Topic: !!Stereo Zoom simplified for PAS!!

[Gutbucket]

[Edit- a new thread with both the simplified table below and a revised and extended table and discussion about it can be found here]


Point mics at stacks and the resulting mic angle determines your mic spacing.  Like dinner in reverse, I’ll start with desert, then explain the why and how I got here if you care to keep eating.

To try this improved, but still relatively simple to use PAS mic setup technique you will need-

1) Two cardioid pattern microphones.
2) A mounting system for the microphones which allows you to adjust both angle and spacing between them.
3) A way to determine the angle between the microphones once you point them at the stacks.  You can use the simple calibrated fist method to approximate angles, or any other method you prefer.
4) This table: (I can't figure out how to use the table formating fuction here, so it's spaced with underscores)

Mic angle___Mic spacing
(degrees)__(centimeters)
__110_________9
__100________14
___90________20
___80________27
___70________36
___60________48
___50________64
 

Here’s how to go about it-

1) Determine where you will setup.  You may not have a choice if this is determined for you by the venue.
2) Setup the mics and point them at the stacks (the PA speakers).
3) Measure the angle between the microphones (or the angle between the stacks, same thing).
4) Consult the table to determine the correct spacing between the microphones for that angle.
5) Adjust the spacing to match.
6) Record.

That’s all you need to know to use it. Give it a try and let me know how it works for you.

If you want to know more, I’ll explain the what & why in a few following posts.
Enjoy!

[Edit- attached is a GIF of the same table.  Note that the setup near the center of the list indicated with grey shading is DIN.  So if the angle between the PA speakers as viewed from your recording position is 90° then running improved PAS is identical to running DIN]

[Gutbucket]

Can anyone explain or point me to something that explains the table format here before I post a couple more extensive tables?

Code: [Select]

[table]
[tr]
[td][/td]
[/tr]
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Or if there is a better way to simply preserve the column spacing for data copied out of Excel and pasted here I'm open to that.

[page]

Mic Angle,

Mic Spacing

---------------------

110	= 9
100	= 14
90	= 20
80	= 27
70	= 36
60	= 48
50	= 64

[page]

so yeah, thats how you do a table, but are you sure about the values? Did you compute that from the stereophonic zoom doc or is that just an example?

[Gutbucket]

Thanks for the insight into the tables, what a PITA.  I think I'll just post an image capture for the next one with mulitple columns.

It's Stereo Zoom data from here- http://www.sengpielaudio.com/Visualization-ORTF-E.htm, since punching in numbers in that on-line calculator is easier than referencing the charts. I'm working on a follow up that explains how I worked this up.  Basically it takes the SRA of DIN and extrapolates that, adjusting spacing to match your PAS angle.  I'm wording the  follow up for users that are unfamiliar with Stereo Zoom, so appologies to you and others here that are already familiar with the concepts.

[DigiGal]

I'm still hoping someone can create an iOS app version of that visualizer.

http://www.sengpielaudio.com/Visualization-ORTF-E.htm

[Gutbucket]

The what & why-

I was explaining the Stereo Zoom concept to someone recently and realized that I could probably simplify usage of it for them to get started by applying it to the popular and straightforward PAS (Point At Stacks) microphone setup technique.  In case you are unfamiliar with the Stereo Zoom concept, I’ll just briefly note that it is a conceptual frame work which explains the underlying relationship between the angle and spacing between a pair of stereo microphones used to record music for playback over a pair of stereo loudspeakers. It allows a recordist to trade level-based stereo cues which are derived from the directionality of the microphone’s sensitivity pattern and the angle between them against time-based stereo cues which are derived from the spacing between the microphones.  The standard microphone configurations such as DIN, ORTF, X/Y, A-B, etc. are specific solutions along a continuum and the relationship between them and how they distribute the sound sources on playback over speakers is explained by the Stereo Zoom.

I recommend reading the Stereo Zoom paper to help understand how that works.  You can download the PDF from here.  I think it is also in the TS reference section.

Playing around with the stereo microphone setup visualizer web app at the Spengpiel audio website http://www.sengpielaudio.com/Hejia.htm, which I believe uses Micheal William’s Stereo Zoom Data for it’s calculations, I plugged in different numbers and wrote down the data to make a few simple tables.  I based all of this on the DIN microphone setup, which is one of the more popular mic setups for concert recording using cardioid microphones.  The DIN setup arranges the two microphones with a 90° angle between them and a spacing between mic capsules of 20cm.  That setup has a ‘pickup angle’ of 102°, meaning that sounds arriving at the microphones from within that angle will be distributed between the two speakers on playback, with sounds at the outer edges of that angle coming directly from the speakers and sounds arriving from within that angle distributed between the two speakers as a phantom image. In Stereo Zoom terminology  the ‘pickup angle’ is referred to as the Stereo Recording Angle (SRA) and expressed as an angle from either side of center- so a ‘pickup angle’ of 102° is expressed as an SRA of +/- 51°. 

