The 2011 Richard C. Heyser Memorial Lecture: "Where Did the Negative Frequencies Go?" Case Study 1: Recording

Case Study 1: Recording

Back in 1987, the AES published the anthology pictured above of historic papers on "Stereo." It includes a document (celebrating its 80th anniversary this year) that pretty much defined the whole field of stereo reproduction, including the 45°/45° stereo groove and the moving-magnet stereo cartridge. That document, a 1931 British Patent Application written by the English engineer Alan Dower Blumlein, is worth quoting at length:

"The fundamental object of the invention is to provide a sound recording, reproducing and/or transmission system whereby there is conveyed to the listener a realistic impression that the intelligence is being communicated to him over two acoustic paths in the same manner as he experiences in listening to everyday acoustic intercourse and this object embraces also the idea of conveying to the listener a true directional impression. . . . An observer in the room is listening with two ears, so that echoes reach him with the directional significance which he associates with the music performed in such a room. . . . When the music is reproduced through a single channel the echoes arrive from the same direction as the direct sound so that confusion results. It is a subsidiary object of this invention so to give directional significance to the sounds that when reproduced the echoes are perceived as such."

In other words, if you can record not only a sound but the direction in space it comes from, and can do so for every sound wave making up the soundstage, including all the reflected sound waves (the reverberation or "echoes"), then you will be able to reproduce a facsimile of the original soundstage, accurate in every detail. In addition, because the spatial relationship between the direct and the reflected sounds will be preserved, that reproduced soundstage will give a realistic illusion of depth.

Incidentally, I mentioned earlier Hermann Bondi, one of Hoyle's collaborators on the Static Universe Hypothesis. Like Blumlein, Bondi had worked on the British development of radar in World War II. When I worked in the research lab developing LEDs, in a corner office was a charming elderly gentleman, Dr. Henry Boot. Only years later did I learn that Henry was one of the people who invented the cavity magnetron, which was fundamental to the British development of radar. I suppose you could therefore say that there are just two degrees of separation between me and Alan Dower Blumlein.

The Blumlein Patent Application mentions that, when recording for playback over headphones, the simplest way of carrying out the preservation of the soundstage is to use two microphones spaced as far apart as the average pair of human ears: the "binaural" technique. This, however, makes headphone listening mandatory; until recently, headphones have been about as popular as a head cold for relaxed, social listening. Blumlein was concerned with a system for playback over loudspeakers, and proposed a method of recording directional information as a ratio of amplitude differences between the two signal channels.

The ear/brain, of course, uses more than amplitude information to determine the direction of sound sources. It uses the amplitude difference between the signals reaching the two ears above about 2kHz, but below about 700Hz, it determines direction by looking at the phase difference between the signals; ie, it uses time-of-arrival information. (Both frequencies are proportional to the head size, so there will be a spread among individuals.)

Things get a bit ambiguous between those two frequencies, but there are two other mechanisms also at work: first, the frequency-response modifications due to the shape of the pinnae differ according to the direction of the perceived sound; and second, the head is in continual lateral motion, sharpening up all the mechanisms by introducing second-order (rate-of-change) information. The result is that human beings—and animals—are very good at determining where sounds come from (unless they happen to consist of pure tones in the "forbidden" region between 700 and 2000Hz, which is why birds, for example, use such tones as warning signals).

Blumlein's genius lay in the fact that he realized that the low-frequency phase information can be replaced by corresponding amplitude information. If you have two independent information channels, each feeding its own loudspeaker, then the ratio of the signal amplitudes between those two loudspeakers will define the position of a virtual sound source for a centrally placed listener equidistant from them. For any ratio of the sound levels of the two speakers, this virtual source occupies a dimensionless point somewhere on the line joining their acoustic centers. The continuum of these points, from that represented by maximum-left/zero-right to that represented by zero-left/maximum-right, makes up the conventional stereo image. If there is no reverberant information, then the brain will place the virtual image of the sound source in the plane of the speakers; if there is reverberation recorded with the correct spatial relationship to the corresponding direct sound—that is, if it is "coherent"—then the brain places the virtual image behind the speakers, the exact distance depending on the ratio of recorded direct sound to recorded reverberant sound.

