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The Phantom Image

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#1
I've read a bit about this but perhaps not enough! Anyway, a question please.

Floyd Toole tells us that there may be a problem with the phantom image as normally created by our brains and two stereo speakers. It suffers from some damage caused by Interaural Crosstalk. This is when the left ear hears not only the sound from the left speaker, but also from the right speaker. This sound from the right, presumably broadly similar to that from the left, has to travel a slightly longer distance related to your head size - around 8". As a result it is slightly delayed and therefore as it meets the sound from the left speaker it can cause comb filtering. This is mostly noticed as a dip around 2kHz of perhaps 6dB.

However, when we hear side wall reflections early enough to cause the Precedence Effect the sound is added to the direct sound but doesn't change the directional information. Do we get comb filtering here too?
 

Cosmik

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#2
The stereo image from speakers needs the crosstalk from each speaker to the 'wrong ear'. A purist stereo recording uses a coincident pair of cardioid mics, so there is no interchannel time difference, only a level difference. When the channels mix acoustically at the ears from speakers subtending the correct angle to the listener (+/- 45 degrees) , it produces a real, physical, measurable inter-channel time difference if you perform a cross-correlation between what arrives at the ears. Such a timing difference simulates a sound emanating from somewhere on a straight line from the listener (in nature it is not possible to determine range from the interaural timing difference). Amazingly, it works out that the angle of this line is the same as would have been correct at the recording i.e. the image is 'correct'. And even more amazingly, it is stable (relatively) with respect to head turning and limited movement of the listener.

This means that the panpot is the 'correct' way to pan stereo artificially. The beautiful image you hear is not a result of level differences at the ears, but timing differences.

Now maybe there's an issue with comb filtering, but... I wouldn't bet that it isn't eliminated by the listener's brain due to the 'complementarity' of what is arriving at the ears from both speakers. To me, the system seems to work outrageously well and I just have a feeling that it is even better than it appears on paper. Half-understanding it would certainly make you doubt your own ears, so I don't propose to worry about the supposed comb filtering. As with 'hearing through the room', attempts to 'correct' it I suspect will be a triumph of measurements at the expense of perceived sound.
 
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sergeauckland

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#3
I wasn't aware of any comb filtering until I read Floyd Toole's book, and wasn't aware of it on music. However, the book prompted me to try it on white and pink noise, and indeed there's a change in noise timbre which varies with head turning and side-to-side position.

Nevertheless, even though knowing it's a real effect, I still can't hear it on music.

Timing differences can be used as a 'balance' control. Meridian do this in their DSP5000 loudspeakers. The balance control delays one channel with respect to the other, which slews the image towards the earlier channel. Level doesn't change, only timing. This also allows a full stereo stage even when listening off-centre, albeit with a restricted stereo width.

S.
 

Thomas_A

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#4
I've read a bit about this but perhaps not enough! Anyway, a question please.

Floyd Toole tells us that there may be a problem with the phantom image as normally created by our brains and two stereo speakers. It suffers from some damage caused by Interaural Crosstalk. This is when the left ear hears not only the sound from the left speaker, but also from the right speaker. This sound from the right, presumably broadly similar to that from the left, has to travel a slightly longer distance related to your head size - around 8". As a result it is slightly delayed and therefore as it meets the sound from the left speaker it can cause comb filtering. This is mostly noticed as a dip around 2kHz of perhaps 6dB.

However, when we hear side wall reflections early enough to cause the Precedence Effect the sound is added to the direct sound but doesn't change the directional information. Do we get comb filtering here too?
The timbral change and dip caused at 1.8 kHz is that deep only in anechoic conditions. In normal rooms you will have reflections that dampens those effects. Still there are a few speaker developers that designs speakers with small corrections 2-8 kHz according those data (+/-1 dB or so) to get a more neutral phantom image.
 

Robin L

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#5
The stereo image from speakers needs the crosstalk from each speaker to the 'wrong ear'. A purist stereo recording uses a coincident pair of cardioid mics, so there is no interchannel time difference, only a level difference. When the channels mix acoustically at the ears from speakers subtending the correct angle to the listener (+/- 45 degrees) , it produces a real, physical, measurable inter-channel time difference if you perform a cross-correlation between what arrives at the ears. Such a timing difference simulates a sound emanating from somewhere on a straight line from the listener (in nature it is not possible to determine range from the interaural timing difference). Amazingly, it works out that the angle of this line is the same as would have been correct at the recording i.e. the image is 'correct'. And even more amazingly, it is stable (relatively) with respect to head turning and limited movement of the listener.

