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Author Topic: !!Stereo Zoom simplified for PAS!!  (Read 6431 times)

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Offline Gutbucket

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #15 on: January 27, 2012, 12:08:29 PM »
Yeah, I realized the capsule spacing would be larger than what is inicated on the bar as I was typing.  The clincher though is the clear mic angle indication which is more of a challenge to meausure accurately.
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Offline alpine85

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #16 on: January 27, 2012, 12:47:12 PM »
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.

Quote
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.

Good points.   Another thing to consider is... Is the best sound that we want our mics to capture actually AT the stacks themselves?  Especially in bigger venues, with longer throw drivers, could the sound be much more balanced and accurate 20 feet out from the stacks?  50 feet?  100 feet?  In general, this is definitely a very helpful guide (and thanks for putting it together, Lee), but it seems like there will be more variables in a lot of situations.   Are you in the sweet spot?  If not, do you want to point the mics towards the sweet spot (which might or might not be directly at the stacks)?  I also tend to agree with Will_S that direct/reverberant ratio is probably a lot more important than angular distortion when you're recording a PA.  (Even if it's a stereo PA).  In fact, if you're only getting angular distortion on the reflections and on the crowd (and not the music itself), is it really a factor at all?
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Offline Brian Skalinder

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #17 on: January 27, 2012, 01:18:28 PM »
Another thing to consider is... Is the best sound that we want our mics to capture actually AT the stacks themselves?  Especially in bigger venues, with longer throw drivers, could the sound be much more balanced and accurate 20 feet out from the stacks?  50 feet?  100 feet?  < snip >  Are you in the sweet spot?  If not, do you want to point the mics towards the sweet spot (which might or might not be directly at the stacks)?

While I agree with your comments for the most part, and recognize there are a variety of factors that come into play, I think there are two notions suggested here (quoted above) that may confuse newbies.  Perhaps it's due to the imprecision of language.  Specifically:

  • The "best sound" we want to capture isn't really AT the stacks, or any other single (or multiple) point(s) source, for that matter.  Generally, the "best sound" exists at a particular spot that suits one's recording / gear / configuration / listening preferences:  typically a specific location / height in the room, i.e. the "sweet spot".
  • Pointing mics "towards the sweet spot" doesn't make much sense, if one defines the sweet spot in the traditional taper sense:  the (or one of the) location(s) in the venue that sounds the best.
And, unfortunately, much of the time we're unable to select our "sweet spot", since our location is decided for us, in whole or in part, by the band, venue management, physical space, etc.  In other words, we often don't have control over the single most important variable -- location -- and must rely on the other variables we can control:  like mic pattern and configuration.  In that sense, I can see the the chart proving helpful as a quick reference.  (FWIW, I carry cut-outs of the SZ graphs in my gear bag.)
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Offline alpine85

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #18 on: January 27, 2012, 02:35:34 PM »
You're right, Brian.  "Pointing at the sweet spot" is probably not the best way to describe it.  I was just trying to say that there are times where PAS is not ideal.  Here are 2 extreme examples:

1)  Indoor venue, long narrow room with PA stacks in the corners pointing towards the middle of the room.  You are in the back of the room.  You probably would get way too much room sound/ reflections by pointing at stacks - a wider spacing and pointing just inside the stacks may work better. 

2) Outdoor venue, large open field with PA stacks on the sides pointing straight forward.  Not much center fill, but you are stuck way up front (in front of the "sweet spot").  You would probably be better off with a tighter pattern and wider angle on the mics, pointing just outside the stacks rather than straight at them. 

Yeah, I know, I'm over complicating things, when the point of the original post was to simplify ; ) 
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Offline Todd R

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #19 on: January 27, 2012, 03:06:50 PM »
Interesting discussion, thanks Gut!

