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Author Topic: Improved PAS setups - better imaging with higher direct-sound/reverberant ratio  (Read 9710 times)

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Offline Gutbucket

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I've reworked the Point At Stacks (PAS) microphone configuration setup table I posted a few years ago to make it more useful for advanced tapers, rather than simply consisting of a few spacing recommendations for the quick-and-easy PAS setup.

The table takes PAS and applies the Stereo Zoom concept and data to suggest a few appropriate microphone spacings determined by whatever angle you end up with between microphones when you point them directly at the PA speakers.  I’ll explain where, why and how to use it in following posts. Here it is below as a GIF.  I’ve also attached it as a PDF.

[edit- first, here's a copy of the original simplified table with only two columns (calculated using cardioids, the shaded row is DIN)]-


[and below is the new extended table with several options and information for each PAS angle]-

[edit again- Try figure 8's in PAS outdoors.  They require the least spacing between mics of any pattern, allowing even narrow-width mic bars to work).  More here- http://taperssection.com/index.php?topic=177050.0]
« Last Edit: April 01, 2016, 02:12:04 PM by Gutbucket »
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Offline Gutbucket

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How to use the table-

1) Determine the angle between the PA speakers as seen from the recording position.

2) Consult the table. Find the angle between microphones in the left column.

3) Choose one of the four spacings indicated in the Mic spacing column immediately to the right.  The safe bet if you don’t know which to pick is the one in the grey row. 

4) Setup the mics with that spacing. If your mic bar doesn’t let you achieve that particular spacing, try to get the microphones somewhere within the range of spacings shown for that microphone angle. If you can’t do that, just get as close as you can.

5) Record. Enjoy the show. Go nuts. Go home.

6) Play it back and listen.  If you prefer a more narrow sounding presentation with the on-stage and PA sound more tightly grouped in the middle between the speakers, choose a wider SRA the next time (which means a narrower microphone spacing).  If you’d prefer something that sounds wider, choose a narrower SRA the next time (a wider microphone spacing).  Once you determine your personal preference you can always default to the same row, or you can pick and choose between them to suit the situation.

That’s it.
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Offline Gutbucket

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The why and where-
 
The table is based on data from the excellent sengpielaudio page which has been linked here at TS many times before.  It’s a great tool.

The inspiration to revisit this was deadheadcorey's recent thread titled- PAS/POoS which got me thinking again about why the PAS configuration makes a lot of sense in difficult recording situations such as rooms with bad sounding ambiences, overloud obnoxious audiences, and recording positions which are further away from the stage and PA than we'd choose, because it helps maximize the direct/reverberant pickup from a given location.  In simple terms that means it focuses as much as possible on the sound from the PA and band on stage (the direct sound), and less on the sound arriving from everywhere else (the reverberant, ambient sound). 

The most effective way to maximize the direct sound and minimize the reverberant room sound is to move closer to the source.  The extreme is stack taping where the majority of the sound arriving in the recording position comes directly from the PA and very little of it is reverberant room sound (at least in proportion to the PA sound).  The opposite extreme is the far back of an arena, where the situation is reversed and most of the sound arriving at the recording position is reverberant room sound, swamping the direct sound.

PAS using supercardioids maximizes the proportion of direct sound picked up verses the reverberant sound as much as possible from a given recording location.  It isn’t a substitute for finding the optimal recording location and it certainly can’t make the back of the room sound like the front.  It simply makes the best of a mediocre situation. From a good recording position in an excellent sounding room, other configurations may be more appropriate, but using the table won’t make a bad recording.  It’s entirely possible to have a direct/reverberant ratio that’s too high.  That’s one problem with a stack tape or a straight SBD, it’s mostly direct sound that often doesn’t have enough good reverberant room information and sounds not so 'live' but rather lifeless.

The problem with the typical PAS setup is that it doesn’t indicate how much spacing is appropriate between the two microphones. Most tapers simply use whatever spacing their mic bar provides. At the narrow microphone angles typical of PAS, the spacing between microphones is often not enough to achieve good playback imaging which evenly fills the space between speakers with phantom images (or sounds open and as if ‘you are there again’ over headphones) and provides an appropriately wide and involving audio illusion with the audience applause wrapping around the listener.


PAS is a dedicated concert tapers microphone configuration and this table quantifies it to make it a more valuable tool with improved stereo imaging.

Here's a link to the original thread discussing the first go-round at this, which partly works through the process of developing the table- !!Stereo Zoom simplified for PAS!!

