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Gear / Technical Help => Microphones & Setup => Topic started by: gratefulphish on February 06, 2007, 10:06:43 PM
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I am fortunate enough to have Neumann TLM 170s, which have five adjustable polar patterns: omni, subcard, card, hyper and figure 8. In looking at the various polar pattern pickup diagrams, and from my own practical experimentation, I have several questions.
1. Why do all hypercard mics have that rear lobe pickup pattern? This seems counterintuitive, as you are trying to narrow the pickup pattern in front of the mic, which is accomplished, at the cost of picking up unnecessary/unwanted sound from the rear, which otherwise is only picked up by the figure 8 pattern.
2. What would happen if you used something like this http://cgi.ebay.com/SE-ELECTRONICS-Reflexion-Filter-Mic-Ambience-Reflector_W0QQitemZ250078926416QQihZ015QQcategoryZ41466QQrdZ1QQcmdZViewItem , or something smaller and homemade, to try to block out some of that rear lobe and pickup. Would it be better to make it convex, rather than concave, in order to try to avoid reflection or echoing back of the sound coming from the front, although the manufacturers of this particular device seem to have gone to great lengths to assure sound absorption and dispersion? What if it were as simple as a piece of heavy felt or fabric several inches behind the mic?
3. From the various diagrams, the figure 8 pattern actually has the narrowest front (and matching rear) lobe. I have M/S capability in my deck and preamp, so what would happen if I ran two mics in figure 8, a la Blumlein, and then completely panned away the rear lobes? Can this be done, and has anyone tried it? Since M/S assumes one figure 8, and one single other pattern mic, would I need additional hardware/software to accomplish this?
Just a taper/geek in search of answers, and this is the place to get them. TIA
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I am fortunate enough to have Neumann TLM 170s, which have five adjustable polar patterns: omni, subcard, card, hyper and figure 8. In looking at the various polar pattern pickup diagrams, and from my own practical experimentation, I have several questions.
1. Why do all hypercard mics have that rear lobe pickup pattern? This seems counterintuitive, as you are trying to narrow the pickup pattern in front of the mic, which is accomplished, at the cost of picking up unnecessary/unwanted sound from the rear, which otherwise is only picked up by the figure 8 pattern.
The polar pattern of a cardioid microphone must have a back port in order to use phase cancellation to create directionality..
2. What would happen if you used something like this http://cgi.ebay.com/SE-ELECTRONICS-Reflexion-Filter-Mic-Ambience-Reflector_W0QQitemZ250078926416QQihZ015QQcategoryZ41466QQrdZ1QQcmdZViewItem , or something smaller and homemade, to try to block out some of that rear lobe and pickup. Would it be better to make it convex, rather than concave, in order to try to avoid reflection or echoing back of the sound coming from the front, although the manufacturers of this particular device seem to have gone to great lengths to assure sound absorption and dispersion? What if it were as simple as a piece of heavy felt or fabric several inches behind the mic?
I use acoustic foam behind mics all the time in the studio.. So this is something that does change the sound it tends to make it warmer sounding. I don't know how it would work for taping live concerts but for vocals it would be sweet..
3. From the various diagrams, the figure 8 pattern actually has the narrowest front (and matching rear) lobe. I have M/S capability in my deck and preamp, so what would happen if I ran two mics in figure 8, a la Blumlein, and then completely panned away the rear lobes? Can this be done, and has anyone tried it? Since M/S assumes one figure 8, and one single other pattern mic, would I need additional hardware/software to accomplish this?
The capsules outputs are combined you would have to separate them in order to achieve this... This is a basic principal of the 6 capsule surround sound mic. it would be very interesting.. You can change the sound quite a bit by changing the way the microphone capsules interact with one another. But again this requires separate outputs from each capsule..
Just a taper/geek in search of answers, and this is the place to get them. TIA
I know I have not really answered all of your questions I hope I was of some help..
