side array the
left-right imaging is the reverse of the front.

This is correct. When you have a signal that hit's each diaphragm equally, you get a cancellation, in the case of a fig 8 this happens at 90 deg to the diaphragm which produces the characteristic fig 8 polar response.

Quote:

So would the results vary much using a MKH 30 mic (single diaphragm) versus another Fig 8 mic (dual diaphragm) in a MS or Blumlein setup?

The only fig 8 mics I have access to is the Neumann KM88i and according to the manual it is dual diaphragm.
Dual diaphragm mics can provide a decent fig 8 pattern providing both diaphrams have a good symmetrical response, mics like the 414 are good examples of this and the ability to vary the amount (and phase) of one capsule against another is how they achieve their variable polar patterns. Personally I have nothing against dual diaphram mics for MS and as they are more readily available they provide an excellent opportunity for almost anyone to have a play with MS!
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