Thus, by recording and playing back just the amplitude information in a two-channel system, we can create a virtual soundstage between and behind the loudspeakers (footnote 6). And if, instead of capturing an original event, we record many individual sounds in mono and assign each one a lateral position in the stereo image with a panpot (along with any added reverberation or echo), when we mix down to stereo, again we have a true amplitude-stereo recording. It is fair to say that 99.99% of all recordings are made in this way. It is so fundamental to how recordings are now made that I doubt if anyone thinks about the fact that it is based on psychoacoustic sleight of hand: the substitution of amplitude ratios for time-of-arrival differences in the midrange and bass.

For many years, I was a hard-line Blumlein purist when it came to classical recording. I was attracted by the theoretical elegance of the M-S technique—a sideways-facing microphone with a cosine or figure-8 pickup pattern is spatially coincident with a forward-facing mike; sum-and-differencing the mike outputs gives you true amplitude stereo—and of two figure-8 microphones horizontally coincident at 90°, each positioned at 45° to the forward direction. Of all the "simple" techniques used to capture live acoustic music, these two, in all their ramifications, are the only ones to produce real stereo imaging from loudspeakers.

I used to dismiss with a snort recordings made with spaced microphones. After all, if the microphones are separated in space by a distance larger than the wavelength of most of the musical sounds—10', say—unless an instrument or voice is exactly halfway between the two microphones, there will be, in addition to the amplitude information, a time delay introduced between the electrical signal that voice or instrument produces in one channel and the signal it produces in the other. Such time information pulls the image of the source farther toward the nearest speaker, resulting in an instability of central imaging and a tendency for sources to "clump" around the speakers. Add to that the fact that the interchannel amplitude differences produced by spaced microphones do not have a linear relationship with the angular directions of the sound sources, and it is hard to see how a pair of spaced microphones can produce any image at all.

Yet . . .

In 1992, we were recording two concerts for Stereophile featuring classical pianist Robert Silverman.

The main pickup was with a single stereo microphone, but I had put up a pair of omnis that I fed to a separate recorder. After the concert, it became apparent that the stereo mike had failed, so I was forced to use the spaced-omni recording for the CD release. Here is a short track from that album, Schubert's Moment Musicaux No.3:

[Play Schubert Moment Musicaux No.3, from Concert CD, Stereophile STPH005-2 (1994)]

There are two things intriguing about this recording. For this lecture, one minute in, I flipped the right channel's polarity. I doubt that anyone noticed—there is so much time disparity between the two channels that it cannot be considered a stereo recording at all; rather, it is two different recordings of the same performance that happen to be played back simultaneously. The second thing is that, despite that theoretical imperfection—for which I was duly castigated on Usenet—the CD sold quite well. People liked the sound.

I wasn't too surprised by that. Theoretically perfect amplitude stereo has served us well, but when I played people some of my classical recordings made in the appropriately purist manner, they often described the sound as "thin" or "cold" or "lacking bloom."

As I said earlier, when people say they like or dislike something, you should take notice. And in this instance, the late Michael Gerzon had discussed the matter in a paper he gave to the London AES Convention in 1987. Specifically, he had postulated that Blumlein's substitution of amplitude for phase differences at low frequencies is inadequate, that people prefer the sound when there is some time-difference information between the channels, presumably because the information their brains use to synthesize a model of the stereo image now has more in common with what they would have heard at the original event.

Gerzon had floated the idea of using two pairs of microphones to capture all the information the brain requires: spaced omnis below 1kHz; coincident figure-8s above, with a crossover between the two. I tried that, with disappointing results. However, after 1992, I used a similar miking technique with which I thought I could get the best of both worlds: the good amplitude stereo from coincident or quasi-coincident mikes, and the lower-frequency bloom from spaced omnis. Both mike pairs were used more or less full-range; the only EQ was a touch of top-octave rolloff on the omnis, and some first-order low-frequency boost on the cardioids to compensate for their premature bass rolloff when used distant from the source.

