In this book, realism in staged music sound reproduction is understood to mean the generation of a sound field realistic enough to satisfy any normal ear-brain system that it is in the same space as the performers, that this is a space that could physically exist, and that the sound sources in this space are as full bodied and as easy to locate as at a live concert. Realism does not necessarily equate to accuracy. For instance, a recording made in Symphony Hall but reproduced as if it were in Carnegie Hall is still realistic even if inaccurate. In a similar vein, realism achieved carelessly does not always mean perfection. If a full symphony orchestra is recorded in Carnegie Hall but played back as if it were crammed into Carnegie Recital Hall, one may have achieved realism but certainly not perfection. Likewise, as long as localization is as effortless as in real life, the reproduced locations of discrete sound sources might not have precisely the same sometimes exaggerated perspective as at the recording site to meet the standards of realism discussed here. An example of this occurs if a recording site has a stage width of 120 degrees but is played back on a stage that is only 90 degrees wide. What this really means in the context of realism is that the listener has moved back in the reproduced auditorium some twenty rows, from the first row but either stage perspective can be legitimately real. Finally, mere localization of a sound source does not guarantee that such a source will sound real. For example, a piano reproduced entirely via one loudspeaker, as in mono, or by two in stereo is easy to localize but almost never sounds real. The mantra goes, Mere Localization Is No Guarantor of Realism. Interestingly, one can have monophonic realism as when you hear a live orchestra from the last row of the balcony but can't tell whether the horns are left, right, or center.

Since most of us are quite familiar with what live music in an auditorium sounds like, we soon realize that something is missing in our stereo systems. What is missing is soundfield completeness and psychoacoustic consistency. One can only achieve realism if all of the ear's hearing mechanisms are simultaneously satisfied without contradictions. If we assume that we know exactly how the ears work, then we could conceivably come up with a sound recording and reproduction system that would be quite realistic. But if we take the position that we don't know all the ear's characteristics or more significantly that we don't know how much they vary from one individual to another or that we don't know the relative importance of the hearing mechanisms we do know about, then the only thing we can do, until a greater understanding dawns, is what Manfred Schroeder suggested over a quarter of a century ago, and deliver to the remote ears an exact replica of what those same ears would have heard if present where and when the sound was originally generated. The old saw that, since we only have two ears, we only need two channels in reproduction has been justly disparaged. I would rephrase this hazy axiom to read, that since humans have only two ear canals, to achieve realism in reproduction, we need only provide the same sound pressure at the entrance to a particular listener's ear canal, even in the presence of head movement, that this same listener would have experienced at his ear canals had he himself been present at the recording session. Fortunately, it does turn out that only two recorded channels are in fact needed for realistic music reproduction (more are actually detrimental) and it is the purpose of this book to show why this is so and how to do it.

While there are some widely held hi-end beliefs that may have to give way to psychoacoustic reality, the basic audiophile ideal that two channel recordings can deliver concert-hall caliber musical realism is not that far off the mark. However, having only two recorded channels does not mean being limited to only two playback loudspeakers. I call the coming replacement for today's stereo 'ambio' optimized but uncompromised for the recording and reproduction of frontal acoustic musical performances such as concerts or operas rather than movies and video. By definition, and as substantiated below, where audiophile purity is concerned, multi-channel recording, especially with a center front channel, not only is not needed but is actually psychoacoustically counter productive. The concert hall genie cannot be squeezed into the 5.1, 6.1, or 7.1 or 10.2 moving picture surround sound bottle.

There are two basic theoretical technologies that are prime candidates to replace stereo in elite high-end music reproduction, where mass marketing and complex technical concepts should not be (but of course are) major stumbling blocks. One is the wavefront reconstruction method often employing hundreds of microphones and speaker walls or, where recording is involved, Ambisonics. The Ambisonic wavefront reconstruction method generates the correct sound pressure and sound direction in a region that at least encompasses one listener's head. The other is the binaural technology method that more directly duplicates the live experience, independently, at each ear. Of course, both tehnologies aim to deliver to the entrance of your ear canal an accurate replica of the original sound field. The Ambisonic method does have the advantage that it can reproduce direct sound sources from any angle and so is quite well suited to non-concert events or movies. But since the Ambisonic wavefront reconstruction method requires a special microphone, a minimum of three (or better four recording) channels and a very complex decoder, is not as accurate as the binaural technology methods, and does nothing for the existing library of LPs and CDs it will not be considered further here.