The first table I made listed all the different combinations of spacing and angle using cardioid pattern microphones that achieve the same SRA of +/- 51° that we get with the DIN configuration. Here it is:




Table of various two channel cardioid arrangements for ~100º SRA (+/- 50º)
(see attached image below - Table of two channel cardioid arrangements for SRA100.gif)




Theoretically, I can choose any of the combinations of mic angle and spacing from the first two columns of that table and get a similar spread of sound sources between the speakers on playback. They won’t all sound the same though for a number of reasons. 

For one thing choice also affects how sounds arriving from outside the SRA angle are picked up and the mic arrays sensitivity to them as a pair.  Configurations that have the microphones arranged more parallel to each other will favor sound arriving from the front more than the back and sides to a greater degree.  Another aspect is the different imaging of time based stereo verses level based stereo, which is often more of a personal preference thing.

Also on a practical level not all of the choices are equally valid, especially those that have the microphones angled very widely apart.  That’s due in part to the off-axis response of many microphones being less smooth and accurate than the on-axis response, so it’s often better to choose a configuration that points the microphones more towards the sources of sound.  Partly for that reason, and partly because I suspect the SRA relationship will break down at the extremes, I didn’t include solutions for mic angles of more than 110° and spacings wider than 64cm from the simplified PAS table in my first post. I’ll post the whole thing later if that helps the discussion.  But next I want to explain how I got there.

The constant SRA table in this post is interesting and sheds some light on developing a gut understanding the relationship, but I wanted one that changes the SRA to match that of the mic angle in an intuitive way.  More on that next..

[Gutbucket]

My first thought in doing that was to simply take the list of PAS angles and plug that into the calculator as the angle between microphones to find spacing solutions where SRA = mic angle.  But is that really what we want?  Most of the time we want a SRA that is slightly wider than the angle of sound sources (called the orchestra angle in that on-line calculator).  We discussed this a few years ago in this thread on understanding the concepts of the Stereo Zoom.  As noted in that thread, the Stereo Zoom paper mentions that many engineers prefer an SRA slightly wider than the total width of the sources (orchestra angle) and suggests + 10° as a general guide.  I’m using DIN as the reference here, and it’s SRA is +/- 51° or 11° wider than the 90° angle between mics, so I simply added 11° to each mic angle down the list to get each target SRA and solved for that.  So on the PAS table SRA=mic angle + 11°.  Sometimes it works out to 12° or so to end up with distances in whole centimeters.

BTW, I also cross referenced DINa’s  SRA= angle + 11° at a few positions and the solutions are similar, but slightly wider spaced.  The simple solution of SRA=Mic angle+0° produces wider still spacings.  It breaks down at the extreme settings because at one extreme mic angle becomes zero when both mics point directly ahead, but SRA can never reach  zero (the calculator tops out at SRAs of around 9°  by entering outrageously ridiculous inputs) and at the other extreme the mics become coincident as mic angles approach 140°.

The moral to the story is moderation in choosing values in the middle of the range which balance time-differences and level-differences. That’s why I only posted the range of microphone angles of  50° – 110°.  Note that near the middle of the range in the PAS table you’ll find DIN, which has a pretty even balance between the two with a level differences making up 53% of the phantom image shift and time differences making up 47%.

Here’s the full PAS table including all the mic angles, the SRA values from the calculator and the relative percentage of time vs level differences for each configuration-
 

[Gutbucket]

A couple points I'd like to make about the PAS technique.

We sometimes tend to think of PAS as a simple yet often good sounding starting point that's easy for new tapers to use before moving onto more advanced mic configs.  It sure is easy to setup.  Part of the problem with the technique is there has never been a specific spacing specified, the idea has alway been to simply point the mics at the stacks, which is a blatantly obvious solution to any novice before developing a deeper understanding of the complex relationship between mic angle and spacing.

But there are very good technical reasons for pointing the mics directly at the stacks which we sort of overlook with other techniques, beyond the easy setup, as long as the spacing between them plays along! And the spacing playing along is what this thread is about. Here's a few that come to mind:

1) As mentioned previously, less than stellar mics often show their warts in their less than ideal off-axis response.  Great mics are much smoother off-axis and hold their patterns far better, but all mics are most accurate on-axis, so why not take advantage of that by pointing them directly at the two primary sound sources?

2) One of the biggest challenges we face in recording bands in rooms from a location out in the audience is optimizing the direct to reverberant ratio.  That's just more fancy talk that basically means getting more of the music coming out of the speakers and less of the echoy room sound and crowd sound on your recording.  Pointing the mics directly at the PA speakers does that better than any other configuration.

