Download full-text PDF. Ambiophonic Principles for the Recording and Reproduction. of Surround Sound for Music. Angelo Farina (1), Ralph Glasgal (2) , Enrico. PDF | Ambiophonics, as one of the most realistic music reproduction methods, requires multi-channel convolution Download full-text PDF. Ambiophonic AP - Experience your records with a different kind of audio reproduction system called Ambiophonics! ATTENTION: Don't use.
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Ambiophonics is a method in the public domain that employs digital signal processing (DSP) . ; ^ Instructions and free downloads available at bestthing.info Create a book · Download as PDF · Printable version. Ambiophonics Book - Download as PDF File .pdf), Text File .txt) or read online. Ambiophonics Book. REAL-TIME PARTITIONED CONVOLUTION FOR AMBIOPHONICS SURROUND SOUND - Download as PDF File .pdf), Text File .txt) or read online. Anders.
Additionally, ambiophonics provides for the optional use of concert-hall or other ambience impulse response convolution to generate hall ambience signals for virtually any number and any placement of surround speakers.
Ambiophonics can usually accommodate more than one listener since one can move back and forth along the line bisecting the speakers. Precisely because of the higher level of envelopment along this line, the loss of realism when one moves away from the center line is more dramatic in the case of Ambiophonics than stereo.
The listening area can be enlarged with ambience convolution, whereby surround speakers mimic the contributions of concert-hall walls. Ambiophonics methods can be implemented in ordinary laptops, PCs, soundcards, hi-fi amplifiers, and even modest loudspeakers with consistent phase response, especially in any crossover regions.
REAL-TIME PARTITIONED CONVOLUTION FOR AMBIOPHONICS SURROUND SOUND
Neither true-binaural dummy head with pinna recordings nor head tracking are required, as with headphone-binaural listening. Commercial products now implement ambiophonics DSP, although tools for use on PCs are also available online. From a practical point of view, there are some non-obvious advantages of partitioned convolution. First of all, as the size of the FFTs exceeds the cache size of the processor, the performance degradation is larger than theoretically expected.
Furthermore, as the number of partitions go up, the major part of the computational load is moved from the FFT calculations, to the simple multiply and add step, which is very easily optimized.
On modern processors this translates into a hand-coded SIMD assembler loop of a few lines of code. The SIMD instruction set allows for executing several arithmetic operations in a single instruction, making the resulting code very efcient. To increase the performance further, this critical loop is extended with cache preload instructions, which improves memory access performance.
The task of optimizing the FFT algorithm is much more complex, and therefore the available implementations seldom make the most out of the given hardware. We use the open-source BruteFIR convolver , which was originally designed with Ambiophonics in mind, but has become a general-purpose audio convolver.
In this case, it is congured for 10 channel Ambiophonics, meaning 2 inputs and 10 outputs, with 2 IRs per output, a total of 20 independent ones having the lengths indicated. The sampling rate is 48 kHz and the internal resolution is 32 bit oating point. Table 1 and 2 show how large part of the processor time available is needed for achieving realtime operation.
Thus, a lower value is better, and a value larger than 1. The performance benchmarks clearly show that the partitioned convolution algorithm actually outperforms unpartitioned convolution in all measured cases, if a suitable number of partitions is chosen. Considering the added benets of reduced latency and more exible IR lengths, partitioned convolution should always be preferred.
Since Ambiophonics uses many more lters than there are inputs and outputs, unpartitioned convolution has an advantage over partitioned. Thus, in cases where there is one lter per input and output even larger performance gains by using partitioned convolution should be expected which actually can be seen in table 4.
However, in the rare cases where an optimal FFT algorithm is available, unpartitioned overlap-and-save will out-perform partitioned convolution in terms of throughput, as observed from informal tests made with Lopez software . Lopez uses a heavily optimized proprietary library from Intel, which unfortunately is both closed-source and limited to the Microsoft Windows platform, and is therefore not employed in BruteFIR.
IR length 32, 65, , unpartitioned 0.
For centrally located mono sound sources, two almost equally loud acoustic signals reach each ear, instead of one as in the concert hall but one of these signals, in the normal stereo listening setup, travels about half a head's width or usec. This produces multiple peaks and nulls in the frequency response at each ear from Hz up known as comb filtering. Since the nulls are narrow, and are muddied by even later crosstalk coming around the back or over the top of the head, and since the other ear is also getting a similar but not precisely, identical set of peaks and nulls, the ear seldom perceives this comb filtering as a change in timbre; but it can and does perceive these gratuitous dips and peaks as a kind of second, but foreign, pinna function and this causes confusion in the brains mechanism for locating musical transients.