The concept that dedicated music listening in the concert hall, theater, jazz venue, or at home is a group activity is superficial. Yes, there may be 2500 people in the opera house, but while the curtain is up there is, ideally, no interaction between them. Each member of the audience might just as well be sitting alone unless you believe in ESP. Listeners in a concert hall are also restricted as to the size of their sweet spot. They can't slouch to the floor or stand up, their permitted side to side or back to front movement is not extensive and there are plenty of seats in most halls where the sound and the view are not quite so sweet.

At home, how often does the gang come over to sit with you for five hours of Die Goetterdaemmerung? Certainly, serious home listening to classical music and to a lesser extent longer popular genres such as Broadway shows, new age, movie scores, jazz concerts, etc., is sad to say a solitary or at most a two person pursuit. Of course we all want to demonstrate our great reproduction systems to friends and family, but since these sessions usually last just a few minutes, one can show off the system to one or two people at a time and after everyone has heard it, at its best, from the sweet spot, the party can go on.

By now, every one in the industry has recognized that when a two channel recording is played back through two loudspeakers that form an 60 or 90 degree angle from the listener, that each such speaker communicates with both ears, producing interaural crosstalk. The deleterious effects of this crosstalk at higher frequencies have been greatly underappreciated. For openers, crosstalk is what almost always prevents any sound source from appearing to come from beyond the angular position of the loudspeakers. This result is intuitively obvious, since if we postulate an extreme-right sound source, and can safely ignore the contribution from the left speaker, we can now hear the right speaker by itself, as usual with both ears, and no matter how we turn our heads the sound will always be localized to the right speaker as in any normal hearing situation. However, if we keep the right speaker sound from getting so easily to the left ear then the brain thinks that the sound must be at a larger angle to the right, well beyond the, say 30 degree position of the loudspeaker, since, as in the concert hall, the lesser sound reaching the left ear is now fully filtered by the head and the left pinna. So, for starters, stereo, because of its crosstalk, inadvertently compresses the width of its own sound stage.

Just as there are optical illusions, so there are sonic illusions. One can create sonic illusions by using complex filters to create virtual sound sources that float in mid air or rise up in front of you. As with optical illusions some people detect them and some people don't. The most prominent audio illusion is in stereo where phantom images are created between two speakers. You may have observed that most optical illusions are two-dimensional drawings, that imply three dimensions. Likewise there is something indistinct about the stereo phantom illusion. This is because the phantom image is largely based on lower frequency interaural cues and barely succeeds in the face of the higher frequency head and pinna localization contradictions.

The fact that earphone systems such as Toltec based processors, a host of PC virtual reality systems, SRS, Lexicon, etc. can move images in circles just by manipulating high frequency head and pinna response curves, even if not of great high-end quality, does show that these hearing characteristics are of considerable importance. Thus the direction from which complex sounds over 1500 Hz originate, particularly from the reproduced stage, should be as close to correct as possible.

Crosstalk elimination is not a new concept to Ambiophonics. Most of the older electronic crosstalk elimination circuits such as those of Lexicon, Carver, Polk etc. assume the stereo triangle and have, therefore, had to make compromises to enlarge the sweet spot size over which they are effective. I would hesitate to class any of them as high-end components, especially as they still promote pinna position errors. But now a new crosstalk canceller from England, called a Stereo Dipole, assumes that the speakers are practically touching. Usually good crosstalk cancellers require complex compensation for the fact that the crosstalk signal being canceled has had to go around the head and over the pinna on its way to the remote ear. Since VMAx, Lexicon, etc. don't know what your particular head and pinna are like, they assume an average response and thus can't do a very good job of cancellation at high frequencies. If they try, most listeners experience phasiness, a sort of unease or pressure particularly if they move about. But when the speakers are in front of you there is not much of the head to get in the way and so the head response functions are much simpler, less deleterious if ignored or averaged, and head motions make little difference. Electronic stereo dipoles are just now appearing but you can easily achieve an inexpensive and truly high-end result using a simple three foot square six inch thick absorbent panel set on edge at the listening position. You get used to the panel rather quickly and it is high-end tweak that needs no cables and produces no grunge. There is already a commercially produced panel from Echobusters that folds up for storage when not in use. Either electronic stereo dipole processors or panels allow complete freedom of head motion without audible effect and afford more squirm room at the listening position than one has in a concert hall. Two people can be accommodated comfortably but usually one need to be directly behind the other for optimum results, not unlike stereo.

