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What is Stereo Recording Angle?

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What is Stereo Recording Angle?

Postby Tomás Mulcahy » Mon Mar 29, 2021 5:46 pm

Had an interesting interaction with a person from a well known developer of spatial audio plugins. He claims that because speakers are set at -30° and +30° there is only 60° "of liberty in stereo sound" (sic). He disagrees that the image is more like 180° despite sending him this link:
http://www.sengpielaudio.com/HejiaE.htm

The fact that XY90 gives an SRA of 195° really threw him.

But it is generally 180° (depending on mic array etc.). Right? Since loudspeakers are basically cardioid, and we're assuming an acoustically treated room. Right? I'm having a moment of doubt!
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Mon Mar 29, 2021 6:14 pm

There is only 60 degrees angle of possible image movement/positioning when listening over stereo loudspeakers. The whole audio environment has to be fitted into that +/-30 degree angle. He's right.

The SRA only applies to stereo mic arrays and only denotes how the space in front of a stereo microphone array is mapped into that 60 degree angle in front of our ears.

So if he's developing processing plugins, he does only have +/-30 degrees of area to play with for a loudspeaker listener.
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Re: What is Stereo Recording Angle?

Postby Tomás Mulcahy » Mon Mar 29, 2021 7:02 pm

Eek. I've totally misunderstood the concept. So what is the point of, say, XY90 which gives 195° if the speakers cannot reproduce the whole image? How is it mapped in to the narrower space?

Are headphones better/ wider?
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Re: What is Stereo Recording Angle?

Postby blinddrew » Mon Mar 29, 2021 7:39 pm

How does that 'wider than the speakers' effect work then Hugh? Not challenging you understand, just curious! :D
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Mon Mar 29, 2021 7:42 pm

Tomás Mulcahy wrote:Eek. I've totally misunderstood the concept.

:-D When I first started as a lecturer I was told that to teach someone 10% of a subject, you need to know 90% of it... And I soon found out how true that is!

So what is the point of, say, XY90 which gives 195° if the speakers cannot reproduce the whole image?

They can reproduce it... Just not with identical relative positions. It's about creating a representative impression of the real acoustic world for a pleasing effect... If you want real world imaging accuracy you need to step up to a 'surround sound' setup.

How is it mapped in to the narrower space?

It's much the same as taking a photo. You could use a 16mm wide angle lens on the camera to capture an image of the whole space, or a 200mm lens to capture an image of a small specific area... But the printed output will end up on the same 5x4 paper whichever lens you use.

Are headphones better/ wider?

Not really... Different angular/spatial distortions, but still distortions...
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Re: What is Stereo Recording Angle?

Postby John Stafford » Mon Mar 29, 2021 7:51 pm

It strikes me as odd when I hear recordings that sound like nobody considered any of this stuff, e.g. some well-known concerto recordings that sound like the piano is wider than the orchestra. :protest:
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Mon Mar 29, 2021 7:52 pm

blinddrew wrote:How does that 'wider than the speakers' effect work then Hugh? Not challenging you understand, just curious! :D

The 'outside the speakers' effect uses intentional opposite-polarity signals in the two channels -- a situation which can't ever happen in real life.

The phase anomalies from the direct and reglected sound from both speakers create peaks and dips in the frequency response which can be similar to those introduced naturally by the curves of the pinnae.

The brain is a bit confused with the conflicting information and so tries its best to make sense of it by creating an image location somewhere outside the speakers... Could be left or right, or it could be up or down... And to make matters worse, different people hear the 'phantom sound' in different places because the dips/peaks correspond to different locations depending on the shapes of different ears -- and we all have different pinnae!
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Re: What is Stereo Recording Angle?

Postby blinddrew » Mon Mar 29, 2021 7:56 pm

Thank you! :thumbup:
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Re: What is Stereo Recording Angle?