Chris Church
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I am not sure about your response to question 3., which could well be the fault of the way the question was written. As I understand it, M/S encoding, recognizes that there are two separate "sides" to the mic, which is perpindicular to the stage. It then allows you to mix the "left" with some of the center mic, and the same with the "right" side. I am assuming that there is something akin to a pan control, to adjust between left and center, and right and center. So what if you went with all left, or in my question, all front, and no back? In other words, can you completely eliminate one side of the figure 8 in the post processing?
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I am not sure about your response to question 3., which could well be the fault of the way the question was written. As I understand it, M/S encoding, recognizes that there are two separate "sides" to the mic, which is perpindicular to the stage. It then allows you to mix the "left" with some of the center mic, and the same with the "right" side. I am assuming that there is something akin to a pan control, to adjust between left and center, and right and center. So what if you went with all left, or in my question, all front, and no back? In other words, can you completely eliminate one side of the figure 8 in the post processing?
OK got you.. Then you would have stereo :) the whole point of MS encoding is simply that there must be a left right and center the Sony 909 or something like that uses a side firing capsule with a front firing capsule the name of this pattern escapes me but it has been done before.
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I am simply trying to figure out if I can use the most limited polar pattern available, which would be half of the figure 8, by eliminating the other half with an M/S decoding process. Thanks. and +T
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gratefulphish, let me try to explain the basics of microphone patterns. Microphone designers can't just make up any old pattern they want; directional patterns result from the basic physics of how sound pressure is used to move the membrane(s) in the microphone. Two basic arrangements exist. All directional patterns come from using one or the other or various combinations of those two, so it's worth knowing what they are and how they can be combined. Then this whole picture begins to make sense.
The first, simpler arrangement is when a membrane simply responds to the sound pressure that reaches its front. The other, more complex arrangement is, the membrane is exposed both in front and in back, and it responds to the difference in sound pressure between its front and back. The first, simpler arrangement ("pressure response") results in an omnidirectional pickup pattern, while the second, more complex arrangement ("pressure gradient response") results in a bidirectional pattern--a "figure 8" shape, with the two lobes having opposite signal polarity from each other.
A hypercardioid microphone is basically a bidirectional microphone with a small degree of pressure response added--a blend of maybe 1/4 omni plus 3/4 figure-8. Adding the omni component increases the microphone's front sensitivity (since signals in the same polarity add together) and decreases the sensitivity of its rear lobe (since signals in opposite polarity tend to cancel). But it still has a front and a rear lobe, even though they're unequal in sensitivity. A hypercardioid is really just a lop-sided ("lop-fronted"?) figure-8.
A cardioid, by comparison, is a 50/50 mix of the two methods. The strength of its omni component exactly cancels the rear lobe of its figure-8 component, while doubling the sensitivity in front. If you mainly want to minimize rear-incident sound, cardioid is the pattern that does this the best. If on the other hand you want the narrowest front pickup pattern, figure-8 has that. Unfortunately you can't optimize both at the same time; those two requirements physically contradict each other to some extent.
Now if you look at the pattern selector of your microphone, you'll see that its five possible settings (apart from the "R") are just points along a continuum from omni to figure-8. As you move the dial, you are selecting the polarization voltage (and thus the sensitivity) for the rear half of the twin-diaphragm capsule. Sensitivity is directly proportional to this voltage, and the polarization on the front half of the capsule remains constant. In the omni setting, the back half of the capsule is charged as strongly as the front half, and with the same polarity. It is charged more weakly, but still in the same polarity, for the "wide cardioid" setting. It isn't polarized at all for cardioid; the front half of the capsule is a cardioid by itself. The rear half is polarized weakly (in inverse polarity) for super- or hypercardioid response, and at full strength (but again, in inverse polarity) for the figure-8 setting.
Some variable-pattern microphones such as the Neumann M 269 used potentiometers for pattern selection rather than switches with definite "stops" to them; maybe now you can imagine how this is possible. For that matter, a few microphones have been designed with one pure pressure transducer and one pure pressure-gradient transducer in them, and the pattern control simply varies the mix between them, allowing any first-order pattern to be produced. In that type of design, the omni setting has full low-frequency response and freedom from proximity effect, wind and breath noise, etc., so it's a rather interesting approach.