One can have video with either arrangement but adding a picture can have its down side. One reason that so many listeners are impressed with the realism of movie surround sound systems is the presence of the visual image. While the research in this field is not definitive, it stands to reason that a brain preoccupied with processing a fast moving visual image is not going to have too much processing power left over to detect fine nuances of sound. Certainly, if you close your eyes while listening to any system, your sensitivity to the faults of the sound field is heightened. Thus when a seemingly great home theater system is used to play music only, without a picture, the experience is often less than thrilling. Adding a picture to Ambio seems to make fine adjustments to the ambient field much less audible, but one must observe that most people keep their eyes open at concerts and so perhaps an image is desirable to provide the ultimate home musical experience.

Like the federal budget agreement, a method of achieving that air, space, and appropriate concert hall ambience at home, has technical devils in its details. The most obvious suggestion, based on movie and video surround-sound techniques, to just stick the ambient sound on additional DVD multi-channel tracks, on closer examination, just can't do it for hi-enders. The problem with using third, fourth or fifth microphones at or facing the rear of the hall and then recording these signals on a multi-channel DVD, is that these microphones inevitably pick up direct sound which, when played back from the rear or side speakers, causes crosstalk, pinna angle confusion, and comb filter notching. It is also pinnatically incorrect to have all rear hall ambience coming from just two point sources even if these surround speakers are THX dipoles. Remember, using rear dipoles implies a live listening room, which will thus also increase unwanted early reflections from the front speakers. Additionally, recording hall ambience directly is really not cost effective or necessary. Unlike movies, the acoustical signature of Carnegie Hall (despite its always ongoing renovations) does not change with every measure so why waste bits recording its very static ambience over and over again? It is much more cost and acoustically effective to measure the hall response once from the best seat (or several) for say five, left, right, and center positions on the stage (If the hall is symmetric, the measurement process is simpler) and either include this data in a preamble on the DVD, store it in your playback system or provide it as part of a DVD-ROM library of the best ambient fields of the world.

The cause of concert-hall early reflection and reverberation tail synthesis by digital signal processors (DSP) in computers or audio products was set back by the late Michael Gerzon, the Oxford Ambisonics pioneer, who wrote in 1974 "Ideally, one would like a surround-sound system (yes, he did use this term in 1974) to recreate exactly, over a reasonable listening area, the original sound field of the concert hall.... Unfortunately, arguments from information theory can be used to show that to recreate a sound field over a two-meter diameter listening area for frequencies up to 20 kHz, one would need 400,000 channels and loudspeakers. These would occupy 8 gHz of bandwidth equivalent to the space used by 1000, 625-line television channels!" This article of faith was quoted recently at an AES meeting in New York by Bob Stuart of Meridian (and the ARA) and is still a widely held belief in audio engineering circles.

An audiophile-friendly approach to ambience reconstruction is to derive the surround speaker feeds by convolution of a two channel recording, preferably made using the microphone technique described below, that limits rear hall pickup. The questions to be asked are these:

There may never be a definitive answer to the first two questions. Just as there is no sure recipe for physical concert hall design, there is no best virtual concert hall specification. But, adjusting the number, placement, and shape of early reflections is easily more audible than changing amplifiers or cables and offers a tweaker delights that can last a lifetime. I can only say that in my own experience, just as there are thousands of real concert halls that differ in spite of being real, so there are thousands of ambience combinations that sound perfectly realistic even if not perfect. How do you get more real than real? Remember, absolute, particular hall parameter accuracy is not essential to achieve realism. By analogy, even if one sits on the side, in the last row of the balcony at Carnegie Hall where the ambience is lopsided, the sonic experience is still real.