So even advanced tapers with stellar gear and a good understanding of other stereo configurations may find this advantageous.  Many here like applying the Stereo Zoom, and this is nothing more than SZ applied to PAS.  Maybe a more fitting tread title would have been: PAS grows from ugly duckling >  swansong.  If you do try this please let me know how it works out.

[page]

If you do try this please let me know how it works out.

http://www.archive.org/details/isd2011-12-31.ck950.flac16

I ran 65 degrees, 26cm because of the SZ curves. The stack angle was about 45 degrees. Not exactly PAS, but fairly close. I wanted to keep the SRA of DINa but get close to PAS.

Basically it takes the SRA of DIN and extrapolates that, adjusting spacing to match your PAS angle.

gotcha, that was the bit I missed from the original bit of the post, my bad.

So one of the neatest things that I think you've done is the chart effect. If someone does up a set of tables (with ortf/dina/din/nos SRA basis), then it would be pack a set of charts, and walk off to the show and get a good 3/4ths of the way there.

[Gutbucket]

http://www.archive.org/details/isd2011-12-31.ck950.flac16

I ran 65 degrees, 26cm because of the SZ curves. The stack angle was about 45 degrees. Not exactly PAS, but fairly close. I wanted to keep the SRA of DINa but get close to PAS.

Nice, thanks, dig the Dusters.

So one of the neatest things that I think you've done is the chart effect.

Yeah, the idea was to make it as easy as possible to use with a very simple table.  The hassle I see with SZ is deciphering the graphs and applying them to real world recording situations.

[Will_S]

My first thought in doing that was to simply take the list of PAS angles and plug that into the calculator as the angle between microphones to find spacing solutions where SRA = mic angle.  But is that really what we want? 

This is somehing I've done many times with usually decent results.  Thanks for putting it in a table.  One intersting further step would be shadings towarn of large angular distortions or unacceptable direct/reverberent ratios as in the original stereophonic zoom.

Most of the time we want a SRA that is slightly wider than the angle of sound sources (called the orchestra angle in that on-line calculator).  We discussed this a few years ago in this thread on understanding the concepts of the Stereo Zoom.  As noted in that thread, the Stereo Zoom paper mentions that many engineers prefer an SRA slightly wider than the total width of the sources (orchestra angle) and suggests + 10° as a general guide.  I’m using DIN as the reference here, and it’s SRA is +/- 51° or 11° wider than the 90° angle between mics, so I simply added 11° to each mic angle down the list to get each target SRA

This is interesting but when recording a mono PA are the two stacks really the edges of the sound source, or (especially at distances where the angle gets small) should the focus instead be a convincing representation of the audience and room?  My approach from far away is often to figure out how to get at least a 120 degree SRA (so numerically, 120 or less) without "missing" the stacks by too much or changing the direct/reverberent ratio too much (angular distortion is not irrelevant, but is my lowest priority).  If set up far back, this often means a good image of the crowd with the PA sound centered - which strikes me as pretty accurate.

[hi and lo]

Thanks, Gutbucket. Another typically awesome contribution to the forums! I will try this tonight with the grace spacebar and see how it goes.

[Gutbucket]

Thanks! The Spacebar is perfect for this.  In the photos I've seen of it, both spacing and angle are indicated with marks on the bar and mounts so figuring both should be a piece of cake.

Will, you raise good points about persepctive.  I think much of it is preferece and I don't think there is any one correct answer.  I suppose the esthetic is similar in some ways to choosing a photographic perspective with zoom and depth of field foreshortening- a wide angle view of the whole scene verses zooming in for a closer perspective.  That's why it's called the Stereo Zoom I guess.  Where the analogy breaks down is that with photography what is cropped out is eliminated completely from the image, where here we still pickup sounds from outside the SRA, but they are simply pushed to the outer edges of the sonic image between the speakers. So the question then isn't as much about cropping out audience and room ambience as much as about how wide we want the music to sound. We can apply the SZ to choose different angle/spacing relationships which all have similar SRAs to help control the reveberant balance and rear sensitivity.

I generally like a wide and forward perspective, with an enveloping audience wrapping around the sides, but I think the best perspective is one which matches the other qualities of the recording that indicate distance.  If the recording sounds somewhat distant instead of close, say due to frequency balance or direct/reverberant balance, then a big, wide image of the stage filling the whole 'sonic scene' may be less appropriate.

I have similar priorities, pretty much this order of importance for me:
good direct/reverb ratio
reasonable frequency balance
appropriate stereo width
minimal angular distortion of stereo image

[hi and lo]

Thanks! The Spacebar is perfect for this.  In the photos I've seen of it, both spacing and angle are indicated with marks on the bar and mounts so figuring both should be a piece of cake.

Yup, should be perfect. The only annoyance is that you still have to measure capsule separation. The cm spacing on the bar is just a reference and doesn't reflect the actual capsule spacing. Nevertheless, I have an interesting show tonight where this method should work perfectly for testing.