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!
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.
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.