I've only read as far as Gut's two long threads -- need to get back to this when I can really digest it.  Mapping PAS configurations/spacings to match DIN (in the first thread) makes total sense to me.  I'm missing how that mapping changes if you want to increase the SRA beyond that of the spread of the orchestra (PA stacks).  Seems like if you apply the need to have a wider SRA for DIN, then you would map to that SRA for PAS, and the mapping would be the exact same (recognizing that the only way to insure the SRA for DIN is exactly 10 degrees more than the orchestra width is to move closer or farther, since the angle and capsule distance for DIN isn't changing by definition).

I'll try to wrap myself around that when I can digest it more. In the meantime, I don't think I've ever cross-posted something on ts.com, but the post I just made in the playback section seems to make more sense in this thread:

I tend to believe that most of our 'standard' configs are too wide, even at 90 degrees. I have always loved the 4v sound run backwards on an ORTF bar at 70 degrees and have been experimenting with a narrow angle quite a bit of late.

Yes, I'm with you on that, or at the very least want to try that sometimes vs always doing DIN.

Probably should at least reference Gutbucket's thread on PAS and stereozoom.  I've only had a chance to glance at that, but alot of what really is going on is probably explained there.

Many years back I used to run on occasions a modified NOS pattern, with caps at 30cm spacing and mic angle at ~75 degrees.  I found it worked good in many instances, esp when you only had cards to work with (I had km140s at the time).  Then I got the jklabs setup for the AKG caps, and both had the option of ck63 hypers available and was also working off a fixed DIN kwonbar -- so I got away from the modified NOS*75 pattern.

Lately I've been trying it again more.  A mod NOS*75 pattern with cards, which often is close to pointing just outside the stacks for where I am taping from, I think gives an SRA that is about the same as DIN, but since it is a narrower pattern, gets more direct PA sound and less room reverb.  So the soundstaging is similar to DIN, with less reverberant sound.

I also think something like this (wider spacing, narrower angle) probably works better with brighter mics.  IIRC, larger spacing can lead to more comb filtering on high frequencies, softening them a bit.  So flat mics don't tend to sound as good with larger spacings since the high frequencies are already flat, but softening the highs on mics that have a high freq bump works a bit better.  (Hopefully I've got that all right, going from my memory of having investigated it 9 years ago or so.)

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Offline Gutbucket

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #20 on: January 27, 2012, 03:07:29 PM »
..it seems like there will be more variables in a lot of situations.

Yes, true of course.  This is just another tool to use when appropriate.  No single setup works for every situation, which is part of the challenge in making great sounding recordings and also what makes it far more interesting than simply patching into the SBD if you enjoy figuring out the puzzle.

You raise a good point about what to do to compensate for less than perfect recording positions.  I think this technique can be helpful for less than ideal positions farther back than would be preferable, and does that by it's nature without having to put much though into it.  That's because in those cases it will automatically produce a setup with less angle and more spacing between mics than it will when setup closer, and that trend helps to get a better direct/reverb ratio which I see as the biggest challenge as the recording position gets more distant.

As for off center locations and otherwise odd sound source locations and distributions, one of the most helpful things for me is simply to close my eyes and listen for a while, trying my best to forget what I know about the situation visually, and just concentrate on the sound itself- where it's virtual center seems to be, how wide it is, and how the different sound sources are distributed within that width.  I then adjust my mic setup with just that in mind.  That can produce a significantly different setup than what I would have arrived at by sight.

2 extreme examples..

OK, however in both of those examples the basic idea of this still works in your favor as a good starting point.  It produces wider spacings and narrower angles from farther back, and the opposite up close.  You can improve upon the solution further if you have a good command over the basic SZ principles. 

And that gives me another idea to possibly take this further with an optional second refining step in the process-

Start with the basic improved PAS table which gets things in the ballpark. Then if you want to adjust further, consult a secondary set of tables like the one in my later post which lists all the configurations that achieve the same SRA.  That would be a more straightforward way to optimize other aspects while keeping the SRA the same.  If you wanted to start with a different SRA than the PAS table (which is always the angle between the stacks +11°), just pretend the PA speakers are at the angle you think is appropriate and the same system works for refining the other aspects from there, without loosing the SRA you wanted.