It was the acquisition of a Sonic Solutions Digital Audio Workstation in 1993 that allowed me to fine-tune this technique, because it became apparent that the two pairs of mikes needed to be time-aligned for the resultant stereo image to lock into place. This time alignment of mikes had been used by Denon and was described in an early 1990s AES convention paper, but I had no way of easily implementing this until I could slide individual tracks backward and forward in time to get the required synchronization.

Since then, I have made all my classical recordings in this manner. Here is a typical example: Minnesotan male-voice choir Cantus singing Eric Whitacre's Lux Aurumque in the glorious acoustic of Sauder Hall, at Goshen College, in Indiana. You can see the two pairs of mikes in this photograph.

Not shown in the photo is a third pair of mikes, omnis on a Jecklin disc, farther away from the singers, which I used in case it turned out that the main pickup was too dry. (When you are on location and the clock is ticking away your money, you cover your bases.)

[Play Cantus: Eric Whitacre, Lux Aurumque (excerpt), from While You Are Alive CD, Cantus CTS-1208 (2008)]

If you listen critically to this recording, you will hear that acoustic objects get a little larger, the farther away they are from the center of the stage. However, their spatial positions in the image are correct.

I tell this tale because it illustrates one of my points: that thinking you are right about something in audio doesn't mean you are right. No matter how much you think you know, there will always be new things that upset your world view. Einstein, for example, would be astonished to find that his "biggest blunder," the Cosmological Constant, turns out to be real—that we now are aware that something completely unknown to science is causing the expansion of the universe to accelerate. Physicists call it "Dark Energy," but that's just scientific shorthand for "We have no idea what it is."



Footnote 6: this creation of a virtual soundstage only works for soundsources to the front of the listener and a two-channel system. However, when the mixing engineer requires a virtual image to be placed to the listener's side in multichannel audio, it fails for the simple reason that we do not have a pair of ears on the front and back of our heads.
Share | |
Comments
JohnnyR's picture
Misguided?

I haven't seen one iota of explanation from yourself yet as to why both of us and Harman Kardon and the other links George posted to are wrong. Still waiting ChrisSy.

ChrisS's picture
Georgie & JRusskie = Harman Kardon?

Has HK hired you guys as DBT consultants?

 

Hey JRusskie,

Can you answer this one?

If A=B and C=B, then A=?

If you pass the test, then perhaps someone will hire you... But you and Georgie might have to fight over the job.

JohnnyR's picture
Duhhhhhhhhhh........

"i think THESE are the sort of differences between individuals that make DBT difficult: everyone hears differently. there is no absolute sound."

The sole purpose of DBT is to see if the person listening can distinguish between A and B. If they can't then for all practical purposes there is no difference in the sound from A and B. You and ChrisSy seem to think it's all about what the person "likes". It's a straight forward test method and "likes" has nothing to do with it.

Please explain to us all how Harman Kardon manages to use DBT all the time and do it well? I will be awaiting your reply Ariel.

ChrisS's picture
Twas the night...

So every household has a Harman Kardon product? And you and Georgie have living rooms that look like anechoic chambers? No fireplaces, of course....

ChrisS's picture
And, and...

Hearing a difference between a Harman Kardon product and another product in a anechoic chamber means what to you, Georgie and JRusskie?

Do you know that Ford makes the best trucks in the world?

ChrisS's picture
That's the Pepsi....No, it's...

Are you sure I didn't say 'licks". You know maybe tasting an audio product will yield just as useful results in a DBT.

ChrisS's picture
It's So Easy, Even A 3 Year Old...

JRusskie,

Now run out to your nearest Boris' Convenience store and get yourself a can each of Pepsi (do you even have Pepsi in the Former-USSR?) and Coca-Cola and set up your own Pepsi (or whatever passes for cola in Russia) Challenge.