--best regards
P.S.: Actually the so-called "hypercardioid" pattern of a TLM 170 is a bit closer to being supercardioid (it has a little more omni and a little less figure-8 than a classic hypercardioid), but that's equally true of most "hypercardioid" microphones, including the Schoeps MK 41 capsule and Neumann's own KM 185 or KM 150. Pragmatically, people mostly find that the rear lobe of a true hypercardioid is just too much, and for various reasons you can also get more extended response at both ends of the frequency spectrum if you move more toward a supercardioid pattern.
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> I am simply trying to figure out if I can use the most limited polar pattern available, which would be half of the figure 8, by eliminating the other half with an M/S decoding process.
The answer is, unfortunately, no.
The matrixing part of your idea is entirely possible; you could cancel out the figure-8's rear lobe by matrixing in a second signal from a cardioid of similar 0-degree sensitivity. But then you'd end up with a front-facing cardioid pattern that is just as broad as any other cardioid would have been in the first place!
(If you want to discuss why this is so, we can, but first I'd want to know whether you remember the cosine function from your math classes. If you do, that would save us an awful lot of verbal hand-waving ...)
Switchable-pattern condenser microphones with multiple membranes and various circuit arrangements to combine their outputs have been around since the 1930s. Everyone, at some point or other, probably wishes for the same thing that you do. If it could be achieved by such means as you are describing, it surely would have been an option in all switchable-pattern condenser microphones since before we were born (even me). The engineers back then were limited in the electronic devices they had available and in the precision of some of their tools, but they understood acoustics very well.
In reality you have to choose which parameter you want to optimize--either the narrowest front-facing pattern or the best null in the back. Or you can split the difference (i.e. use a super- or hypercardioid pattern)--but you can't optimize both characteristics in the same microphone at the same time.
--condolences and best regards
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Thanks for this fine post...
DSatz - 7 of the best posts ever on TS.com!
Right up there with SparkE! for concise, informative posts!
Thanks...
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Thanks for this fine post...
DSatz - 7 of the best posts ever on TS.com!
Right up there with SparkE! for concise, informative posts!
Thanks...
I completely agree... + T and another in twelve
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Thanks for this fine post...
DSatz - 7 of the best posts ever on TS.com!
Right up there with SparkE! for concise, informative posts!
Thanks...
I agree too I wish I could be that concise in my posts! Its in my brain I just cant get it out in print lol. T+ To this guy he knows his shit!
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DSatz Super +T for the response, and detailed explanation. This is the link to the interactive mic pattern page for the TLM-170s http://www.neumann.com/zoom.php?zoomimg=./assets/diagrams/tlm170r_diagrams.htm&zoomlabel=Diagram&w=878&h=278
Looking at the lobes of the patterns, it almost seems as if the subcard has a smaller lobe than the card pattern. The figure 8, is the narrowest of all. You may have fully answered my question, but I may not have phrased it completely correctly, now that I have read your responses. So, with your permission, I'd like to ask one more.
What if I did as I indicated, and ran both mics set at figure 8, in M/S mode, but in decoding, mixed in "nothing" in the other channel. In other words, have the figure 8 of each mic panned to the fullest of the opposite directions, without actually mixing in a center signal. Would the M/S decoder allow you to do this, or will it insist on some signal, which theoretically could be another mic, with the gain all the way down. The part of your response that I really did not understand was thyis:
"The matrixing part of your idea is entirely possible; you could cancel out the figure-8's rear lobe by matrixing in a second signal from a cardioid of similar 0-degree sensitivity. But then you'd end up with a front-facing cardioid pattern that is just as broad as any other cardioid would have been in the first place!"
What if I matrixed in nothing in this situation? If you are saying that the lobes for the figure 8 and standard card are identical, then I get it, but do not understand the true meaning of the diagrams on the Neumann site. Otherwise, I am still trying to figure out whether there is some way to isolate the signals from the two figure 8 mics, so that I am only getting what appears to be the smaller, narrower frontal lobes of the figure 8, and canceling out the rears.