That would just require a set of additional tables, one for each common SRA.
« Last Edit: January 27, 2012, 03:36:29 PM by Gutbucket »
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Offline Chuck

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #21 on: January 27, 2012, 03:18:40 PM »
Great stuff Gutbucket. A couple of years ago, I made myself a laminated sheet that I carry in my bag with the correct angles and spacing to simulate DIN according to the Stereo Zone article. I have found this to be very useful as you mention when pointing mics at PA stacks. It isn't perfect, but it is a useful starting point when encountering unusual taping conditions.
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Offline Gutbucket

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #22 on: January 27, 2012, 03:28:10 PM »
Thanks Chuck, it's encouraging to hear that basically the same idea has worked for you in the real world.

I'm missing how that mapping changes if you want to increase the SRA beyond that of the spread of the orchestra (PA stacks).  Seems like if you apply the need to have a wider SRA for DIN, then you would map to that SRA for PAS, and the mapping would be the exact same..

Todd, I'll have to digest that a bit to better understand what you're getting at.  But maybe this helps- the difference between the orchestra angle and the SRA is always ~11° on the PAS table.

Quote
(recognizing that the only way to insure the SRA for DIN is exactly 10 degrees more than the orchestra width is to move closer or farther, since the angle and capsule distance for DIN isn't changing by definition).

Right. So what I've done here is keep the SRA to mic angle relationship of DIN (which is 11°) as the constant. The orchestra angle is dictated by the speaker angle + that 11°, and the SZ tell then tells us what the spacing should be.
« Last Edit: January 27, 2012, 06:22:50 PM by Gutbucket »
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Offline Gutbucket

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #23 on: January 27, 2012, 03:46:27 PM »
Todd, I think what may be helpful for what you are talking about in the post from the other thread quoted above is a set of secondary tables- one for each SRA of interest, each listing the different configurations which can achieve that particular SRA.  I mentioned that a couple posts above as possibly being a good secondary refining step to further adjust things.  The second table I posted in this thread does that to list all the cardioid configs that achieve the SRA of DIN, but ideally I'd make a similar table for each of the SRAs I typically use.

Here is is again, same table-
Table of two channel cardioid arrangements for SRA100.GIF


[edit- heading over to check out that other tread]
« Last Edit: January 27, 2012, 06:32:00 PM by Gutbucket »
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Offline Gutbucket

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #24 on: February 15, 2012, 07:01:24 PM »
Tech note bump.

[snip] 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. [snip]

Looking for some possible insight into the "SRA wider than the actual source" preference thing I came across this paper by Wittek & Theile, "The Recording Angle - Based on Localisation Curves", a 112th AES Convention paper from 2002 down-loadable from here:
http://www.hauptmikrofon.de/HW/AES112_Wittek_Theile.pdf

It explains that in the various sets of experimental data upon which the calulations are based, different stereo mic configurations which should have similar Stereo Recording Angles 'by the numbers' actually are quite similar within the central 75% of the total playback image range between speakers. The larger variations between sets of data happen mostly outside the central 75% region, which is where these general calulations begin to break down, so the authors suggest comparson between various stereo mic configurations is best done by looking at at their +/-75% SRA points, not the full SRA.

Not sure exactly how applicable that is to the question of what SRA is appropriate for a given Orchestra Angle, but it's an interesting read and at least explains the somewhat mysterious (to me anyway) 75% SRA indications in the Image Assistant 2.0 visualizer: http://www.hauptmikrofon.de/ima2-folder/ImageAssistant2.html
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Offline MIQ

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #25 on: February 16, 2012, 04:01:37 AM »
Hi Lee,

 :o Unbelievably I have also been reading the Wittek paper over the past few days and was thinking about the comments about setting the SRA slightly wider than the actual "orchestra".  The main issue seems to be the localization curves different researches have developed, and specifically, the test signals and listening rooms used during the research. 

