Topic: How split do split omnis need to be?

[lsd2525]

Getting ready to do a few outdoor shows over the next couple of weeks. First trip out for the new DR-60D, so I gots 4 channels to work with. Going to run my AT853's and a pair of omnis I picked up in the yard sale. Was going to run the 853's in a regular DIN/ORTF-ish config, and was thinking about trying to rig up something (yardstick, tent pole, whatever I can come up with) to mount on top of the stand to split the omni's at 3 feet or so. Is 3 feet enough to even matter when you're 50 feet back from the stage? What kind of distance do you need to even call something a "split omni"?. I guess I could run a 2nd stand to get more of a spread, but that would be ton of hassle. Just trying to figure out if this is a stupid idea to try out. Thanks, Dan

[Chuck]

I have experimented with omni spacing a lot and I use 24 - 28" for a pair of Audix Micro omni's I have. But, I have heard excellent recordings with much larger spacings too.

This is a recording I made that I really like the sound of: https://archive.org/details/furthur2012-09-22

That's with 24" spacing at Red Rocks.

[ts]

I have experimented with omni spacing a lot and I use 24 - 28" for a pair of Audix Micro omni's I have. But, I have heard excellent recordings with much larger spacings too.

This is a recording I made that I really like the sound of: https://archive.org/details/furthur2012-09-22

That's with 24" spacing at Red Rocks.

Chuck, what are using for a bar?

[Gutbucket]

From 50' back outdoors, I'd try to space a stero pair of omnis about 3' apart minimum.  If you plan to mix them with the AT853s, double that to 6' if you are able to.  Both wider and narrower will work as well, I just find those distances seem to be more or less optimal most of the time and present few potential problems.  If you have a single mic or stereo pair in the center, or alternately a SBD you will be matrixing in, you can space them significantly wider than a stereo pair of omnis alone, without worries.  It might work fine to go much wider than that with a stereo pair alone and sound great, but it's more of a gamble.

For an improptu, clamp-able 2nd rig setup at a festival, I used a shortened section of a discarded tent pole with a windtech clamp in the center to support miniature omnis (a couple sections of typical light-weight snap-together fiberglass or aluminum pole sections connected with the internal bungee).   Worked well for a sufficiently wide spread with miniature omnis while not being too heavy or overly visually intrusive.  I think it was about 4' overall.

[Chuck]

Chuck, what are using for a bar?

I made a DIY bar using rigid plumbing pipe aluminum tubing that works well. It's light and easy to set-up.

Edit to add:

I got the idea from this thread.

http://taperssection.com/index.php?topic=161914.msg2040793;topicseen#msg2040793

[chinariderstl]

I have experimented with omni spacing a lot and I use 24 - 28" for a pair of Audix Micro omni's I have. But, I have heard excellent recordings with much larger spacings too.

This is a recording I made that I really like the sound of: https://archive.org/details/furthur2012-09-22

That's with 24" spacing at Red Rocks.

That sounds good as hell!  Nice work!

[lsd2525]

Thanks for the info. Gutbucket, spacing the omnis at 3-4' at that distance, would you run the 853's ORTF, NOS, or X-Y? The omni's are Naiant X-X's; was planning on running them straight ahead or PAS.

[Gutbucket]

Assuming they'll be mixed with the omnis, I'd lean towards runing the cardioids X/Y in the center.  A coincident pattern in the  center cuts down on phase interaction complexities which may or may not sound good when mixing a near-spaced pair with the omnis.  It allows you to adjust the width of the central image cardioids by panning that pair however it sounds best without any comb-filtering between that pair itself, even if they are both panned fully center (mono) letting the omnis do the stereo work.  Without the omnis I usually prefer a typical near-spaced configuration for a cardioid stereo pair.  However, the phase interactions between four seperate physical microphone positions mixed down to two channels over that of three microphone positions mixed down to two channels is significantly more complex as mentioned earlier in the thread Chuck linked. 

The spaced omnis produce plenty of phase difference information which completely counters the tendancy of a coincident X/Y center pair to sound sort of narrow and without spatial dimension when listened to on it's own.

[lsd2525]

For argument's sake, what would happen if I put both cards together (side by side or one on top of each other) and pointed them dead center? Would I get like 2 channels of mono? (or, "stereo" mono?)

[Fried Chicken Boy]

I know the OP is asking about split omnis, but what about using a jecklin disk?  Better results with split omnis?  Are there better circumstances than others to use one (outdoors vs. indoors, etc.)? 

[lsd2525]

If they are split, don't think a Jecklin disk would do much. Think they just act as your noggin if you're going for binaural. That's the way I understand it anyways.......I once used a foam rubber coozie trying to emulate one lol.

[Gutbucket]

I'll mention a few things about baffled omnis next, first to answer the previous question-

For argument's sake, what would happen if I put both cards together (side by side or one on top of each other) and pointed them dead center? Would I get like 2 channels of mono? (or, "stereo" mono?)