I truly appreciate your extremely knowledgeable response. I am just trying to seek knowledge, and you seem like an excellent source. BTW, if we need to get into the mathematical discsussion, those files were lost in a previous cranial hard drive failure, but my daughter has aced every calculus course there is, so maybe I need to have you speak with her.
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If rear rejection is your goal.. why not just run MS with card mid and dial down the sides(fig8) to your liking. I would think you could eliminate the rear "allmost" completely..
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If rear rejection is your goal.. why not just run MS with card mid and dial down the sides(fig8) to your liking. I would think you could eliminate the rear "allmost" completely..
What I am trying to do is run both mics in figure 8, with midside encoding, say in an X/Y pattern, rather than the typical M/S setup, where the single M/S mic is perpindicular to the stage. Then I want to be able to process out the entire rear lobe of each, if, the big if, this can be done, and will give me the narrower pickup pattern that seems to be acheivable from the figure 8.
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Then I want to be able to process out the entire rear lobe of each, if, the big if, this can be done, and will give me the narrower pickup pattern that seems to be acheivable from the figure 8.
The matrixing part of your idea is entirely possible; you could cancel out the figure-8's rear lobe by matrixing in a second signal from a cardioid of similar 0-degree sensitivity. But then you'd end up with a front-facing cardioid pattern that is just as broad as any other cardioid would have been in the first place!
If rear rejection is your goal.. why not just run MS with card mid and dial down the sides(fig8) to your liking. I would think you could eliminate the rear "allmost" completely..
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The answer to your question seems to be in the quoted parts above. Probably not what you are looking for though.
Your other option for ultra directionality is shotguns.
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> Looking at the lobes of the patterns, it almost seems as if the subcard has a smaller lobe than the card pattern.
[reply rewritten] Neumann's Web page for the TLM 170 R has the patterns displayed in an odd sequence, with cardioid stuck in between omni and wide cardioid. If you click them in their real order { omni, wide cardioid, cardioid, hypercardioid, figure-8 }, you can see the pattern become progressively narrower.
> The figure 8, is the narrowest of all. [ ... ]
> If you are saying that the lobes for the figure 8 and standard card are identical, then I get it ...
No, you just said that the figure-8 pattern's front lobe is "the narrowest of all," and you were right! Consider the 1 kHz curves for the front of the microphone. The cardioid's response is down 3 dB at around 65 degrees off axis, and down 6 dB at 90 degrees. A figure-8 pattern, being narrower, has greater attenuation than the cardioid does as you get away from 0 degrees. Its response is down 3 dB already at 45 degrees off axis, and at 90 degrees it has (theoretically) no response at all.
Or think of it this way: The TLM 170's figure-8 pattern is the result of subtracting the output of a backward-facing cardioid from the output of a forward-facing cardioid. (Technically the signals are added, but the rear-facing diaphragm is reverse-polarized so the voltages subtract.) A cardioid still has some response at 90 degrees, so two back-to-back cardioids will both put out signals when sounds arrive at that angle. But due to their opposite polarization, the net result is zero. That creates the null of the figure-8 pattern.
Now, what if you move the sound source slightly forward from the 90-degree null? The output of the front-facing cardioid will increase, while that of the rear-facing cardioid will decrease. The result is an increasing signal of positive polarity. The farther you move toward 0 degrees, the more the front-facing cardioid's output will increase while the rear-facing cardioid's output will decrease. When you reach 0 degrees, the sound source is in the null of the rear-facing cardioid, so nothing much is subtracted from the output of the front-facing cardioid--which is fully on axis at that point.
Move the sound source back to 90 degrees and this time work backwards; you'll get the same progression with the signal polarity reversed. As you approach 180 degrees, the front cardioid puts out less of a signal while the rear cardioid puts out progressively more. But the rear cardioid's signal is being subtracted from the front cardioid's signal--so while the net sensitivity is increasing, the output is inverted in polarity relative to the sound source.