I ran into this fact a while back when the SRA I computed using the Williams Zoom did not match the SRA given by the Sengpiel web site for a coincident pair.  Since a coincident pair uses only interchannel level differences to steer the image, the difference in SRAs is linked to what interchannel dB difference is needed to image at the speaker locations and not inbetween them (Wittek calls it 100% source shift).  Williams relies on psychoacoustic curves developed by Simonsen to determine this.  Wittek uses curves he developed in his own testing and Senpiel has his own curves it appears.  Below is a little summary I wrote down yesterday when I was wrapping my head around this.  It is the inter channel level difference (ILD) and inter channel time difference (ITD) needed to place the image at the speaker used by the Sengpiel Visualizer, Wittek Image Assistant, and Williams Sonic Zoom.  These values assume that only interchannel level differences exist or only interchannel time differences exist without the influence of the other.

100% Source Shift Sengpiel                    ILD = 18dB      ITD = 1.5ms
100% Source Shift Wittek                       ILD = 15.8dB   ITD = 1.0ms
100% Source Shift Williams (Simonsen)  ILD = 15dB      ITD = 1.12ms

The ILD differences are really different between Williams and Sengpiel and were the source of the coincident mic SRA differences I ran into.  I also looked the SRA numbers each set of curves would give for ORTF and NOS:

Sengpiel ORTF SRA = 96.4 degrees  NOS SRA = 81 degrees
Wittek    ORTF SRA = 102  degrees  NOS SRA = 82 degrees
Williams  ORTF SRA = 100 degrees  NOS SRA = 80 degrees

I like Wittek's approach and goal. He is trying to come up with a "key value needed for directional balancing with stereo microphones" that "avoids confusion or room for different interpretations".  My understanding is that if you used two different mic configurations, both with the same 75% Recording Angle, the distribution of the sound sources should be very similar between your speakers.  It's not until you get to the last 25% of the sound image, played back near each of the speakers, that the imaging would differ considerably.  Of course there are many other variables that make the sound of two different mic configurations with the same 75% Recording Angle sound different, but at least the imaging would be comparable over most of the region between your speakers.  ;D

A couple of other AES papers are making me lean more heavily to using the Wittek psychoacoustic curves over any other.

"Localization Curves In Stereo Microphone Techniques - Comparison of Calculations and Listening Test Results" - Plewa and Pyda
The authors recorded several different instruments as well as white noise at different source positions in an anechoic chamber using several mic techniques.  These recordings were played back over speakers and listeners were asked to judge the location of each sound source.  The results match those calculated using Wittek's localization curves.

"A New Time and Intensity Trade-off Function for Localization of Natural Sound Sources" - Lee
Here the author asked listeners to adjust just the time difference between speaker signals or just the level difference between speakers to place the image (from the center of the speakers) at 1/3rd, 2/3rd, and at the speaker location.  The results lead to localization curves that are very close to Wittek's and if you apply Wittek's localization functions to the data, it fits a little better than the numbers the author came up with IMHO. 

The cumbersome part about using the RA_75% is that it is a 75% playback angle.  So how do you choose what stuff you want to have smeared out to the edges of the speakers.  If you choose too conservatively (put all the instruments inside the RA_75%) you narrow the sound stage between the speakers (all the instruments are accurately distributed but none of them are played back at/near the speakers).  If you choose to put more of the instruments outside the RA_75% you end up with strange imaging for the stuff outside the RA_75% near the speakers.  The reality is that using the Williams SRA is also leading to strange imaging as the playback locations near the speakers.  We want to think of stereo mic configs as taking a clear picture of the sound stage in front of them (perfect lens).  The reality is that when you listen to their recordings over speakers, the outside edge of the lens is distorted and smeared.  You can clearly see this in the Image Assistant in the "Show Localizations" screen.  The center (+/- 75% = no shit, it's based on the RA_75% right  ;D) of the localization curve is basically a straight line.  Beyond that area the line starts to curve.  We wish it was straight the whole way, but in fact it is not. :(

Sorry for the crazy long and intense post but I've been tossing this around in my head for the last two days and have not disucssed it with anyone yet.  Looking forward to hearing what other people think.