To the extent that the two microphones are truly identical, mounted as close to coincident as possible and pointed in the same direction, you'll end up with two channels of identical information.  Call that mono or dual mono or whatever.  The only practical difference between doing that and recording a single channel with one microphone, then duplicating that to get two channels will be the slight real world response differences between the two microphones, the evitable but minor differences due to imperfect coincidence mounting and any differences in the two signal paths (a gain match being the most obvious).  But none of that stuff is likely to be significant enough to make much of an audible difference in regards to what we're talking about here.

If you mix those two identically pointed X/Y cardioids together at equal signal levels, the resulting polar pattern response will resemble a single forward facing cardioid with the same pattern shape as the two individual microphones, and the resulting signal level will be about 3dB higher than the two individual channels.

If you were to point those X/Y cardioids 180 degrees apart so they are facing in opposite directions and either mix them together to a single channel or pan them both to center (which is essentially the same thing), you end up with an omnidirectional polar pattern response. 

If you arrange them in a more typical X/Y angle and either mix them together to a single channel or pan them both to center, the combined response of the two will be a forward facing semi-cardioid. The degree to which it is either more cardioid-like or more omni-like is determined by the angle between microphones.

[Gutbucket]

So the overall combined sensitivity pattern of a pair of X/Y cardioids is always more or less semi-cardioid in shape, specifically to an extent determined by the angle between the two microphones. 

If you place that X/Y pair of cardioids in the center between spaced omnis, and pan both channels of the X/Y pair to center, the practical result is the same as if you had used only three microphones and placed a single semi-cardioid pattern microphone facing directly forward between the two omnis. 

If you pan the X/Y pair outwards away from center, the shape of the overall combined sensitivity pattern of the X/Y pair won’t change, but level differences between the Left and Right channels are introduced.  Because the two X/Y pair microphones are mounted in the same physical location in space, there is no phase difference between them, so the phase relationship doesn’t change regardless of how they are panned. That’s not the case for a near-spaced pair.

Because the phase relationship between the two X/Y channels never changes, regardless of their panning, the phase relationship between the X/Y pair and the omnis doesn’t change either.  It only changes if you physically change the spacing between the microphones.. or if you pan the omnis in towards center but don’t do that.  Just leave them routed hard-left and hard-right.  The X/Y pair will fill-out the center nicely.

If you use a near-coincident pattern in the center instead of X/Y, the phase relationships are considerably more complicated even if you don’t try to pan anything around and simply mix the left cardioid directly with the left omni and the right cardioid with the right omni, both hard-panned left and right.  If you want to adjust the panning of either the central near-spaced cardioid pair or the omnis (not sure why you’d want to pan the omnis), then you’ll introduce comb filtering artifacts similar to what you get when attempting to mix a near-spaced pair down to mono. 

The mono-compatibility of X/Y (or any other coincident configuration) is what makes it a very good choice to use between spaced omnis, not because the end result we want is a  mono compatible mix (it won’t be, because of the spaced omnis), but because the same problems encountered when mixing stereo microphone configurations down to mono are encountered in a multi-channel scenario when mixing more than two physical microphone locations down to two-channels.

With three microphone locations it’s manageable and not that problematic mixing to two channel stereo; with four locations the phase relationships become quite complicated.  In addition to being more potentially problematic to begin with, that limits the degree of freedom you have when mixing it.

[lsd2525]

Sounds like my best bet is going to be trying the X-Y. Hopefully, it that ends up sounding like ass I'll have the omni source to fall back on. Now I just got to figure out some way to mount the AT853's X-Y. Never tried that. Guess I need to figure out some home brew solution. Last question: If it were you, would you X-Y the cards or just use 1 and point it straight at the center and do a 3-track recording? And is this something I can mix down in Audacity?

[Gutbucket]

I suggest trying X/Y in the middle first if you can rig that.   If that's a PITA you can always try just the single forward facing cardioid in the center. But you'll have that extra channel so you may as well use it, and X/Y is pretty much as non-problematic mixing-wise as the single forward facing cardioid (although you could use that 4th channel for a SBD feed).

If it were me, I'd point one cardioid directly forwards and the other one backwards, spaced about 20" front/back between 6' spaced omnis.  But that's another step along the path away from trusted taper setups, through the looking-glass down into the rabbit hole. 

If the X/Y setup or single cardioid works well for you, I'll point you to a few threads explaining why you might want a backwards facing cardioid.

I've been considering adding a coincident figure-8 to the single center facing cardioid of my surround recording setup based upon this same technique, to form a Mid/Side coincident pair in the center providing similar center width adjustment as a pair of coincident X/Y cardioids.  Depending on what you are recording and the situation, the X/Y pair may be more useful to you than the rear facing cardioid.