> I am still trying to figure out whether there is some way to isolate the signals from the two figure 8 mics, so that I am only getting what appears to be the smaller, narrower frontal lobes of the figure 8, and canceling out the rears.
The answer to that is still no. What makes the figure-8's front lobe narrow is the subtracting of signals of the rear-facing cardioid within your TLM 170. If you cancel that rear lobe, you also cancel its narrowing effect on the front lobe--and then you've got yourself a cardioid all over again.
Whether you do this with an M/S matrix or a mixing board with a polarity switch or in any other way, that's what you'll get. I'm sorry; it's pretty much a 2 + 2 kind of thing, and you're going for 2 - (-2) as a variation, but it still adds up to 4.
--best regards
P.S.: I was out walking and thinking about your initial question; I have the feeling that we're still not quite addressing it. I think this may be because your point of reference is a Sony stereo microphone with three cardioid capsules, one of which points forward (the "M" signal) and the other two of which are used to synthesize a sideways-facing figure-8 (the "S" signal). Maybe you didn't realize that the signals from the two side-facing elements are combined in opposite polarity--not simply added or mixed together. And maybe you wanted to subtract the signals of those left- and right-facing elements from the "M" microphone so as to narrow the front pickup pattern of "M".
The signals of the left- and right-facing elements aren't available separately--but if they were (as with some quad microphones having a "clover-leaf" pattern of cardioid elements, e.g. the Neumann QM 69 from the 1970s), this would indeed narrow the front pattern of the "M" microphone. At the same time, however, the overlap between the two side-facing elements (centering precisely behind "M") would create a rear lobe in opposite polarity to "M"'s front lobe. So again, it's the same result as what you get by simply dialing in the hyper- or supercardioid pattern of your TLM 170, or very nearly so.
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P.S.: I was out walking and thinking about your initial question; I have the feeling that we're still not quite addressing it. I think this may be because your point of reference is a Sony stereo microphone with three cardioid capsules, one of which points forward (the "M" signal) and the other two of which are used to synthesize a sideways-facing figure-8 (the "S" signal). Maybe you didn't realize that the signals from the two side-facing elements are combined in opposite polarity--not simply added or mixed together. And maybe you wanted to subtract the signals of those left- and right-facing elements from the "M" microphone so as to narrow the front pickup pattern of "M".
The signals of the left- and right-facing elements aren't available separately--but if they were (as with some quad microphones having a "clover-leaf" pattern of cardioid elements, e.g. the Neumann QM 69 from the 1970s), this would indeed narrow the front pattern of the "M" microphone. At the same time, however, the overlap between the two side-facing elements (centering precisely behind "M") would create a rear lobe in opposite polarity to "M"'s front lobe. So again, it's the same result as what you get by simply dialing in the hyper- or supercardioid pattern of your TLM 170, or very nearly so.
Now, you are sort of getting to what I was thinking, but not quite. My point of reference had nothing to do with any Sony mic. I was thinking classic M/S, with a TLM-170, or other similar mic perpindicular to the stage, and a center mic (TLM 170, or any mic with the polar pattern of your choice, most likely card, as the center.) As I always understood it, M/S processing split the signal of the figure 8 mic in two, for lack of better terms, the left side, and the right side. In doing the mix, you then started with one side, and then mixed in a portion of the center mic. Same with the other channel. But I always, (and maybe well incorrectly) understood the left and right to be discrete signals, and that when you were mixing one, you were for practical purposes, only "getting the signal from that side," plus any center channel that you mixed in.
Hence my question, if these two lobes of the figure 8 are really discrete in M/S mode, then why could you not separate them, whether you call it left/ right, when the mic is perpindicular, or front/back, if the mic is facing forward. As I understood your last answer, the effect of completely removing the opposite signal, whether in M/S or not, effectively "changed" the polar pattern of the original figure 8, to something closely resembling, or equal to, a cardiod.
So, after all of this, the question is, for processing, are those signals separate, and if so, can they be separated from one another, without causing an effective change in the polar pattern. I know this sounds confusing, but my brain keeps telling me that if you can do M/S, then you can do this.