-MIQ

3/5/12 - edit for ILD numbers
« Last Edit: March 06, 2012, 02:24:38 AM by MIQ »

Offline Gutbucket

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #26 on: February 21, 2012, 11:00:28 AM »
Apologies for the delay in responding, I've been working a tradeshow for the past week and wanted to digest this a bit and respond when I had more time..

Big thanks for the info on the various studies and differences in data sets.  I simply assumed Sengpiel used the Williams data.  I'd like to look more deeply into the Wittek curves and data.

Seems to me we have two separate issues here, and I'm not sure they are related to each other: The first is the question of what SRA is appropriate for a particular 'Orchestra Angle'.  I realize that there may not be a good hard-and-fast answer to that.  It probably depends on a number of things like the type of music, the size of the ensemble, the room, the recording position, the desired image perspective, as well as the personal preferences of the recordist.  The second is the interpretation and comparison of the data from various imaging studies, where things seem to match up more or less within the central 75% of the SRA, so comparison of various techniques is best done by looking at their behaviors within that region.

It's probably a false conclusion to take that to mean that as a general rule, the appropriate SRA is one which is 25% wider than the Orchestra Angle.



A couple comments-
One analogy to this 'distortion of angular source distribution' is image distortion in camera lenses.  Many lenses distort the image at the outer edges of the frame in comparison to the center, higher quality lenses less so.  The difference is that camera lenses 'hard-crop' everything outside the frame, so a lens manufacturer can choose whether to minimize the size and cost of the optics and accept that distortion at the edges, or use larger and more complex optics and use only the less distorted central region.  Microphone arrays may or may not have reduced sensitivity outside the SRA, but they don’t hard-crop or exclude those sound sources, they just radically distort their apparent position on playback. It seems that they begin to do so outside the middle 75% region of the SRA.  But the analogy holds in that both photographer and recordist can chose to deliberately distort (compress or expand) the live image to better fit the desired end result. ‘Stereo Zoom’ as well as stereo fish-eye.

I generally like a very wide stereo distribution verses a more narrow, mono-sounding recording, as long as the image is solid across the center and there isn’t an apparent ‘hole in the middle’ with nothing there, or more commonly an  ‘island in the middle’ with nothing to the immediate left and right of center.  I really don’t mind image distortion of the distribution of sources out near the speakers, other than overly mono recordings which have much too little out there, as long as the central region is solid and blends out seamlessly to the sides.  Many live concert recordings I hear err to the safe side of a narrow, more mono sound, which admittedly is better than wide with a weak middle.

But I really don’t care much about imaging accuracy.  I care more about how I can manipulate the imaging to improve the end result.  I find it interesting to think about and understand the ‘objective’ image distortions of the scene as heard live from the recording position verses the recorded image played back on speakers, and I enjoy going home and listening to hear how close the recording matches what I heard, but I don’t consider duplicating that distribution accurately to be important at all.   Many times I want to purposefully distort the image to something more pleasing if the live distribution was less than optimal.  That might be because there was a less than ideal distribution of live sound sources even if I was able to choose the best microphone location for stereo imaging, or I may have deliberately chosen a different recording position based on things more important than imaging- such as one which better balances the relative levels of the individual sources, or one that has a better direct-to-reverberant sound ratio, or better overall timbre, or any number of things which are far more important to me than playback image accuracy.
« Last Edit: February 21, 2012, 11:05:24 AM by Gutbucket »
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Offline MIQ

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #27 on: February 22, 2012, 11:32:00 PM »
Interesting comments, especially on imaging accuracy vs optimizing the sound at playback.  No stress on the speed of response.  I wrote from Japan and am now back in the States, so I know how difficult it can be to stay on top of forum posts.   :)