This means that the panpot is the 'correct' way to pan stereo artificially. The beautiful image you hear is not a result of level differences at the ears, but timing differences.

Now maybe there's an issue with comb filtering, but... I wouldn't bet that it isn't eliminated by the listener's brain due to the 'complementarity' of what is arriving at the ears from both speakers. To me, the system seems to work outrageously well and I just have a feeling that it is even better than it appears on paper. Half-understanding it would certainly make you doubt your own ears, so I don't propose to worry about the supposed comb filtering. As with 'hearing through the room', attempts to 'correct' it I suspect will be a triumph of measurements at the expense of perceived sound.
1200px-ORTF-Stereo.svg.png


Note that "ORTF" [a "purist" mode of recording from Radio France, two cardioid microphones, capsules about 7" apart, spread out at a 110° angle] tends to have better stereo imaging than coincident miking. My guess is that the little distance between the two capsules mimics the distance between the ears. In any case, always liked the results of ORTF better than coincident miking in recordings I made.
 
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Cosmik

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#6
View attachment 33210

Note that "ORTF" [a "purist" mode of recording from Radio France, two cardioid microphones, capsules about 7" apart, spread out at a 110° angle] tends to have better stereo imaging than coincident miking. My guess is that the little distance between the two capsules mimics the distance between the ears. In any case, always liked the results of ORTF better than coincident miking in recordings I made.
Better for headphone listening. Speakers and headphones require different mic techniques.
 

Robin L

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#8
Just for the record, in most sessions where I assisted another [better] audio engineer, they usually started with an ORTF pair as the primary pair before adding spaced omnis [for room sound] and spot mics to taste. This was usually Orchestral or Choral recordings, where capture of room sound was an important element in the overall sound. [With keyboard recordings, all bets are off]. I did use a pure coincident pair [Neumann SM69] early on, KPFA had one for remote recording of concerts:

SM69-color.jpg



One of the cool things about these microphones is they adapted well to the middle-side technique:

Unknown.png


Quoting from the Universal Audio website:

"What You Need
While XY recording requires a matched pair of microphones to create a consistent image, M/S recording often uses two completely different mics, or uses similar microphones set to different pickup patterns.

The "Mid" microphone is set up facing the center of the sound source. Typically, this mic would be a cardioid or hypercardioid pattern (although some variations of the technique use an omni or figure-8 pattern). The "Side" mic must be a figure-8 pattern. This mic is aimed 90 degrees off-axis from the sound source. Both mic capsules should be placed as closely as possible, typically one above the other.
How to Do It
The signal from each microphone is then recorded to its own track. However, to hear a proper stereo image when listening to the recording, the tracks need to be matrixed and decoded.

Although you have recorded only two channels of audio (the Mid and Side), the next step is to split the Side signal into two separate channels. This can be done either in your DAW software or hardware mixer by bringing the Side signal up on two channels and reversing the phase of one of them. Pan one side hard left, the other hard right. The resulting two channels represent both sides of what your figure-8 Side mic is hearing.

Now you've got three channels of recorded audio — the Mid center channel and two Side channels — which must be balanced to recreate a stereo image.

Now, if you listen to just the Mid channel, you get a mono signal. Bring up the two side channels and you'll hear a stereo spread. Here's the really cool part — the width of the stereo field can be varied by the amount of Side channel in the mix!

Why It Works
An instrument at dead center (0 degrees) creates a sound that enters the Mid microphone directly on-axis. But that same sound hits the null spot of the Side figure-8 microphone. The resulting signal is sent equally to the left and right mixer buses and speakers, resulting in a centered image. An instrument positioned 45 degrees to the left creates a sound that hits the Mid microphone and one side of the Side figure-8 microphone.

Because the front of the Side mic is facing left, the sound causes a positive polarity. That positive polarity combines with the positive polarity from the Mid mic in the left channel, resulting in an increased level on the left side of the sound field.

Meanwhile, on the right channel of the Side mic, that same signal causes an out-of-phase negative polarity. That negative polarity combines with the Mid mic in the right channel, resulting in a reduced level on the right side. An instrument positioned 45 degrees to the right creates exactly the opposite effect, increasing the signal to the right side while decreasing it to the left.

What's the Advantage?
One of the biggest advantages of M/S recording is flexibility. Since the stereo imaging is directly dependent on the amount of signal coming to the side channels, raising or lowering the ratio of Mid to Side channels will create a wider or narrower stereo field. The result is that you can change the sound of your stereo recording after it's already been recorded, something that would be impossible using the traditional XY microphone recording arrangement.