This new version of the table expands on it by using supercardioid microphones as a starting point. Cardioid mics will work too, and probably image similarly for the direct sound, the data doesn’t change that much substituting that pattern (notice that the same angles on the simple table which is  based on cardioids, and the new extended table which is based on supercardioids have spacings that differ by only about 1cm), but using supercardioids helps maximize the direct/reverberant pickup ratio.
« Last Edit: April 03, 2014, 10:40:56 PM by Gutbucket »
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Offline Gutbucket

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More details on each step and on the other stuff in the table-

1) With PAS, the angle between PA speakers is always the same as the angle between microphones.  Using supercardioid (or hypercardioid) pattern microphones and pointing them directly at the PA is what maximizes pickup of the direct sound component as much as possible from the recording position.  The first step after choosing a recording position is to measure that angle.  Since they are the same, you can either meausure the angle between PA speakers or the angle between microphones.  There are a few ways to measure.   I’ve learned to measure distant angles quickly and relatively accurately by using what I call the “backyard astronomer / poor sailor method”.  A balled fist held at arms length and viewed through one squinted eye covers approximately a 10 degree arc.  Practice measuring a few known angles and you can perfect it pretty quickly.  I need to kind of stick my thumb knuckle out a bit to get a full 10 degrees.  Yes it looks funny when doing it.  Smart-phone apps now exist which make measuring angles easily without looking like you are shaking your fist at the stage.

Alternately you might simply go ahead and set up the microphones, point them directly at the PA speakers, measure the angle between the microphones themselves, then readjust their spacing once you determine what that should be.  You can measure microphone angles directly by whiping out a protractor or using an angleometer, use some creative oragami folding techniques to estimate angles, or a smart-phone app.

2) Consulting the table- If you understand the Stereo Zoom concept, here’s a few things going on here: The PA angle is the Orchestra Angle and on this table that’s always the microphone angle as well.. that’s what makes it PAS.  The grey rows provide a Stereo Recording Angle which is 10 degrees wider than the Orchestra Angle (and the stacks), which is generally a good starting point.

The options in the other rows for the same microphone angle are SRAs which are 20 degrees wider than the PA speakers, the same width as the PA and 10 degrees narrower than the PA.  That information and the total SRA angle are indicated in the columns to the right of the one indicating the closest “standard” mic setup, which is just for reference.  The only standard setup which is exact is DINa (which maybe only a standard to tapers and not the rest of the world anyway), the others are approximate.  Yes, it is true that ORTF is only ORTF if it uses small diaphram cardioids which have well behaved polar patterns angled 110 degrees apart and spaced 17cm.  The squiggly line in front indicates “approximately” and the indication simply provides a general point of reference many users will be familiar with.

3) Playback width is how far the sound sources within the SRA window extend outwards from the center towards the speakers on playback.  It describes the relationship between the SRA and the “playback window between the two speakers”. If it’s 100%, sounds originating from the outer edges of the SRA should spread out to the speaker locations, if less than 100% the outer edges of the SRA won’t extend all the way out to the speaker locations and things will be more tightly grouped in the middle.

4) The Level and Time delta columns indicate what proportion of the phantom stereo imaging is based on level differences between channels due to the angle of the microphones and how much is due to time of arrival differences due to the spacing between them.

5) Notice how wide the microphone spacings become with narrow microphone angles.  As the angle between microphones gets increasingly narrow, the level differences between them decrease rapidly and they begin behave more like omnis with regards to stereo imaging, though not like omnis at all in terms of direct/reverberant pickup ratio.  It might be difficult to setup the wide spacings indicated on the table due to practical considerations such as the limited length of the microphone mounting bar, but since decent spaced omni recordings can be made with less wide spacings it may be acceptible for you to err towards spacings which are somewhat less wide than what the table suggests in those situations.  However I doubt I’m the only one who likes omnis a few meters wide when recording from far enough away that the angle between PA speakers is only measures about 40 degrees wide or so.  When you consider it from that perspective, setting up supercardioids about a meter apart with a 40 degree angle between them will begin to seem much more reasonable than it might have at first glance, even if it’s not always so easy to achieve. 
« Last Edit: May 21, 2014, 03:34:50 PM by Gutbucket »
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Offline MIQ

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Hi Gut

Excellent post on applying the Stereo Zoom concept as usual. Thank you for posting this!

I think there may be a small cut and paste error in the 50 degree row.

(Trying to walk a fine line between shameless self promotion and adding useful info) >:D
Like you wrote there are smartphone apps that can increase the choices beyond the 10 degree increments you've listed. One nice thing about the Stereo Mic Tools app we developed is that it allows you to measure the angle of your mics when pointed at the stacks in the "bombsight" screen then quickly flip to the recording angle calculator screen to determine the exact mic spacing that makes the recording angle match the mic angle. Also we provide selection of many popular directional mic patterns including the cardioid and supercard patterns you've shown in these tables. Of course a printed copy of the table you've created will work even when your iPhone battery is dead.  ;D

FYI your original post on this is the first reference listed in the "info" page of the Stereo Mic Tools app. Much respect.  :)

Miq
« Last Edit: April 04, 2014, 06:01:25 AM by MIQ »

Offline deadheadcorey

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thanks for putting this together gutbucket!
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Offline Ultfris101

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this is great. many thanks for the effort. I need to get much wider spreads in the small, narrow club where I typically tape from the back of the room near the soundboard. going to try that tonight.

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Offline Gutbucket

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Thanks for the thanks. 