There are plenty of times when the "orchestra angle" is quite a bit larger than the "playback" angle could ever be when listening in the sweet spot of a conventional 60 degree speaker set up.  It's not too easy to get the image outside the speakers  ;D, so you are stuck "compressing" the entire panorama of recorded sounds into the 60 degree playback system.   It's not likely that a two speaker playback system could ever faithfully reproduce the sound image we hear with the 3D ear-brain sensory system we are equipped with.  The thing I'm curious about is how much closer you are able to get with your surround sound recordings.  I know from reading some of your other posts that you've done a fair bit of this kind of thing.  I know it's getting a little (lot) off topic but when you record with a set-up that dedicates mic channels (or tetra mic decodings) to the rear playback channels, is it quite a bit more convincing?  Slightly better?  Just different and cool? 

I had a chance to listen to the DTS NeoX 11.1 surround sound demo room at CES this year.  I didn't hear any "live" music recordings so I'm not sure if having all those speakers would make it more "real" but it was neat. 

Back to the RA_75%, you wrote: "It's probably a false conclusion to take that to mean that as a general rule, the appropriate SRA is one which is 25% wider than the Orchestra Angle."  I bet you are right.  I think though, that Wittek and Theil are proposing that you could get pretty good results by choosing a mic system with an RA_75% that is about 25% less than the orchestra angle.  This corresponds to Fig 9 in their paper.  At least you would have the majority of the sound sources nicely distributed within the speakers and some stuff less accurately distributed out toward the speakers.  Like you wrote, there won't be "one rule fits all situations", especially if there is a lot going on at the edges of the "orchestra". 

-MIQ


Offline Gutbucket

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #28 on: February 23, 2012, 12:37:15 PM »
Not much talk on-topic in the thread at this point. I’m more than happy to jump track to interesting off topic tangents.

Any playback system outside of a high-order ambisonic or WFS system is going to distort the location of sound sources compared to what is heard live to some extent, it’s just a question of how and if the distortions are acceptable or not. 

I’m of the mindset that it is essentially impossible to reproduce the exact event, and trying to do so is interesting only as an intellectual goal, not an artistic or musically enjoyable one.  I have no qualms about setting up my recording rig or adjusting things later to optimize the recording for how it will be played back, or to ‘fix’ problems with the sound at the live event.  At one level that simply means editing out obnoxious noises, rumble, pops, et cetera; correcting timbre with EQ, and choosing microphones and setups that sound closer to what I consider ‘good’.  Leaving in the hiccups would be more accurate to what was heard there, it’s a judgment call and I aim for better than live in the ways it can be, understanding that some things will always be better live.  Usually the goal is one of transparency and striving for the platonic ideal of what I think it should sound like, often an improvement on the live sound I heard at the recording position in some ways.

What is important in a mono recording?  With mono there is just one channel so in geometric terms the recording is ‘zero dimensional’ like a point. What is important in a mono recording is overall level, timbre, instrument balance, and the direct/reverberant ratio. There is no left/right imaging, although depth can be implied by differences of timbre, level and reverberence. 

Stereo doesn’t make any of those core things less important, it expands things by distributing the sound sources along the line between the speakers.  It’s a line between two points, so it’s geometrically one-dimensional.  Overall level, timbre, level balance and reverberation are still king.  Anyone will notice those things immediately if they are off.  Unless the spatial L/R imaging is grossly messed up, most people wouldn’t hear a distribution problem, especially if they were not at the live event and had an idea of the actual arrangement.  Some more discerning listeners will listen for things like how wide the apparent image is, how even the distribution of those sources are within that width, and how distinct the placement of individual sounds sources are within it.  Yet they still only have an imaginary idea of what that should be, unless they were there at the live event (an even in that case I’d argue that what they want is something pleasing, something close to what they think the distribution should be, rather than something that is as geometrically ‘accurate’ from the actual recording position as possible).