Remember, in real halls the ear can hear a one degree shift in angular position, but not if strong comb-filter effects occur in the same kHz region where the ear is most sensitive to its own intrapinna convolution effects and interpinna intensity differences. As long as this wrongful interaural crosstalk is allowed to persist, the sound stage will never be as natural or as tactile as it could be and for some people, such listening is fatiguing after awhile and all 60 or LRC degree stereo reproduction sounds canned to them.
Pinna-Sensitive Front Speaker Positioning 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 ephemeral 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. Thus the direction from which complex sounds with energy over Hz originate, particularly from the frontal stage, should be as close to correct as possible.
Most stage sounds, particularly soloists and small ensembles, originate in the center twenty degrees or so. Remember that we want to launch sounds as much as possible from the directions they originate. Thus it makes much more sense to move the front channel speakers to where the angle between each of them to the listening position is perhaps ten degrees. This eliminates the pinna processing error for the bulk of the stage.
But, of course, if the speakers are so close together, what happens to the separation? The answer is that with the crosstalk eliminated, as is necessary anyway, separation, as in earphone binaural, is no longer dependent on angular speaker spacing. The Ambiodipole Crosstalk elimination is not a concept new to just Ambiophonics; but most of the older electronic crosstalk elimination circuits such as those of Lexicon, Carver, Polk etc. I would hesitate to class any of them as high-end components, especially as they still promote pinna position errors.
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 Carver, Lexicon, etc. 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.
Ambiodipoles 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 a high-end tweak that needs no cables and produces no grunge. Either electronic 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 needs to be directly behind the other for optimum results, not unlike high-resolution stereo.
Earlier crosstalk cancellation systems were less than satisfactory because they were not recursive. That is, in the earlier systems the unwanted signal from the left speaker at the right ear was cancelled by an inverted signal from the right speaker and that was the end of it; but this right speaker cancellation also reaches the left ear ansd so one has a new form of crosstalk.
In Ambiophonics, this later crosstalk is cancelled over and over again until its level is inaudible. A comparison can be made with reverberation time in concert halls. Normally, the reverberation time is specified as the time it takes for a sound to decrease by 60 decibels. This implies that the human ear is sensitive to concert hall reverberation at this low a level. Likewise, crosstalk is still deleterious even if its level gets to be quite low after several cycles of successive cancellation.
One can have video with crosstalk cancellation XTC but adding a picture can have its misleading 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. Nothing we have done to make the front stage image more realistic and psychoacoustically correct has required any extra recorded channels. I call all these changes to standard stereophonics, Ambiophonics or Ambio.
Ambio, does not rely on the fluky phantom image mechanism. But there still remains one further difficulty with the stereo triangle and that is that we need a proper ambient field coming from more directions than just those of our now crosstalk-free, pinna-correct, front speakers.
The Case For Ambience By Reconstruction Like a 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 use of ambience reconstruction would obviate the need for DTS or Dolby Digital multi-channel recordings at least where classical music is concerned. Unlike frontal sound, ambience can and should come from as many speakers as one can afford or has room for.
Crosstalk, and comb-filtering are not problems with ambient sound sources if these signals are uncorrelated unrelated closely in time, amplitude, frequency response, duration, etc.
An Uphill Political Struggle 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 "Ideally, one would like a surround-sound system yes, he did use this term in 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 , channels and loudspeakers.
These would occupy 8 gHz of bandwidth equivalent to the space used by , line television channels! Indeed the complete impulse response of a hall can be measured and stored on one floppy disk by placing an orthogonal array of three microphone pairs at the best seat in the house and launching a test signal from the stage during the recording session or at any time.
Convolution to The Rescue 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.
How many channels of late reverberation do we absolutely need to generate and where should these speakers be placed? There may never be a definitive answer to the first question.
Replacing Stereophonics to Achieve Concert-Hall Realism
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 10 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 a simple mouse click will yield a selection. With enough hall impulse responses stored, you could even select a seat and a stage width.
If it's a solo recital one wants only central derived early reflections, if a symphony orchestra, the works, etc. There are already over impulse responses of concert halls available on the Internet.
While I may not be the best one at executing my own theories, I have gotten startlingly good results using the new 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. 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 2 large area speakers at the sides and rear to provide a reverberation field as diffuse as possible.