Try some experimenting with this—listen to just the Mid channel, and you'll hear a direct, monophonic signal. Now lower the level of the Mid channel while raising the two Side channels. As the Side signals increase and the Mid decreases, you'll notice the stereo image gets wider, while the center moves further away. (Removing the Mid channel completely results in a signal that's mostly ambient room sound, with very little directionality — useful for effect, but not much else.) By starting with the direct Mid sound and mixing in the Side channels, you can create just the right stereo imaging for the track.

Another great benefit of M/S miking is that it provides true mono compatibility. Since the two Side channels cancel each other out when you switch the mix to mono, only the center Mid channel remains, giving you a perfect monaural signal. And since the Side channels also contain much of the room ambience, collapsing the mix to mono eliminates that sound, resulting in a more direct mix with increased clarity."


https://www.uaudio.com/blog/mid-side-mic-recording/
 
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#9
One of the main benefits of MS for small sources is that the M mic is pointed directly at the source, avoiding off-axis colourations that exist on most mic capsules (notably not on some of the smaller DPA mics, eg 4060).

To try to push this thread back to phantom image, the main cues are Interaural Level Differences and Interaural Time Differences, and they brain trades these off between each other. The best information I have read about this is in Doug Self's book "Audio Engineering", but there may be good internet links on ITD vs. ILD trading.

This theory then plays into the various stereo mic techniques, from coincident MS, Blumlein etc (all ILD based), to AB and Decca Tree (ITD based), to near coincident techniques like ORTF, EBU, NOS etc (using both ITD & ILD).
 
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#10
Also, I just remembered this site which gives a fabulous explanation of the phantom image produced in stereo reproduction with different microphone tehcniques - select the stereo mic technique on the top left.
http://www.sengpielaudio.com/Visualization-EBS-E.htm
It is most illuminating to play with the parameters on the right hand side and see how it will affect the stereo image - it hi-lights (for example) the hyped stereo image from widely spaced omni microphones.
 
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#11
Thanks everyone.

The stereo image from speakers needs the crosstalk from each speaker to the 'wrong ear'.
Not sure about that. Headphone listening and Ambiophonics suggest otherwise - I've not tried the latter, yet.

I wasn't aware of any comb filtering until I read Floyd Toole's book, and wasn't aware of it on music. However, the book prompted me to try it on white and pink noise, and indeed there's a change in noise timbre which varies with head turning and side-to-side position.

Nevertheless, even though knowing it's a real effect, I still can't hear it on music.
This is exactly my position. Although Toole says that side wall reflections partially fill in the dip I've tried to remove these and still get a really strong centre image.

My question was really why interaural crosstalk can lead to comb filtering that may be an issue whereas early side wall reflections do not, theoretically anyway.

Another question occurred to me. We civilised humans are used to living in rooms and therefore our hearing has presumably adapted to them. Perhaps that is why we can appreciate stereo and hear things such as the precedence effect. Have any studies been done on tribal type groups on their psychoacoustics?

We can also see stereoscopically too of course, but we need suitable eyeware to do this. I've looked at wartime aerial photos of ships through a stereoscope and when you get the pictures lined up exactly the stereo vision becomes visible. Ships masts poke you in the eye! There is even crosstalk with stereo imaging!

https://en.wikipedia.org/wiki/Stereoscopy
 

anmpr1

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#12
Floyd Toole tells us that there may be a problem with the phantom image as normally created by our brains and two stereo speakers. It suffers from some damage caused by Interaural Crosstalk. This is when the left ear hears not only the sound from the left speaker, but also from the right speaker.
This was the premise of the Polk SDA speakers, I'm pretty certain. Maybe Bob Carver's Sonic Hologram thing, but I'm not sure about the latter. Paul Klipsch argued that two channel stereo really needs a center channel speaker to 'fill in' the phantom image. His argument was that stereo is basically two mono tracks synthesized artifically in order to create a realistic front image. He devised a kludge for it, using a center speaker, with mixed results.
 

Robin L

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#13
This was the premise of the Polk SDA speakers, I'm pretty certain. Maybe Bob Carver's Sonic Hologram thing, but I'm not sure about the latter. Paul Klipsch argued that two channel stereo really needs a center channel speaker to 'fill in' the phantom image. His argument was that stereo is basically two mono tracks synthesized artifically in order to create a realistic front image. He devised a kludge for it, using a center speaker, with mixed results.
That was also a premise of "Golden Age" three channel stereo recordings from Mercury—"Living Presence"—and RCA—"Living Stereo". Those recordings have been reissued as SACDs with three discreet channels. I've played them back via a 5.1 system, so the sound came out of the front half of an Infinity Primus [360s front, 250s back and C25 center] speaker system. Yes, more solidity in the image, particularly if one's head moved, but not as huge a difference as one might suspect. Full surround made a bigger difference.
 