In my opinion the best software for this purpose is based on impulse response measurements made in actual concert halls as was done by JVC and Yamaha some ten years ago for consumer products and is being done all the time by acoustical architects tuning auditoriums. Others, such as Dr. Dave Griesinger at Lexicon, create ambience signals using an imaginary model. I am not talking here about professional effects synthesizers that generate artifacts never heard by anybody in any physically existing space. Someday, I presume, we will have a DVD-ROM that contains the ambient parameters of Leo Beranek's 76 greatest concert houses of the world and with a simple mouse click, tweaking will yield to selection. With enough hall impulse responses stored, you could even select a seat and a stage width. (If its a solo recital one wants only central derived early reflections, if a symphony orchestra, the works, etc.)

While I may not be the best one at executing my own theories, I have gotten startlingly good results using the relatively primitive convolvers available. It is a rare AES convention that does not describe advances in the state of this art. Another important point is that ambience regeneration is scaleable. As computers get faster, and cheaper and as convolution software gets better, it is easy to upgrade or add more ambience speakers. The hall ambience storage method is also inherently tolerant of speaker type and the precise location or speaker response matter little and are akin to repainting the balcony or curving a wall in the concert hall.

The fact is that the brain is not all that sensitive to whether there are 30 early reflections from the right and only 25 from the left or whether they come from 50 degrees instead of the concert-hall ideal (according to Ando) of 55 degrees. If the reverberant field is not precisely diffuse or decays in 1.8 seconds instead of 2.0 seconds, that may only mean you are in Carnegie Hall instead of Symphony Hall. I make no claim to be an authority on setting ambience hall parameters, and I am sure many audiophiles could do better at this game. I now use two large area speakers at +/- 30 degrees two at +/- 55 degrees, two at +/- 70 degrees and two at +/- 90 degrees to launch left and right early reflections. I tilt these long speakers so that they really span some five degrees about these points to better mimic real halls. I similarly use eight, large area, tilted speakers to the sides and rear to provide a reverberation field as diffuse as possible.

The NHK Science and Technical Research Laboratories in Japan have developed a method of reproducing concert hall ambience using walls of small loudspeakers covering the rear and the sides of the listening room. These prefabricated panels can contain hundreds of small loudspeakers depending on the size of the room. The panels also serve as room treatment and are easy to install.

To feed the speaker walls, the Japanese have devised a new mathematical randomizing method for generating many uncorrelated reverberation trains from a single real impulse response measured in a concert hall. Normally, (information theory again) one would have to measure a separate impulse response at hundreds of locations in the hall for these hundreds of loudspeakers in order to ensure that the reverberant field produced by all these speakers was truly diffuse (the same power but not identical in all directions). If too many speakers are fed exactly the same signal there will be comb filter effects and little interaural excitement. Some forty ambient signals were generated, using a scrambling algorithm, and applied to groups of physically close speakers. Even if one does not want to buy into speaker walls, the measured wave pressures throughout their room show that it is possible to simulate a realistic, diffuse reverberant field from a stored room impulse response starting with a simple stereo signal. They have also conclusively demonstrated that one can have realistic ambience without requiring gigabits of storage.

While audiophiles do not often concern themselves with recording techniques over which they have little control, but almost any LP or CD made with spaced microphones is greatly enhanced by Ambio playback. But one can heighten the accuracy, if not gild the lily of realism, by taking advantage, in the microphone arrangement, of the knowledge that in playback, the rear half and side part of the hall ambience will be synthesized, that there is no crosstalk, that the front loudspeakers are relatively close together and that listening room reflections are minimized.

To make a long story short, exceptionally realistic "You are there" recordings can be made by using a head shaped, pinnaless ball with holes at the ear canal positions to hold the microphones. The Schoeps KFM-6 is a good example of such a microphone even though it is a sphere and an oval would be slightly better. However, for best results, this microphone should be well baffled to prevent most rear hall ambience pickup. KFM-6 recordings are a feature of the PGM label, produced by the late Gabe Wiener who was a staunch advocate of this recording method, first expounded by Guenther Theile. As expected, these PGM recordings are exceptionally lifelike when played back Ambiophonically so as to be free of crosstalk or pinna distortion.

Audiophiles must get over their fear of room treatment (anechoaphobia) and embrace it enthusiastically. Stereo, Ambiophonics, Ambisonics, VMAx, surround-sound, multi-channel, etc. can all be substantially `improved by eliminating early reflections and reverb that conflict with what is on the recording. Finally, after the multi-channel furor has subsided, I believe some form of two-channel binaural technology, if not Ambiophonics per se, will emerge to serve audiophile music lovers.