Wiki has a nice explanation of how to do a DBT...

Once you've done your very own Peps(k)i Challenge, please send us your conclusion. We're curious...

The next step now is to get everyone in your subsidized housing project to participate in your Pepski Challenge.

Gather up that data, compare it your own conclusion and let us know how useful that information is.

I'm sure you'll enjoy the challenge of your doing your very own DBT's! (You won't even have to ask Harman Kardon to use their anechoic chamber!)

John Atkinson's picture
Some knowledge of statistics required

JohnnyR wrote:
The sole purpose of DBT is to see if the person listening can distinguish between A and B. If they can't then for all practical purposes there is no difference in the sound from A and B.

And that's the problem with these tests. If a formal blind test gives results that are indistinguishable from what would be given by chance, formal statistical analysis tells us that this result does _not_ "prove" there was no difference in the stimulus being tested, only that if there _was_ a difference, it was _not_ detectable under the conditions of the test. No more general conclusion can be drawn from the results. And as I have said, it is very difficult to arrange so that those conditions don't themselves become interfering variables. Even the fact that it is a test at all can be an interfering variable, as I explain in this lecture preprint.

JohnnyR wrote:
Please explain to us all how Harman Kardon manages to use DBT all the time and do it well?

I have visited Harman's facility in Northridge and their blind testing set-up is impressive. They have worked hard to eliminate interfering variables and their testing is time- and resource-consuming and painstaking. Even so, they have to make compromises. Blind testing of loudspeakers, for example, is almots always performed in mono. And despite the rigor of their testing, you still have anomalous results, like the Mark Levinson No.53 amplifier, which was designed with such testing but fared poorly in the Stereophile review.

While formal blind tests are prone to false negative results - not detecting a difference when one exists - sighted listening is prone to false positives, ie, it detects a difference when none exists or perhaps exaggerates the degree of difference. As neither methodology is perfect, we go with the one that is manageable with our resources. We therefore offer our opinions for readers to reject or accept in the context of their own experience and I believe Stereophile does  a better job of that than any other review magazine or webzine.

If you are uncomfortable with that policy, then you should not read the magazine. And if I remember correctly, JohnnyR, you admitted in earlier discussions on this sute that you neither subscribe to Stereophile, nor do you buy the magazine on the newsstand. So why should anyone pay attention to your opinions on how the magazine conducts itself?

John Atkinson

Editor, Stereophile

JohnnyR's picture
Silly Boy

So you are saying that if in a DBT the listeners could NOT tell a difference between two amps using music of their own choice then that doesn't prove the amps sound alike?  Funny stuff there Atkinson. For one who thinks you should trust your ears to evaluate components you just bashed the ONE single TRUE way to test by USING YOUR OWN EARS in a DBT.

"And that's the problem with these tests. If a formal blind test gives results that are indistinguishable from what would be given by chance, formal statistical analysis tells us that this result does _not_ "prove" there was no difference in the stimulus being tested, only that if there _was_ a difference, it was _not_ detectable under the conditions of the test."

Your own words are saying that "if there was a difference it was not detectable under the conditions of the test"........oh you mean like letting the listener use the music of their own choice and switch back and forth endless times between two amps and then guess wrongly enough times so that they can't tell which one was which? LMAO if that's not proof that both amps sound alike then what sort of test WOULD prove that they do?  Come on Atkinson you just don't like DBTs because they would show up so many components that people think sound "oh so better than the rest"

Opinions from you and your reviewers are the Gospel now folks. No need to test anything really just trust good ol'JA and his flunkies. Yay.

"If you are uncomfortable with that policy, then you should not read the magazine. And if I remember correctly, JohnnyR, you admitted in earlier discussions on this sute that you neither subscribe to Stereophile, nor do you buy the magazine on the newsstand. So why should anyone pay attention to your opinions on how the magazine conducts itself?"

