Topic: Non Fig-8 mic MS?

[page]

I was thinking about this last night. Would it be possible to have an MS setup with 3 mics instead of the figure 8 for the sides and one other for the mid? For example, run two mics (cards or maybe even hypers) in coincident for the figure 8 pattern and then run something of a similar style for the mid. I was just thinking of the possibilities with standard equipment. If you used hypers, I wonder what the typical hole in the back would do, or would a jecklin disk between them resolve that. On that note (in theory), if you wanted to you could widen your mid and slightly tilt the side mics in a tighter back config then strict fig8. I think about it because there are many mic sets which terminate into a single 1/8th jack and you could attach a stereo2mono adaptor to get the single figure 8 image on a single track, so 3 mics on 2 channels.

Has anyone tried this? If you had an R-4 or something else that was 4 track, you could then do a standard XY on ch 3 and 4 and then matrix the post-MS decoding and the XY to your tastes to get just enough stereo from the XY with the main mid as a base... What would be the flaws to doing this (other then a mic stand that looks like a mutant)?

[Alexandru Petrescu]

The inconvenient would be the need of a third mic?  The two S mics would have to be phase-inverted, not sure how to achieve that.

I think there exist figure-8 mics with two membranes, I'm not sure.

[boojum]

A MS approximates a card XY.

[j.mart]

M/S without a fig8 as the S capsule is oxymoronic. you can switch up the M capsule for a omni or a narrower cardiod pattern, but the fig8 is pretty much irreplacable. of course, there are mics with two capsules in one body, such as the LSD2.. maybe that's what you meant? a M/S matrix works in a very complex way, where phase summing/difference determine how much info you'll get from the M and the S. an S capsule (third mic) can't possibly be on the exact same place as the other S capsule, so sound would get to both microphones in a total unrelated phase relationship, making the M/S useless... scratch that, that wouldn't be M/S at all

A MS approximates a card XY.

i respectfuly disagree. there is no way you can achieve a proper capture on axis (M = 0º) with XY, without seriously compromising the stereo imaging of your capture. cardiods lose a lot of it's acoustic properties off-axis, especially on high frequencies. if you close up the angle of the XY, you'll lose side information, yet you'll get a narrow, less than desirable stereo image. not to mention that you can adjust the M/S matrix on post-production.. it's up to you the ammount of S information you want in the mix. nothing, *nothing* as of 2008, can approach what M/S does.

i know people (me ) who'll go further and say that you can only achieve true stereo with M/S or blumlein. i'm a hardcore fan of M/S, but i won't say that other pseudo-stereo formatsrther and say that you can only achieve true stereo with M/S or blumlein. i'm a hardcore fan of M/S, but i won't say that other pseudo-stereo formats are are useless, especially for our purposes (live music recording).

[page]

M/S without a fig8 as the S capsule is oxymoronic. you can switch up the M capsule for a omni or a narrower cardiod pattern, but the fig8 is pretty much irreplacable. of course, there are mics with two capsules in one body, such as the LSD2.. maybe that's what you meant? a M/S matrix works in a very complex way, where phase summing/difference determine how much info you'll get from the M and the S. an S capsule (third mic) can't possibly be on the exact same place as the other S capsule, so sound would get to both microphones in a total unrelated phase relationship, making the M/S useless... scratch that, that wouldn't be M/S at all

I see my posting skills are neither mad nor l33t tonight. Let me try to give some reasoning where I was headed:

I have always had a set of mics. Unfortunately, I've never had a set the capability to do figure 8. So one evening I thought about it, and if you stacked the two separate cards together pointed 180* of each other, the resulting pattern appears similar to the figure 8. So if I summed the signal from both of those together into a single channel, I could come close to the figure 8 pattern for processing in post. I thought about this some and remembered an earlier discussion on how some mics can do the figure 8 pattern, but require additional post processing (such as inversion) before the MS can be done.

I guess based on your reaction, that the physics involved in such a feat would be just outside of what's possible with 2 (small) mics. I guess I'm curious as to why the 5-10cm involved of vertical space (between the left S with mic A and the right S in mic B where they are stacked) makes a difference in the S, but not in the M.

Why go to all of this trouble? Well, I like the mental gymnastics, even if I'm not really familiar with it all. Second, and a real driving factor is: I'm poor when it comes to gear. Looking back on my original question the "What are the flaws" bit should have been in the paragraph above. Sorry for the poor organization.

[page]

I see one mistake, the two sides are in the same polarity, so you end up with a tighter mid, or a more spaced mid instead of a left MS combo and right MS combo (by duplication and inversion of the figure . My bad, carry on.

[boojum]

A MS approximates a card XY.

i respectfuly disagree. there is no way you can achieve a proper capture on axis (M = 0º) with XY, without seriously compromising the stereo imaging of your capture. cardiods lose a lot of it's acoustic properties off-axis, especially on high frequencies. if you close up the angle of the XY, you'll lose side information, yet you'll get a narrow, less than desirable stereo image. not to mention that you can adjust the M/S matrix on post-production.. it's up to you the ammount of S information you want in the mix. nothing, *nothing* as of 2008, can approach what M/S does.

i know people (me ) who'll go further and say that you can only achieve true stereo with M/S or blumlein. i'm a hardcore fan of M/S, but i won't say that other pseudo-stereo formatsrther and say that you can only achieve true stereo with M/S or blumlein. i'm a hardcore fan of M/S, but i won't say that other pseudo-stereo formats are are useless, especially for our purposes (live music recording).