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anmpr1

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#14
That was also a premise of "Golden Age" three channel stereo recordings from Mercury—"Living Presence"—and RCA—"Living Stereo". Those recordings have been reissued as SACDs with three discreet channels.
Discrete 3 channel was never an option for consumer reproduction. Maybe with tape. Open reel tape could accommodate it. But records, which were the major format, were just getting going with two channel, in the late '50s. Four channel was a small radar blip in the '70s, but only lasted a few years. Some folks claimed that multi channel would end two channel stereo, but that has not happened. For various reasons.
 

sergeauckland

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#15
Discrete 3 channel was never an option for consumer reproduction. Maybe with tape. Open reel tape could accommodate it. But records, which were the major format, were just getting going with two channel, in the late '50s. Four channel was a small radar blip in the '70s, but only lasted a few years. Some folks claimed that multi channel would end two channel stereo, but that has not happened. For various reasons.
Quadraphonics in the 1970s failed for various reasons. Firstly, there were three competing and incompatible systems, two of which were matrix (SQ and QS) which didn't work very well and the third, CD-4 which was discrete using an ultrasonic carrier, worked only under laboratory conditions, not in domestic use, and didn't work for long as the HF modulation was very easily damaged.

Secondly, and as a consequence of the first, one had to choose the system according to which artist(s) one followed. If you liked both Bob Dylan (CBS) and Elvis (RCA), tough. They required different decoders and the few multi-standard decoders didn't do any of them very well.

Thirdly, few people had space or could accommodate even two loudspeakers for stereo in a proper arrangement, I saw many with one behind the sofa and the other on top of a bookcase. Trying to get four loudspeakers properly arranged was beyond anyone except real enthusiasts. The cost of extra amplification and loudspeakers didn't help much either.

Since the failure of Quadraphonics, surround has been more popular for movies with the advent of cheap multichannel AV receivers and pod-type 'speakers, but those don't work very well for music, OK for speech and loud bangs, not much subtlety.

I don't know what proportion of the public have decent multi-channel home-cinema systems, but I expect it's small. Probably still more than have decent music-only systems these days.

S.
 

Robin L

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#16
Quadraphonics in the 1970s failed for various reasons. Firstly, there were three competing and incompatible systems, two of which were matrix (SQ and QS) which didn't work very well and the third, CD-4 which was discrete using an ultrasonic carrier, worked only under laboratory conditions, not in domestic use, and didn't work for long as the HF modulation was very easily damaged.

Secondly, and as a consequence of the first, one had to choose the system according to which artist(s) one followed. If you liked both Bob Dylan (CBS) and Elvis (RCA), tough. They required different decoders and the few multi-standard decoders didn't do any of them very well.

Thirdly, few people had space or could accommodate even two loudspeakers for stereo in a proper arrangement, I saw many with one behind the sofa and the other on top of a bookcase. Trying to get four loudspeakers properly arranged was beyond anyone except real enthusiasts. The cost of extra amplification and loudspeakers didn't help much either.

Since the failure of Quadraphonics, surround has been more popular for movies with the advent of cheap multichannel AV receivers and pod-type 'speakers, but those don't work very well for music, OK for speech and loud bangs, not much subtlety.

I don't know what proportion of the public have decent multi-channel home-cinema systems, but I expect it's small. Probably still more than have decent music-only systems these days.

S.
I had one, but it was relegated to the garage/man cave. As of 2010 or so, it's a lot easier to get gear that will really work well in surround but harder to find a place where it can live in peace. One has to be a dedicated audiophile to get into Hi-End [or decent] surround, same as it ever was.

I realize that this is getting far away from the question: " . . . why interaural crosstalk can lead to comb filtering that may be an issue whereas early side wall reflections do not, theoretically anyway." And I guess 2 channels would be more likely to produce that effect than, say, 5. But I don't see any reason why room reflections wouldn't be part of the mix.
 

anmpr1

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#17
Since the failure of Quadraphonics, surround has been more popular for movies ...
Definitely movies. I think the subwoofer thing can be attributed to movies, too. Special effects. Back in the days of quadrophonic, there were no home movies--well, I guess Super 8. But you know what I mean. Star Wars and that genre really got surround going.
 

