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What is Stereo Recording Angle?

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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Sat Apr 17, 2021 12:53 pm

Tomás Mulcahy wrote: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 perhaps you're overthinking this and confusing yourself with possibly misunderstood terminology and unhelpful format comparisons!

Conventional stereo was invented for loudspeakers. Full stop. Forget headphones -- that's a completely different experience. (And, while you can play two-channel audio material over both formats, the experiences are inherently very different. To get the best from each format, you need audio material recorded specifically for each format.)

So, back to speakers: the stereo speaker format creates a stereo image stretching between the speakers. That's your 'window' into the recorded world, in the same way that the TV screen is a 'window' into telly-land.

Stereo speakers are not designed to provide an immersive experience. A two-dimensional sound stage is laid out in front of you (width and depth). If you want an immersive experience of sound all around you have to go to one of the surround formats (each with their own compromises and benefits).

The Stereo Recording Angle of a stereo mic array simply relates how far to the left and right of the mic array you need to go to create the impression of moving from the centre to the edge of your stereo speaker image. The SRA is chosen to suit the spread of sound sources around the mic array, and to create the stereo image width wanted from the speakers.

...why can't I find visualisations of stereo or surround speaker array dispersion patterns?

If I knew what you were talking about I might be able to help... but I don't.

Most manufacturers normally mention 'dispersion' in the context o a speaker's polar pattern -- showing how the different frequencies are radiated in different directions. Most monitor speakers try and provide a uniform spread of sound at all frequencies over about 120 degrees or so... but most also tend towards being omni-directional at low frequencies just because of the physical size o the bass driver and cabinet.

But the idea is to disperse sound into the room as spectrally uniform as possible. This has nothing to do with the stereo imaging, per se -- that's predominately down to the on-axis sound being thrown at your ears.

I think what you might be looking for is a map of how the sound image locations can be varied depending on placement and number of speakers. I've seen such things for quad formats and 5.1 I think... I'll try and dig some out next week.

5.1 cannot do sides
.

It can to a degree, but the imaging accuracy is poor (just as it was with quad). But to be fair, the consumer 5.1 format is a ridiculous bodge of misunderstanding and should never have been entertained!

7.1 can.

Yes, it is less bodged...

But do both have a "hole" in the middle of the rear?

Not really... stereo speakers don't have a "hole" in the middle (unless the recording technique is crap)... but for large spaces (ie cinemas) an extra centre rear speaker is available in some formats for better defined imaging for off-axis listeners.

And why do a lot of vectorscopes default to a semicircle display?

It's just an alternative way of displaying the stereo information that some people find easier to comprehend.

It makes more sense (to me) than, say, the diamond shape.

Fair enough...

Some 'vectorscopes' (eg, The Box) show only the top quadrant of the 'diamond' display, and that portion is what is effectively being stretched out into the semi-circle display.

The 'diamond' display is derived from the Lissajous display shown on scopes back in the day, and it effectively shows the positive and negative portions of the waveform in separate areas, so it actually conveys more information once you know how to interpret it -- such as waveform asymmetry, DC offsets and so forth.
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Re: What is Stereo Recording Angle?

Postby ManFromGlass » Sat Apr 17, 2021 1:33 pm

Great stuff! So theoretically in order to get an immersive audio experience that is as close to real life as possible one would have to be suspended in the centre of a large ball made of speakers?
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Sat Apr 17, 2021 1:38 pm

Yes.

But it's not a very popular format with interior decorators or domestic managers... :lol:

Speaker stereo is an illusion. It relies on the brain making things up, and it's not dissimilar in that regard to the trickery of showing 24 still images each second while we perceive natural movement (mostly) across the cinema or tv screen.

The 60 degree angle of the speakers generates false time of arrival differences at the ears based on interchannel level differences at the speakers, but the fakery is enough to create believable, stable imaging information across that limited angle in front of us.

The same imaging trickery doesn't work for speaker's at the sides, and it's not as accurate for speakers behind either. So for stable and believable surround imaging you really need to create more natural time of arrival signals at the ears, and that typically requires a lot more speakers in a lot more places. Hence Dolby Atmos and all the other large multichannel formats.

5.1, 7.1 etc were designed for the cinema where the rear channel signals were duplicated across a lot of speaker's with the idea of creating a sense of envelopment rather than accurate rear imaging. Entirely different intention and outcome.
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Re: What is Stereo Recording Angle?

Postby ManFromGlass » Sat Apr 17, 2021 1:45 pm

I’m not even allowed to connect a stereo to the tv so I get that!
But what an ideal man cave a speaker ball would make! but then the tv screen would probably cause weird reflections. The physics of things can be so inconvenient at times!
:geek:
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Re: What is Stereo Recording Angle?

Postby blinddrew » Sat Apr 17, 2021 2:05 pm

Physics, ruining things since the 4th century BC.
;)
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Sat Apr 17, 2021 2:23 pm

:bouncy:
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Re: What is Stereo Recording Angle?

Postby James Perrett » Sat Apr 17, 2021 2:32 pm

ManFromGlass wrote:Great stuff! So theoretically in order to get an immersive audio experience that is as close to real life as possible one would have to be suspended in the centre of a large ball made of speakers?