There is one thing I'll ask of those of you who use this which will help refine it-
Once you've used this enough to determine which SRA width choice you like most of the time (+20°, +10°, same as the stack angle, -10°), please post here with feedback to let me know your general preference.  And I mean your preference for the resulting playback results, rather than those based on setup constraints like whatever your mic bar width happens to allow.

That question is something I've wondered about and it can only be determined through trial and error.  The +20° to -10° range is a simply a best guess at an appropriate range of options.  The best answer may be mostly personal preference, may be situation dependant, or a combination of things.  I may add a row for +30 as well, to extend the options to less wide spacings which are closer to typical PAS setup as commonly practiced, and which can be more easily done with less wide bars, even though I suspect that will be a wider SRA (and narrower resulting image) than is optimal.  I doubt there is any need to go to finer than 10° angular increments.  Similarly the tolerance on the fractional centimeter and inch measurements on the table are more precise than they need to be.  Get it to within an inch or so and that's probably close enough, especially at wider spacings.

Thanks to Michael Williams for the Stereo Zoom data and research.  This is just a re-interpretation of that to present it in a way that's specific to what we do.
« Last Edit: April 04, 2014, 01:28:57 PM by Gutbucket »
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Offline dyneq

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Hey All,

I'd like to experiment with this method and had a couple of questions:

To those of you who have tried this in the field: what method did you use to determine the angle between the PA speakers? The chart has 10 degree increments, and that seems like a tricky distinction to make in the field. Since the spacing is quite different for the different angles, I'd want to try and get fairly close to the actual angle, especially as I experiment to find my spacing preferences (as repeatable as possible).

Anyone experimented with it outdoors? If so, which part of the spacing spectrum did you run/prefer?

I plan to try this at Rockygrass where it was a gentle slope from the OTS down to the stage (not a well-defined bowl). After last September's flooding, it may look completely different this year.

Offline Gutbucket

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questions:

To those of you who have tried this in the field: what method did you use to determine the angle between the PA speakers? The chart has 10 degree increments, and that seems like a tricky distinction to make in the field. Since the spacing is quite different for the different angles, I'd want to try and get fairly close to the actual angle, especially as I experiment to find my spacing preferences (as repeatable as possible).

[self quote]
There are a few ways to measure.   I’ve learned to measure distant angles quickly and relatively accurately by using what I call the “backyard astronomer / poor sailor method”.  A balled fist held at arms length and viewed through one squinted eye covers approximately a 10 degree arc.  Practice measuring a few known angles and you can perfect it pretty quickly.  I need to kind of stick my thumb knuckle out a bit to get a full 10 degrees.  Yes it looks funny when doing it.  Smart-phone apps now exist which make measuring angles easily without looking like you are shaking your fist at the stage.

Alternately you might simply go ahead and set up the microphones, point them directly at the PA speakers, measure the angle between the microphones themselves, then readjust their spacing once you determine what that should be.  You can measure microphone angles directly by whiping out a protractor or using an angleometer, use some creative oragami folding techniques to estimate angles, or a smart-phone app.
[/self quote]
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Offline Ultfris101

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I've been using the outstretched fist method. Never actually measured to see how close it is but I've heard it mentioned a few times.
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Offline voltronic

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The best tool I have found for this is a compass bearing app with camera overlay.  You point center, note the bearing.  Pan to the far left or right of your included angle, note the second bearing.  Couldn't be easier.  I have found that the "large viewfinder" overlay (second image) is the most accurate. 
https://play.google.com/store/apps/details?id=com.appyhand.bearingcompass
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Offline dyneq

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Thanks, folks!

Since I'm after repeatability as I test/tweak this technique, I'll give the app a try. I'm the same way about cooking; follow the recipe as it is written the first few times, then adjust to my tastes!

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The best tool I have found for this is a compass bearing app with camera overlay.  You point center, note the bearing.  Pan to the far left or right of your included angle, note the second bearing.  Couldn't be easier.  I have found that the "large viewfinder" overlay (second image) is the most accurate. 
https://play.google.com/store/apps/details?id=com.appyhand.bearingcompass

Thanks!

I've been using "Smart Tools" in my android phone for some time (paid app with a whole bunch of stuff in it) but they have a free "Smart Compass" app which is part of that.  Works really well, too.  I'm still gonna give the AB Bearing app a shot next time I'm out.

Smart Compass is here:
https://play.google.com/store/apps/details?id=kr.sira.compass

Offline voltronic

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The best tool I have found for this is a compass bearing app with camera overlay.  You point center, note the bearing.  Pan to the far left or right of your included angle, note the second bearing.  Couldn't be easier.  I have found that the "large viewfinder" overlay (second image) is the most accurate. 
https://play.google.com/store/apps/details?id=com.appyhand.bearingcompass

Thanks!

I've been using "Smart Tools" in my android phone for some time (paid app with a whole bunch of stuff in it) but they have a free "Smart Compass" app which is part of that.  Works really well, too.  I'm still gonna give the AB Bearing app a shot next time I'm out.

Smart Compass is here:
https://play.google.com/store/apps/details?id=kr.sira.compass

Very cool!  It does the same thing as the AR Bearing Compass app, but looks a bit slicker.  Thanks!
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