Postby Tomás Mulcahy » Mon Mar 29, 2021 8:03 pm

Hugh Robjohns wrote:It's much the same as taking a photo.
OK that I understand. I'm struggling to find any diagrams of it. Or at least, ones that I understand. I've been wading through Michael Williams's books. Maybe I don't understand the diagrams there! :lol:

I didn't think the angular distortion would be more than 5 degrees on a good system?

I hear you on the lecturing :D
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Mon Mar 29, 2021 8:28 pm

I'll try and dig out some pics (or draw some) tomorrow.

But another way to think about it is this: if you have an array like XY fig8s, say, which has an SRA of 90 degrees or so, a single sound source in front of the stereo mic array has to move just 45 degrees to one side or other to move fully across the sound stage from the speakers to reach one speaker or the other.

But if you had XY cardioids, with an SRA or 180 degrees or more, the same source would have to move around 90degrees to reach the speaker!

In a recording situation you have a sound source (orchestra, ensemble, soloist, etc) that occupies a certain physical width, and you'll want that to occupy a certain proportion of the sound stage between the speakers, possibly with some room acoustic extending to the sides. So that's the end goal to keep in mind.

You'll also want a certain direct/ambience/reverb balance which obviously depends on the distance between mic and source, combined with the polar pattern.

So once you know where the mic needs to be, you'll also know the SRA you need to create the sound stage you desire. There are a number of different stereo mic arrays that can create the desired SRA, with different polar patterns, mutual angles, and capsule spacings... So you pick one with the characteristics you want that works with the right angles.

It's really no different, conceptually, from standing a set distance from a subject with a camera, and choosing a lens (or adjusting a zoom lens) to frame the subject in the way you want.
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Mon Mar 29, 2021 8:38 pm

Tomás Mulcahy wrote:I didn't think the angular distortion would be more than 5 degrees on a good system?

It can get up to about 8-10 degrees in some cases, but with most stereo arrays it is under 5 degrees most of the time.

But the term 'angular distortion' in this context is about a departure from linearity -- the equivalent of pin-cushion or barrel distortion in a camera lens.

So consider five sources equally spaced in an arc in front of an XY cardioid stereo array. So one is at -90, one at -45, one at 0, one at +45, and one at +90 degrees.

They should be portrayed from the speakers at -30, -15, 0, +15, +30 degrees so that their relative positions are in the same proportions.

But all stereo mic arrays have some small non-linearities -- angular distortions -- so in practice you might find the perceived angles are -30, -20, 0, +20, +30 degrees. In most cases, sources near the edges are pushed wider, so tend to bunch near the edges, while those nearer the center tend to pull wider apart than they should be. It's a pretty subtle effect, though, and generally goes unnoticed.
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Re: What is Stereo Recording Angle?

Postby Tomás Mulcahy » Mon Mar 29, 2021 8:54 pm

Hugh Robjohns wrote:I'll try and dig out some pics (or draw some) tomorrow.
Fantastic, thank you. However, your explanation there makes it a lot clearer thank you!
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Mon Mar 29, 2021 9:05 pm

:thumbup:
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Re: What is Stereo Recording Angle?

Postby Tomás Mulcahy » Sat Apr 17, 2021 12:18 pm

Hmm maybe I do need a diagram :headbang: . I can kind of see how headphones give a genuine sides signal but not so much an impression of things being in front. Whereas speakers can do in front, but not sides. I think I can imagine the shape of the angular distortion in each. But why can't I find visualisations of stereo or surround speaker array dispersion patterns?

5.1 cannot do sides. 7.1 can. But do both have a "hole" in the middle of the rear?

And why do a lot of vectorscopes default to a semicircle display? Obviously a kind of compormise but I never really thought about it before. It makes more sense (to me) than, say, the diamond shape.
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Re: What is Stereo Recording Angle?

Postby dbfs » Sat Apr 17, 2021 12:46 pm

Very interesting. Would love to see the diagrams.
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