Which is the idea behind higher order Ambisonics as I understand it. A couple of years ago I went to a demo of it at the ISVR in Southampton. They have created a 32 channel ball of speakers with a microphone array to go with it and the results, from a spatial audio point of view, are impressive.
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Re: What is Stereo Recording Angle?

Postby The Elf » Sat Apr 17, 2021 3:14 pm

ManFromGlass wrote:I’m not even allowed to connect a stereo to the tv so I get that!
£$%^& that!

Once Coronation street had been heard in (at first) stereo, then (later) in surround there were no complaints! :lol:
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Re: What is Stereo Recording Angle?

Postby ManFromGlass » Sat Apr 17, 2021 11:34 pm

It’s almost like being there . . . . . . . . . .
Aaaaaaaaaaaah, run away, run away. :smirk:
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Re: What is Stereo Recording Angle?

Postby Dynamic Mike » Sun Apr 18, 2021 1:38 am

I sort of get how the brain evaluates sounds heard in the left & right ears and calculates where the source is in a lateral plane.

Stereoscopic vision is similar, although it can be thought of as though one dominant eye looks directly at an object & the brain calculates the distance from the angle other eye would have to deviate to see it (it only actually deviates at very short distances but the brain knows whether it's off-centre on the retina.) But the brain also knows when we're looking up or down so can estimate a 3D point in space where the object is likely to be. Biologically speaking that's grossly over-simplifying steropsis, but the principle is fairly sound.

But back to sound, do we have height perception of a sound source & if so how given we don't have a top ear & a bottom ear? (Although I do seem to recall Captain Kirk having a third ear. ;) )
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Re: What is Stereo Recording Angle?

Postby RichardT » Sun Apr 18, 2021 9:32 am

Dynamic Mike wrote:I sort of get how the brain evaluates sounds heard in the left & right ears and calculates where the source is in a lateral plane.

Stereoscopic vision is similar, although it can be thought of as though one dominant eye looks directly at an object & the brain calculates the distance from the angle other eye would have to deviate to see it (it only actually deviates at very short distances but the brain knows whether it's off-centre on the retina.) But the brain also knows when we're looking up or down so can estimate a 3D point in space where the object is likely to be. Biologically speaking that's grossly over-simplifying steropsis, but the principle is fairly sound.

But back to sound, do we have height perception of a sound source & if so how given we don't have a top ear & a bottom ear? (Although I do seem to recall Captain Kirk having a third ear. ;) )

We do. I’m no expert, but I think it’s due to the shape of the pinnae. The brain learns how to correlate the effects of the pinnae on the sound with its location. Or at least that’s one theory. I think there are others.
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Re: What is Stereo Recording Angle?

Postby Wonks » Sun Apr 18, 2021 10:03 am

You can always test your height perception empirically by shutting your eyes and get a friend to make noises all around your head.

You can try clicking your fingers yourself around your head, but as you always know where your hand is, that will colour the results.
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Re: What is Stereo Recording Angle?

Postby Hugh Robjohns » Sun Apr 18, 2021 10:26 am

Dynamic Mike wrote:But back to sound, do we have height perception of a sound source & if so how given we don't have a top ear & a bottom ear? (Although I do seem to recall Captain Kirk having a third ear. ;) )

Ah yes, his final front ear... :lol:

Yes, we have some height perception thanks to the asymmetrical shape of the pinae and the unique comb filtering effects imposed on sounds from different directions.

But it's not very accurate over position and we instinctively turn our heads to gather more information with the aim of helping guide our eyes onto the sound source.
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Re: What is Stereo Recording Angle?

Postby Tomás Mulcahy » Sun Apr 18, 2021 1:52 pm

Hugh Robjohns wrote:I think perhaps you're overthinking this and confusing yourself with possibly misunderstood terminology and unhelpful format comparisons!
Yes I can accept that :)

...why can't I find visualisations of stereo or surround speaker array dispersion patterns?

Hugh Robjohns wrote:If I knew what you were talking about I might be able to help... but I don't.
Maybe speaker polar pattern might be the right term? The opposite of this:
http://www.sengpielaudio.com/HejiaE.htm
The inclusion of speakers on that page is what confused me in the first place.

I think what you might be looking for is a map of how the sound image locations can be varied depending on placement and number of speakers. I've seen such things for quad formats and 5.1 I think... I'll try and dig some out next week.
Great thank you!

Not really... stereo speakers don't have a "hole" in the middle (unless the recording technique is crap)... but for large spaces (ie cinemas) an extra centre rear speaker is available in some formats for better defined imaging for off-axis listeners.
Right. Makes sense. I was thinking the wider spacing might cause it but I didn't consider the angle and distance.

Some 'vectorscopes' (eg, The Box)
Is that the old LED one you've criticised before?

Lissajous display
[/quote]
Had forgotten about those, thanks for the reminder :)
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Re: What is Stereo Recording Angle?

Postby Tomás Mulcahy » Thu May 20, 2021 12:56 pm

Hugh, thank you for the excellent diagram and explanation in Q&A this month :)
https://www.soundonsound.com/sound-advi ... -mic-array
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