Perhaps I was not clear enough; first, "to approximate":

v. (-mt) ap·prox·i·mat·ed, ap·prox·i·mat·ing, ap·prox·i·mates
v.tr.
1. To come close to; be nearly the same as: This meat substitute approximates the real thing.
2. To bring near.
3. To bring together, as cut edges of tissue.
v.intr.
To come near or close, as in degree, nature, or quality.

Second, if you observe the decoded pattern of a MS recording you will see it very much approximates a card XY.    The left lobe of the figure 8 and the center of the card being the "X" and the right lobe of the figure 8 and the card being the "Y." 

Cheers

[DSatz]

saitoh, theoretically if you place two coincident cardioids back-to-back, and sum their signals with the polarity of one cardioid inverted, the result will be a figure-8 pattern. In practice this rarely results in a fully symmetrical pattern or one that's smooth at all frequencies of interest, but it's how the figure-8 pattern is generated in most switchable-pattern condenser microphones. For dedicated figure-8 microphones or capsules, a symmetrical single-diaphragm construction is preferable.

One definite problem is that placing two physical objects in the exact same place at the same time is forbidden by the laws of our universe as enforced by the physics police in your quadrant. The compromise is only made worse by adding a third microphone. The functioning of any directional microphone is affected by other solid objects nearby; if the microphones are too far apart they don't function as a coincident pair, but get them too close and they distort each other's directional patterns. There's no real solution to that dilemma--but the available compromises are better with two microphones than with three.

This is also one of the problems with so-called "microphone shootouts"--if the microphones are far enough apart not to interfere with one another's functioning, then they're also far enough apart to sound different because they're placed differently from one another.

--best regards

[Gutbucket]

I started this reply yesterday and left the window open on this computer.  Since then, DSatz has posted a nice concise answer just as I hoped he would.  I'll post anyway even though I'm really just parroting his more knowlegable remarks with more words.

Why go to all of this trouble? Well, I like the mental gymnastics, even if I'm not really familiar with it all. Second, and a real driving factor is: I'm poor when it comes to gear. Looking back on my original question the "What are the flaws" bit should have been in the paragraph above.

Well, there is the theoretical mathematical world and the physical world of applied ideas.  I hope I can help satisfy your mental gymnastics quest and I think the understanding is important, but the real world engineering challenges of doing this satisfactorily in the physical world are hard to overcome.

Mathematically, a cardioid is the sum of a coincident omni and a figure-8.  The positive polarity lobe of the 8 sums with the omni, increasing it's sensitivity on one side, and the reverse polarity lobe of the 8 also sums with the omni, decreasing it's sensitivity in the opposite direction.  Overall the sum of the omni and fig-8 patterns is what creates the cardioid response shape. 

Mathematically you can use the same technique with two cardioids to cancel out the omni part leaving the fig8.  To do so you'd take two cardioids and place them coincidentally, with a 180 degree angle between them, invert the polarity of one mic and sum the outputs equally.  You would be doing the following:

sum the outputs of: cardioid facing left + reversed polarity cardioid facing right
 
{substitute the omni + fig8 equivalent and read that as}

sum the outputs of: omni + Fig8 facing left + reversed polarity (omni + Fig8 facing right)

The omni terms cancel out and the fig8's are add together (because one fig8 has reversed polarity but is facing the opposite direction as the other).

But in the real wold there are problems:

>The cardioid mics aren't perfectly cardioid over the whole spectrum especially off axis and to the rear of the pattern.
>The two mics cannot be arranged to be perfectly coincident. The bodies of the mics get in the way.  Unlike lining up capsules vertically to get them coincident for X/Y recording, even a small misalignment between them screws up the phase cancellations that are needed to generate a fig8 pattern, especially the 'null' at 90 degrees off axis.  You would really need to get them as close together and as perfectly aligned as the back to back cardioid capsules in switchable pattern mics that use this technique to derive various patterns.

In addition you use up two preamp channels before summing the outputs to create one virtual fig8.  So to make a M/S pair you then would need 3 preamp channels and a summing network for the two side facing mics.

However, if your are a bit of an electronics DIY type. I know of at least one M/S microphone kit that uses cheap cardioid capsules on opposite sides of a project box to derive the S signal.  The capsules are in-line with each other but are separated by the width of the box, so it would seem the distance between them isn't the most critical aspect. I have no idea how well it works though.  Here's a link to the PAiA M/S mic kit. The schematic can be viewed here.

[page]

+Ts for humoring me with the detailed responses folks.

I spent a lot of my early taping days experimenting. I knew about NOS/ORTF/etc, however I enjoyed the experimentation. Not a lot was pretty, (read: spaced Cards...) but it was fun, even if I don't have a lot to show for it.

In some ways I still am...