Multi-channel surround is two-dimensional, expanding the one-dimensional line between speakers to a horizontal plane around the listener.  Same hierarchy of importance: Level, timbre, level balance between sources, direct/reverberant ratio.. then farther down on the list envelopment in the ambience and directional source distribution.  But like everything else, the question is not really one of directional or revererent accuracy as much as about presenting something believable and pleasing.  As far as the sound source distribution aspect of multichannel, I love hearing a more solid image across the front, the crowd reaction wrapping around the back, the more tangible sonic fingerprint of the room reverberence. 


But it’s not only better source distubution that makes stereo superior to mono and muti-channel superior still.  Things sound more ‘real’ in stereo than mono, and more so in good multi-channel.  The timbre of many instruments is better and ‘more real sounding’, the direct/reverberant balance clearer and less critical to get just right.  And if it’s not just right I can adjust it afterwards to some extent.  In some ways it’s actually easier to make a decent stereo recording than a mono one, and easier to make a multi-channel one than a stereo one.  There is less compromise required in fitting the recording situation to the limits of the medium.  For example, a recording that would be way over reberberant from a less than ideal mic position becomes more listenable because the listener can direct her attention to what she wants to hear, more like she can at the live event, and ignore the room reverberence behind, even without the limited adjustments I can make to improve it after the recording is made.  It’s somewhat ironic that recording in multichannel is more forgiving in that way.  Of course it’s technically more complex to setup, record, mix and playback, but the challenge of ending up with a good sounding recording is actually made easier.  Same for stereo over mono.

I think it’s quite a bit more convincing, more real sounding, interesting and cool as well.  I wouldn’t make the effort otherwise.  It’s also helpful in making the best stereo recordings I can, assuming I have the time to mess with mixing it down, which I rarely do.

To get the 3-dimentions we’d need to record height information. My recordings with the ambisonic mic capture that to a basic extent, but I have no way of playing back the height information currently, other than pointing the virtual microphones up or down a bit to better optimize the 2D sound.

I haven’t heard any of the 11 channel matrix surround decoders, but I can image that a good implementation, properly set up might work quite well with live recordings by distributing the decorelated ambience information (mostly the L-R difference signal) more evenly throughout the room for a stronger feeling of envelopment and ‘your are there-ness’, even though it makes no improvement in the location of actual sound sources.  If so, that would make it a good example of the value in improving the ‘suspension of disbelief’ even though there is no improvement in actual accuracy.
« Last Edit: February 23, 2012, 12:39:05 PM by Gutbucket »
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Offline MIQ

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Re: !!Stereo Zoom simplified for PAS!!
« Reply #29 on: February 24, 2012, 10:19:26 AM »
Great post Lee!  Thanks for taking the time respond and explain your approach and insights.  A couple of thoughts:

I agree with the hierarchy of importance you've laid out - overall level, octave to octave balance, relative level of the instruments to each other, direct to reverberant levels, spatial distribution of instruments (between 2 or more speakers) and envelopment.  Like others have mentioned though, the performance trumps all of this if it gives you goose bumps. 

Imaging is a funny thing.  A lot of listeners don't pay much attention to it, but once you start to listen for it, it takes on more importance.  To me, I much prefer recordings where the instruments are distinct, and clearly focused in the horizontal line between the speakers (stereo).  I also like the localization I get with the HRTF recordings I've made, but I wouldn't necessarily say they were exactly like being there.

I wish it was more common and easier to distribute/play-back multi-channel recordings.  Like you wrote the results are often very pleasing even if they are not an exact replication of the live event.  I even like the results of some of the surround up-mixes (Dolby Pro Logic II, DTS Neural, etc) on (some) stereo sources.  It may not be "real" or what the artist/engineer originally intended but it can be nice to listen to. 

The problem I see with striving for the exact replication of the live event using WFS or ambisonics is that you end up needing an infinite number of speakers to get it "just right" at maybe a small listening area.  I haven't heard a lot of that kind of stuff but that's my impression. 

-MIQ

 

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