Oh just maybe because  a lot of people care for this little thing called the TRUTH? When magazines like your's take liberties with the truth by having shoddy reviews instead of in depth testing, then it's everyone's and anyone's responsibility to speak up when crappy falsehoods are published and the readers are supposed to take it all on faith. That's why. I for one do not take your opinions on anything audio related as worthwhile at all for the simple reasons that you show so much promise when you measure speakers but fail to even bother with the snakeoli products that you let slide under the radar yet let your reviewers give them glowing reviews sans any testing what so ever. Maybe that's the sighted listening bias you just spoke about yet you fail to even try with those type of products to get to the real TRUTH.

ChrisS's picture
Inconclusive?

JRusskie,

If you like Harman Kardon marketing, but you're not sure if Ford makes the best trucks in the world, then get yourself an F-150 and whatever truck you used to rumble across Afghanistan with, do your DBT (just like  the Pepski Challenge) and let us know what you come up with...

You are marketing TRUTH now? How pure is it?

I know some construction workers who might be interested...

ChrisS's picture
A War In Your Head...

JRusskie, Just looking at your response to John's post and comparing word-for-word what John wrote and what you think he says, there's such a huge world of difference!! There's a war in your head!

[Flame deleted by John Atkinson].

GeorgeHolland's picture
"Even so, they have to make

"Even so, they have to make compromises. Blind testing of loudspeakers, for example, is almots always performed in mono."

Well Mr Atkinson the reasoning behind testing speakers in mono is to eliminate the dreaded comb filter affect that would otherwise show up if a stereo pair were auditioned and the listener moved their head even a couple of inces. I'm surprised you didn't mention that fact but then again you think DBTs are hard to do, so if you don't know how to do them then indeed they are hard to do. *Chuckle*  Any DBT done should be auditoned is such a manner. The rest of your "excuses" for not doing them is the same old same old from you, nothing surprising there.

ChrisS's picture
"Kiss the girls..."

So Georgie Porgie,

Let's say JRusskie is DBT'ing a $1500 speaker and a $500 speaker and can't hear a difference, and you are DBT'ing a $4500 speaker and a $4000 speaker and you happen to have enough working neurons to hear a difference... Which set of speakers should the ex-shepherd construction worker buy?

John Atkinson's picture
Mono vs Stereo testing

GeorgeHolland wrote:
John Atkinson wrote:
Even so, they have to make compromises. Blind testing of loudspeakers, for example, is almost always performed in mono.

Well Mr Atkinson the reasoning behind testing speakers in mono is to eliminate the dreaded comb filter affect that would otherwise show up if a stereo pair were auditioned and the listener moved their head even a couple of [inches].

That is a consideration, of course, but in my opinion a minor one. As I had understood from Floyd Toole back in the day, the additional complexity required  of  Harman's physical speaker shuffling apparatus to do blind speaker testing in stereo was not justified by the results, ie, they felt that the stereo performance could be predicted from the mono results.

I don't agree with that, but more importantly, this illustrates the thesis offered in my lecture, that when you move the testing situation a step away from how the product is going to be used, you can't be sure that the assumptions you make haven't invalidated the test. As I write in the abstract to the lecture, "perhaps some of things we discard as audio engineers bear further examination when it comes to the perception of music."

BTW, I am still waiting for you to acknowledge that the criticism you made of my lecture, that it was not about Richard Heyser, was incorrect.

John Atkinson

Editor, Stereophile

GeorgeHolland's picture
More excuses?  I see what

More excuses?  I see what Johnny meant. You are never wrong. I am pretty sure Haram Kardon knows what they are doing. Please address any criticisms to them not me.

I am afraid that comb filtering IS a big deal. That would explain why cables "sound" different. It's not the cable but the listener changing where their head is between "testing"

You will be waiting a long time for any ackowledgement about your "lecture". Stop being the primadonna already.

Regadude's picture
Prima donna supreme

Georgie wrote:

"Stop being the primadonna already."

