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.
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COMMENTS
ChrisS's picture

Who's doing the listening in your tests, JRusskie? Do you know any 18 year old musicians? Oh, of course not...But you probably keep company with a bunch of construction guys (lucky you!) with damaged hearing. They should all find that there's no difference between any products.

ChrisS's picture

Once again, comrade JRusskie, you are on your Quixotic journey down that twisty, winding path for "truth"... Being an upstanding citizen of the former-USSR, you should know about "truth", right?

DBT is SCIENCE, and SCIENCE is ALWAYS RIGHT, especially during the heyday of the Union of Soviet Socialist Republics and you've been hanging on to this idea since the dissolution of the USSR. Hanging on to something you really want to be TRUE.

Comrade JRusskie, just because you want it, doesn't make it so.

Ariel Bitran's picture

just wanted to let you know i haven't forgotten about you.

i've been south of the equator spending time with my father and brother, but now that I'm back in the Stereophile office, i'll answer your question in full a little later.

peace out homeslice.

be nice.

Regadude's picture

Asking Johnny to be nice... Wow, you are very optimistic Ariel! 

GeorgeHolland's picture

Ariel, how about doing your job and deleting post and start placing a 30 day BAN on Regadude and ChrisS for spamming this whole article day after day with nothing less than pure drivel and taunts? I've seen kids frums better moderated than this fiasco but it's already turned into a child's forum the way they act. You would think they had some dignity but we haven't seen any yet.

Ariel Bitran's picture

I believe the internet is pretty much open range, so I'm happy to just watch folks speak their minds using whatever manner to express themselves.

one thing that bothers me is when a member is unneccessarily rude to someone who is being respectful or even innocently posting unknowing of the snark they may receive. in those instances, I say something so new users are not discouraged to continue posting.

On the other hand, if you're being provocative with your initial comment, I can only expect others to react to your provocation. 

******

The other policy I encourage and have stated many times is as follows 

"don't feed the trolls"

******

Finally, we do delete comments from ANY member we feel is totally off-topic and insulting to an individual member, as we want the conversation to continue and bring light to the subject  at hand rather than turn into a name-calling game. That being said, I'd like to delete Regadude's comment above since it just makes fun of Johnny. Also, I probably shouldn't have typed be nice. i dont know what i was thinking. I think i just wanted a rhyme in there, and also to include a general suggestion of positivity, but deleting RegaDude's comment would delete your reply, and this is a conversation I'd like to keep around.

Regadude's picture

What did I do? In response to your comment of "be nice" to Johnny, I merely joked that you were asking a lot. A lot, because Johnny is often not nice. He is that way, that is a fact. I have no responsibility for his behavior. I just pointed it out.

Don't shoot the messenger!

ChrisS's picture

What is reliability?

Think, Georgie, think.

Ariel Bitran's picture

the two links provided earlier giving examples of some DBTs.

I found the matrixhifi test to be ignorable: who is the sample? how did they select these people? how are they representative of a population of listeners as a whole? in order to gather significance from these these tests, the first and most important step is determining your sample, sample size, and how you select your sample. this just seems like a bunch of friends having fun. also, since there were multiple components being switched at the same time, system synergies could have been the cause of the weaker sounding more 'hi-fi' system. maybe those components weren't right for each other, but the cheaper system just sounded better. at least in ABX, they only changed one piece at a time

what i found interesting in the ABX test was the user's ability to control the change of system component themselves. this helps eliminate the idea that the listener might feel like they are getting 'duped' or constantly searching/guessing for the difference.

Also regarding the ABX method, the # of times a difference was heard was 33. the # of times no difference was heard: 29. Interestingly, cables were the least discernable. 

DBT is time consuming and for signnificant results you need a large sample size (to represent a large population of listeners). With a small sample size, as in both of these tests, you risk a greatly flawed hypothesis and will lack confidence in your results. 

I don't want to whip out my textbooks, b/c i have other stuff to do, but 17 listeners is not nearly large enough of a sample size to even represent a population of 70,000 Stereophile readers (for example). Then we run into an even bigger problem of "who" is selected-->ie what type of sample you are trying to represent.