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Ah, the core misunderstanding has finally revealed itself, I think. In M/S matrixing, the front and back halves ("lobes") of the figure-8 signal are not processed separately! They can't be separated (as everyone's been trying to tell you), and fortunately, M/S doesn't need them to be separated.
One key fact which I think may have eluded you (or perhaps you just haven't quite realized its implications yet) is that the two lobes of the figure-8 microphone have opposite signal polarity from each other. If you fire a pistol in front of an omni mike--shooting blanks, I hope, and wearing earplugs--the output signal voltage should initially swing in a positive direction. The same is true if you fire the pistol in front of a figure-8. But if you fire the pistol in back of the figure-8, the signal from that same microphone will initially swing negative. Are we clear about that?
If so, OK--and M/S works like this: You set up a forward-facing "M" microphone of any type or pattern that you like, and you place a sideways-facing figure-8 "S" microphone at (ideally) the same point in space--at least in the horizontal plane which conventional stereo playback systems are concerned with. The "M" microphone's output, by itself, is a mono pickup of whatever you're recording, and should be usable as such directly. And now that you have a Ph.D. in mono compatibility with M/S, let's see how stereo works with it.
A sound source that is perfectly in front of the M/S microphone pair in a completely dead room will be picked up by the "M" microphone on axis--but it will fall right into the 90-degree null of the "S" microphone and generate no signal there at all. A sound source that is 45 degrees to the left of center (as the microphones "see" the world) will still be picked up by the "M" microphone, but now it is no longer in the null of the "S" microphone, so it will be picked up by both "M" and "S" at the same time. The two signals will have the same polarity, since by convention, the front (= positive polarity) lobe of the figure-8 is turned to the left for M/S.
A sound source that is 45 degrees to the right of center will be picked up by both "M" and "S" microphones, too. But in this case, the sound will reach the rear (right-facing) lobe of the figure-8, causing the "S" signal to have inverse polarity (it will be 180 degrees "out of phase") in relation to its "M" counterpart.
You can see, therefore, that if you simply add the M and S signals together, all sound sources that were in the center or on the left will be included in the result--but all the sound sources that were to the right of center will tend to cancel out because of the polarity confict. On the other hand if we invert S before adding the signals, the reverse will occur: All sound sources that were in the center or on the right will now be included in the result, while all sound sources that were to the left of center will tend to cancel out because of the polarity conflict.
If this is unclear, please stay with it and see whether it makes more sense after you mull it over a while. It is the key point in understanding M/S--because all that an M/S matrix does is simply to add M + S to derive the stereo L signal, while to derive the stereo R signal it inverts S and adds that to M (or you could say that it subtracts S from M; same thing).
The above explanation is simplified since, unless you're on location at Wimbledon, there usually are sound sources between the extreme left and right (which may well be farther apart than +/- 45 degrees, depending on the pattern of the "M" microphone). There is also reverberant sound to consider, and some nifty tricks that can be done by processing the M and S signals separately before combining them in the matrix. (By the way, I liked the first movie a lot better than the second one; did you?)
But the main point is that an M/S matrix does nothing more than (M + S -> L) and (M - S -> R); it never does any separate handling of signals from the front and rear lobes of the S microphone, which are only theoretical constructs anyway--not signals that could exist as separate real entities. When I go out walking in the daytime, I have a shadow; it is measurable and everyone agrees that it is there, but I can't weigh it or eat it.
Does this help? If not, please keep asking. It's well worth developing a solid, gut-level feel for how this all fits together; that can be very useful to you in future situations.
--best regards
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+T again. I want to thank you again for your patience, knowledge, and abitlity to communicate it, so that someone like me can actually begin to grasp the concept. (BTW, so it wasn't just me, the Neumann site had the card and subcard patterns out of order?) I am trying to study this whole subject, in-depth, and you have been greatly helpful. If there are any written treatises that you would suggest, not involving too much advanced math or physics, I would love to read one. I will reread your explanation again tomorrow ( I have to head off to a show) and respond with any further questions. Thanks again, you are a credit to this board.