Look in the mirror and repeat those words!!!! laugh

ChrisS's picture
Oops! My head moved...

So how does one differentiate speakers that sound differently, amplifiers that sound differently, pre-amps, turntables, tonearms, cartridges, DAC's, etc., if a turn of one's head makes that much difference?

Where's your reliability, Georgie? Doesn't science depend on reliability?

JohnnyR's picture
For The Lazy and Mentaly Deficient.......

I can see what George is up against in here with Tweedle Dum and Tweedle Dee tag teaming and showing their ignorance.

http://www.ethanwiner.com/believe.html

Golly look what he meant.I think Ethan was banned from here ages ago for showing up Fearless Leader and his cronies and out right showing how REAL science works. BWAHAHAHAHAHAH loser boys.

Regadude's picture
For Johnny, about Johnny...
John Atkinson's picture
Ain't that the truth

Just bookmarking the link for future reference.

John Atkinson

Editor, Stereophile

JohnnyR's picture
Atkinson..........

......when you start doing a single DBT or even a SBT then you can talk about the "truth". Have you EVER designed and built your own speakers? Nahhhhhhhh you are too lazy or too "busy". Still finding plenty of time though to post online all the time though strangely enough.cheekyTill then you aren't an engineer so take your own advice and don't comment on speaker design anymore.

If you are going to save the link then please also save this link where discussion about it unfolded.

http://forums.audioholics.com/forums/loudspeakers/83412-diy-loudspeakers...

As you can see the original post was just one of many OPINIONS about the topic that is if you bother to read it at all. There are various OPINIONS about the topic and notice just how many of the so called "hobbyists" ended up being professional speaker builders. If you just pick and choose certain OPINIONS from the thread then you are guilty of leaving out facts.

For starters read the sixth post down by Jinjuku regarding Jeff's post that pretty much sums up where DIY has progressed.

ChrisS's picture
You think, therefore it's true...

Not real science either...

GeorgeHolland's picture
Frick and Frack strike again.

Frick and Frack strike again. Regadude and ChrisS always come up with strawman replies and ignore the links posted."Not real science "? How pompus can you get? Mr Winer measured the effects of comb filtering, what did you measure ChrisS the length of your nose when you typed that reply? You dismiss anything people link to yet show us nothing in return. Regadude, posting opinions isn't real science just so you both understand. Now run along lil boys and study real hard, maybe in another 20 years you might be able to hold your own in a discussion.

ChrisS's picture
Georgie thinks....

Has Winer's results been verified?

Did you know that Harman Kardon makes the best audio products in the world? And Ford makes the best trucks, right?

JohnnyR's picture
Do Some Testing Yourself Genius

Or is that above your abilities like thinking?

Go ahead, put on a pink or white noise source and move your head about and tell me the sound doesn't change. You won't bother so forget it ChrisSy.laugh

ChrisS's picture
ADHD

When moving a microphone while recording a person's voice, the sound changes. Did the voice change?

GeorgeHolland's picture
You never answer a question ,

You never answer a question , you just put forth silly questions of your own. That's what people do when they don't know or are scared to try.

Moving a microphone while recording a person's voice? If that's how you do things then no wonder you don't know what Johnny was talking about. Yes the sound changes as recorded by the microphone so what?  Genius.angle

ChrisS's picture
Dear Georgie...

I'll answer you this one... You and JRusskie always answer your own questions that you pose to everyone in these discussions. There's no need to provide any answer to you. As well, your attitudes and limited knowledge of the application of research methodology make civil and thoughtful discourse impossible.

So my questions to you and JRusskie are formed to reveal how each of you think whenever you provide a response.

You provide enough information for me to say that I find the "best" use of my time in these discussions is to make fun of you and JRusskie.

Ariel Bitran's picture
And Chrissy

makes some sense of the whole darn thing.

Regadude's picture
Tweedle dee and tweedle dum...

The only duo that strikes here George, is you and Johnny. You both STRIKE OUT!

Site Map / Direct Links