Cosmik

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#18
(Acoustic crosstalk creates the stereo image with speakers)
Not sure about that.
Seeing as the crosstalk is significant, real and measurable, it would be very useful if we could be sure that it did have something to do with creating the stereo image! :)

Quoting an article on Blumlein 'intensity stereo':
Blumlein realised that sound reproduction using multiple speakers inherently means that both ears hear all of the speakers. Consequently, trying to reproduce time‑of‑arrival differences captured by spaced microphones would be extremely problematic: the physical spacing of the speakers relative to the listener would add further time‑of‑arrival differences, compromising the accuracy of the imaging.

Blumlein saw that this apparent problem (both ears hearing both speakers) could be used to advantage, if only level or intensity differences were relayed from the two speakers. If the physical placement of the speakers was controlled, the inherent time‑of‑arrival differences between the speakers and ears could be used to fool the human hearing system into converting the source‑signal intensity differences into perceived time‑of‑arrival differences — and hence creating believable and stable stereo imaging. For this reason, Blumlein's stereophonic system was originally referred to as producing 'Intensity Stereo.'

...The interaction of the signals from both speakers arriving at each ear results in the creation of a new composite signal, which is identical in wave shape but shifted in time. The time‑shift is towards the louder sound and creates a 'fake' time‑of‑arrival difference between the ears, so the listener interprets the information as coming from a sound source at a specific bearing
Amazing that this is almost unknown amongst audiophiles, who labour under the burden of 'knowing' that stereo over speakers is flawed and that the image is all messed up because of the crosstalk. It isn't.

For this system to work, it relies on coincident cardioid microphones so there is no time difference between the channels, only level. And this, in turn, must be heard over stereo speakers.

This means that recordings optimised for headphones will not be the same when heard over speakers and vice versa. Ditto ambiophonics which aims to reproduce the isolation between channels of headphones.

*Any* stereo microphone technique will 'work' of course, but it won't necessarily give 'correct' results; results that accurately reproduce the correct bearing of the acoustic sources and remain (relatively) stable if the listener turns their head or shifts position. Audiophiles are wowed by stereo 'soundstage' so they're not too bothered, but only the Blumlein technique reproduced over conventional stereo speakers is going to give you the ultimate stable, correct, 'holographic' image.
 
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#19
Seeing as the crosstalk is significant, real and measurable, it would be very useful if we could be sure that it did have something to do with creating the stereo image! :)
One way of course is to remove crosstalk and listen if you still get a stereo image. That is surely what ambiophonics, and maybe BACCH, does.

Quoting an article on Blumlein 'intensity stereo':

Amazing that this is almost unknown amongst audiophiles, who labour under the burden of 'knowing' that stereo over speakers is flawed and that the image is all messed up because of the crosstalk. It isn't.

For this system to work, it relies on coincident cardioid microphones so there is no time difference between the channels, only level. And this, in turn, must be heard over stereo speakers.

This means that recordings optimised for headphones will not be the same when heard over speakers and vice versa. Ditto ambiophonics which aims to reproduce the isolation between channels of headphones.

*Any* stereo microphone technique will 'work' of course, but it won't necessarily give 'correct' results; results that accurately reproduce the correct bearing of the acoustic sources and remain (relatively) stable if the listener turns their head or shifts position. Audiophiles are wowed by stereo 'soundstage' so they're not too bothered, but only the Blumlein technique reproduced over conventional stereo speakers is going to give you the ultimate stable, correct, 'holographic' image.
My own experience is that crosstalk doesn't mess up the stereo image and that seems to be most peoples' view. At least it doesn't mess up the image enough to be a major issue but the idea of ambiophonics (and BACCH, the Trinaural processor etc.) is that improvements are to be had. Reading about BACCH, that seems to offer gains too but again it depends on the recordings - well recorded apparently sounds good, questionable studio efforts sound weird.

Indeed that is the problem - we are reliant on the recording techniques which seem varied - part of Toole's circle of confusion.

Again, all I really wanted to know is why interaural crosstalk can lead to comb filtering that may be an issue whereas early side wall reflections do not, theoretically anyway.
 

Cosmik

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#20
One way of course is to remove crosstalk and listen if you still get a stereo image. That is surely what ambiophonics, and maybe BACCH, does.
As I said earlier, you will always get a stereo image; some 'soundstage'. Audiophiles are particularly impressed by weird phasey stuff, so if a pair of spaced microphones gives a whole load of soundstage when listening over speakers, they'll be happy. If, when they turn their heads, it doesn't remain stable, they're not going to complain. BACCH probably sounds even more exciting.

But you're not at all intrigued by the idea that stereo over speakers is one thing, and headphones/BACCH another..? That Blumlein stereo over speakers should have no time difference at the mics? That crosstalk is a necessary part of the stereo image when listening to stereo speakers..? That Blumlein might have been even cleverer than it first appears? To me, it's fascinating.
 
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