I've heard repeatedly that H/K does have a successful DBT model. I'm sure it takes them years to perform each experiment, and it is wildly expensive and time consuming. You need a large sample size for any of this stuff to matter, not a few dudes in a basement.

GeorgeHolland's picture

More excuses from Stereophile?  Why am I not surprised. You scoff and blow off any attempts at doing a DBT by the people I linked to yet when it comes to "reviewing" cables and amps it's suddenly okay to accept a sighted biased review as being true?  Laughable sir but then I suppose you being an emploee have to toe the line. So be it.

I give up. Go on and trust your sighted "tests" while ignoring a DBT or even a SBT. I know your boss won't let you do any. He "knows" it all. *eyeroll*

Regadude's picture

Hey George! Ariel provided you with a more than satisfactory answer. So why are you still complaining? You never heard the expression "agree to disagree"?

Ariel is right; sample size is crucial. Anyone who has a basic knowledge of statistics understands this. 

JohnnyR's picture

George was talking about simply doing a DBT or SBT among friends like he linked to. Let's say you and 5 pals think the "Humungo" amp  blows away every other amp you have listened to. So you set up a simple DBT or even a SBT to see if you can RELIABLY pick which amp is which WITHOUT sighted bias. In other words using your EARS???? I have read on the forums here how much people rely upon "What I heard" yet don't trust a simple test to prove that they can.

So lets say the "Humungo" amp is just simply "liquid, lifts 400 veils and makes the back ground blacker", what ever silly terms you wish to use, If so then you should be able to pick it out 100% of the time doing a DBT. If not and lets say you only choose right 50% of the time then that's proof you were only guessing and couldn't tell which was which. So much for saying sample size is critical. If no one can pick the "Humungo" amp from the other one then where are your rationalisations for saying it's better?

I can tell though that none of you have even bothered to try a SBT or DBT so it's all a waste of time speaking to the peanut gallery. I love how much money you both have WASTED on pricey products that only look good and sound the same. Funny as hell, go on spending YOUR money I love ityes

ChrisS's picture

JRusskie just beat himself with his own thinking...

Regadude's picture

 

Johnny wote:

"I can tell though that none of you have even bothered to try a SBT or DBT so it's all a waste of time speaking to the peanut gallery. I love how much money you both have WASTED on pricey products that only look good and sound the same. Funny as hell, go on spending YOUR money I love ityes"

How do you know what I've bought with my money? I will go on spending my money (but not on your plywood speakers you make in your basement), that's what it's made for!

 

JohnnyR's picture

.a fool and his money are soon parted.

Regadude's picture

Well I am not that much of a fool with my money. I have not yet bought any of your JohnnyR brand speakers! 

ChrisS's picture

"...came out to play,

Georgie Porgie ran away!"

Delusions of absolute truths cloaked in "science" aren't much to hide behind, eh Georgie...

I hear our Man of La Mancha, JRusskie, is looking for his Sancho Panza to accompany him on his journey in search for TRUTH.

Happy trails!

ChrisS's picture

Georgie, Look at all those scientists backing you up!

If you take a college level research methodology course, Georgie, you may not appear so laughable. At this point, there appears to be very little knowledge in what you say about reviewing audio products.

GeorgeHolland's picture

Here is how you review audio products........ looks in Stereophile and buys whatever the product of the month is. Spends too much money but doesn't care. Goes online and talks like a 5 year old and spews insults and THINKS he's smart. Case closed.

Regadude's picture

George, name ONE SINGLE product that I have bought. Just one. You and Johnny have this little fantasy in which you think you know everything and everyone. 

How about you list your gear? If you are so good and knowledgeable about buying audio products, do share with us the products that are George Holland worthy. If you actually have any audio gear...

GeorgeHolland's picture

ZzzzzzzzzZZzzzzzzzzzzzzz sorry but conversing with the likes of you is boring and makes my brain hurt considering the amount of BS you spew,

ChrisS's picture

Take one course of college-level research methodology and call us in the morning...

rl1856's picture

Do you like what you are hearing ?  If no, move on until you do.  If yes, then shut up and relax.  This is a hobby focused on the enjoyment of the creative output of artists.  It is not about how many proverbial angels can dance on the head of a pin.