Since central early proscenium reflections come from the recording via the main front speakers, these need not be regenerated and, of course, by definition they are natural and are coming from the proper directions.
For side, overhead, and rear ambience using the left channel to recreate left leaning early reflections some of which may end up coming from the right and the right channel to produce a set of right reflections, the early reflection patterns for different instruments on the stage have enough diversity to exceed the threshold of the brain's reality barrier.
Whither Recording In An Ambiophonic Hi-End World While audiophiles do not often concern themselves with recording techniques over which they have little control almost any LP or CD made with either coincident or 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 parts of the hall ambience will be synthesized, that there is no crosstalk, that the front loudspeakers are relatively close together, and that thus 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. The Ambiophone is a microphone array specifically designed to make recordings optimized for Ambio playback. The reason such a microphone is optimum is that particularly for central sounds the sound rays reach the ears almost as they do in the concert hall.
That is, one ray from a central instrument reaches the left ear of the microphone, goes to the left speaker where it is sent straight ahead to the left pinna and ear. The fact that the head response transfer function of the microphone is not the same as the listener's is not significant for central sound sources that don't cross either head.
For side sources the microphone ball becomes a substitute for the listener's HRTF but at least there is still only one HRTF and one real pinna in the chain. Perhaps the hardest part of migrating to Ambio will be to convince recording engineers, who are usually rugged individualists, to use microphones and positions that are Ambio compatible.
Law of the First Impression No matter how many great stereo systems I listen to, they still never have the impact that my first Emory Cook stereo disc had. The moral of this phenomena is that the first time anyone hears a major upgrade in reproduction, particularly when going beyond two speakers for the first time, they are always very favorably impressed.
Dissatisfaction with systems like the Hafler arrangement, SQ, Dolby pro-logic etc only set in later.
This is the scenario with the new discrete multi-channel format for music as well. At first 5. Chapter 1 Ambiophonics, considering music rather than video for the moment, is the logical replacement for stereophonics and a technical methodology which, if adhered to closely, makes it possible to immerse oneself in an exceedingly real acoustic space, sharing it with the music performers on the stage in front of you.
Ambiophonics can do this, even using ordinary standard and existing two channel recordings. We will show in the chapters that follow that, as hard as this may be to believe, there is nothing to be gained as far as realism in acoustic music reproduction is concerned by using more than two recorded channels as opposed to multi-speaker and that the complex microphone arrangements that multichannel recording implies are actually deleterious and wasteful of bandwidth that could be put to better use.
Ambiophonics is like a visit to a concert hall and is for serious listeners who do not often read, talk, eat, knit, or sleep in their home concert halls, any more than they would at a live performance. Ever since when Clement Ader ran signals from ten spaced pairs of telephone carbon microphones clustered on the stage of the Paris Opera via phone lines to single telephone receivers in the Palace of Industry that were listened to in pairs, practitioners of the recording arts have been striving to reproduce a musical event taking place at one location and time at another location and time with as little loss in realism as possible.
While judgments as to what sounds real and what doesn't may vary from individual to individual, and there are even some who hold that realism is not the proper concern of audiophiles, such views of our hearing life should not be allowed to slow technical advances in the art of realistic auralization that listeners may then embrace or disdain as they please.
What is Realism in Sound Reproduction? Realism in staged music sound reproduction will usually be 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 in real life.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.
Many also believe that audiophile-grade equipment, properly selected and tweaked, combined with signal path minimalism is more likely than simple acoustic listening room treatments to produce a higher fidelity sound field with enhanced width and depth. There is a perceivable hole in the stereo center The sound is bound by the width of the speaker placemen and you can identify where the speakers are as you listen.
Ambiophiles everywhere can take comfort in the fact that it is both theoretically possible, possible in practice, and reasonable in cost to achieve the formerly impossible dream of recreating a "you-are-there" soundfield from standard unencoded LPs, CDs, MP3s, or DVDs in virtually any properly treated room at home. In Ambiophonic parlance, when we say "real" we mean that an acoustic space of appropriate size and stage width has been created that is realistic enough to fool the human ear-brain system into believing that it is within that space with the performers on stage clearly delineated in front.
Kirkeby, P. One can also argue that a home version of Symphony Hall, where one is apparently sitting tenth row center, is more involving that the live experience heard from a rear side seat in the balcony with obstructed visual and sonic prospect.
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