Go listen to MUSIC !

hnipen's picture

Thanks John for a very interesting and exciting presentation, lots of interesting information here and I'm surprised, to say the least, from the lack of positive feedback.

There are many who are skeptical to some of the ways of doing measurements in Stereophile and in some ways I'm one of them too, especially the way speakers are measured so close, large array speakers like the bigger Dunlavy's and some others will not sum up very nicely in this way. We do, however, not live in a perfect world and Stereophile cannot afford an anechoic chamber, so this is probably the best they can do.

I wish John would share more of this kind of information as he has gathered lots of knowledge during a long interesting career at Stereophile and other places.

Go on John :-)

Merry Christmas

Cheers harald

absolutepitch's picture

John, thanks for getting this lecture pre-print available for us to read. I have been looking forward to this.

I agree that there is a lot fo information combined into one lecture that anyone would need a lot of time to learn and understand the details. Pardon me for paraphrasing some of your words below.

Regarding the null result of DBTs, your description of the interpretation is in agreement with what I remember from statistics classes. I might add that a statistician would include a probability value or confidence band with the interpretation (something to the effect that 'the null hypothesis of no-difference-detctable is accepted with high probability'), and equally for the case when a difference is detected with high probability. I personally think DBTs should be done for product reviews, but agree that valid DBT's are difficult and time consuming (expensive) to do correctly, as Dr. Toole has shown in his writings.

The example of the 'backwards' impulse being not agreeable to listeners is something I have noticed in reference to digital recording. It's a wave form that does not occur naturally in music production, so reproducing it should sound 'bothersome'.

I also agree with the previous post, that more articles like this would be welcomed, to further highlight how complicated this field really is.

bernardperu's picture

I have read your essay with great pleasure (all of it!) and I think it is a great example of the Liberal Arts and Science coming together. In the end, it feels like a piece of applied music philosophy, which I find fascinating. It also seems to be free of busines-oriented interests, as your opinion on cables clearly suggests. It is awesome and very unsual to meet an accomplished person who gives priority to his passions and principles over financial interests (as also expressed on your 2012 writing on the CES and Las Vegas). 

I consider myself to be an audiophile that turns off the lights and tries to connect his emotions with the music with a very relaxed mind (this seems to be a category in itself, as the un-relaxed passive listeners who cannot focus on the music on a mid to long term basis tend to be very opinionated). Having said this, I recently purchased a pair of Class D mono amps that can clearly connect me to the music (Hephaestus brand). I have not ever listened to amps which are over 15k. Within similar prices, class D seems to be the better choice (but how relative this can be, Jon!)

I will continue to follow your writings with deep admiration and I thank you for making a difference on my musical experience (which is passed on to my girlfriend and my child). 

 

Bernard

GeorgeHolland's picture

Clueless you are if you think Mr Atkinson is something special angle

ChrisS's picture

Yoda you are?

Andreasmaaan's picture

It's a pity that some proponents of DBT as the only valid methodology have used the comments thread here to launch personal attacks against JA. Personally, I found the lecture fascinating and thought-provoking, and I thought that the nuggets of personal history provided a powerful context for the thoughtful opinions expressed. For better or worse, Stereophile doesn't restrict itself to DBTs as their only reviewing tool, but JA does measure every piece of gear his reviewers review - a practice which ensures that the opinions of the reviewers are grounded in objective data, or otherwise as the case may be. I'm not sure why this approach, coupled with a reliance on an income stream from advertising, seems to place JA in line for so much personal vitriol. If similar attacks were levelled at me in my professional life, I'd be mortified and enraged.

To cut what is risking becoming a lengthy expression of indignation short: thank-you JA for a wise and thought-provoking read.

JohnnyR's picture

WRONG!

"For better or worse, Stereophile doesn't restrict itself to DBTs as their only reviewing tool, but JA does measure every piece of gear his reviewers review - a practice which ensures that the opinions of the reviewers are grounded in objective data"

Do you even READ the reviews? Seriously dude. Stereophile DOESN'T do DBTs at ALL! Atkinson has said repeatedly that they are "too difficult to do". Show me ONE single DBT he has done in a review please. Plus he does NOT "measure every piece of gear his reviewers review". Cables, power cords, record demagnitizers, just to name a few. You obviously have been reading another publication NOT Stereophile.

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