|The Science of Domestic Concert Hall Design|
by Ralph Glasgal
Ambiophonics, 2nd Edition
Replacing Stereophonics to Achieve Concert-Hall Realism
By Ralph Glasgal, Founder Ambiophonics Institute, Rockleigh, New Jersey www.ambiophonics.org
Surround Ambiophonic Recording and Reproduction
Note: Chapter 9 is in the form of an Audio Engineering Society paper. It may reiterate some of the points made in early chapters, but in a different context.
Ambiophonics, Panorambiophonics, and Periambiophonics are related surround sound paradigms that reliably deliver up to full 360-degree spherical localization for both direct and ambient sound via two, four, or six DVD/SACD/MLP/DTS/Dolby/ADAT/etc. coding/media channels. They reproduce old or new, standard, 2, 4, 6, or ITU 5.1-channel music discs with unprecedented spatial realism and binaural-like localization accuracy via direct sound radiating front/rear/overhead stage-producing Ambiodipoles and virtually any desired number of ambience surround speakers. Alternatively, superior acoustic recordings can be made using the described Ambiophones (or using convolvers, if fabricated) to capture images of startling depth and presence for music in the round, 3D movie sound tracks, virtual reality, or electronic music soundscapes. Six-channel Periambiophonics adds elevated direct sound to the fully spherical hall ambience vectors already provided by basic Ambiophonics which drives essentially any number of hall ambience speakers regardless of their positions. All the versions of surround Ambiophonics easily deliver a you-are-there, psychoacoustically correct, home listening experience, via home theater media, albeit best limited to just a few listeners.
Ambiophonics is a comprehensive sound recording/reproduction methodology, that like or unlike Stereophonics, Ambisonics, THX 5.1, or Wavefield Synthesis, prescribes hardware/software that scrupulously insures that the well known tenets of human binaural hearing (see Appendix B) are rigorously catered to so as to achieve psychoacoustic and physiological verisimilitude for a normal group of home listeners/viewers who seek and value "you-are-there" realism. Ambiophonics combines crosstalk-free speaker pairs (Ambiodipoles), surround speaker ambience derived from measured hall impulse responses via a convolver (Ambiovolver) and room/speaker correction/treatment to generate a binaurally correct sound field similar to wavefield synthesis. Ambiophonics creates a concert hall stage and hall from just two media channels as found on CDs, MP3s, LPs, etc. feeding a single Ambiodipole. Panorambiophonics requires four media channels as provided by multichannel DVDs or SACDs each pair feeding its own Ambiodipole. Periambiophonics uses six media channels as in DVD-A, DTS-EX, etc. feeding three Ambiodipoles. In each type of system additional hall ambience surround speakers may also be driven via a single Ambiovolver and this is strongly recommended where music is concerned.
A single Ambiodipole in front easily produces a stage of more than 160-degrees in width. A single Ambiodipole to the rear of the listener produces a similar rear stage width. A remarkable property of the Ambiodipole software (RACE) we have developed is that when both front and rear Ambiodipoles are working together, they blend and the front and rear stages widen to a full 180-degrees. Thus, 360-degrees of horizontal localization becomes easily attainable for recordings made with Ambiophones or synthesized. A third or even more Ambiodipoles can be elevated over the front and/or rear Ambiodipoles to add full width stages high in the air and again there is vertical fill between the stages although the extent of this phenomenon has yet to be fully investigated. The most basic Ambiophonic system is meant to allow previously recorded two channel media such as CDs, MP3s, and LPs to be reproduced without the well known limitations of the traditional 60-degree stereo triangle (see Appendix B), to deliver an uncompromised full width direct sound stage from two center-front speakers (an Ambiodipole) and to provide real diffuse but still directional hall ambience to almost any number or location of surround speakers including elevated speakers.
It became obvious in the early development of Ambiophonics that existing stereo microphone techniques could be revised to produce better two channel recordings. Thus, on the recording side, the Ambiophone, a novel, baffled microphone arrangement, takes advantage, when recording, of the knowledge that the playback will be Ambiophonic. (not via the stereo equilateral triangle although Ambiophone recordings are actually backward compatible and sound quite normal in standard stereo) The Ambiophone also assumes that both the amplitude and the directional attributes of the early reflections and reverberant tails of the hall will be properly directed to the appropriate frontal Ambiodipole and surround speakers. (Indeed, this is possible even in the case of non-Ambiophone recordings if the recorded or added reverb, unfortunately mixed into the direct frontal sound, is not too intrusive.)
After a brief review of the basics, this paper is devoted to advanced versions of Ambiophonics which take into account the 5.1, 6.0, 7.1, Dolby/THX and DTS coding/media/speaker arrangements. Standard 5.1 discs may also be played Panorambiophonically (described in detail below) in a manner analogous to the Ambiophonic playback of ostensibly stereo CDs or LPs, but, in this case, using front and rear Ambiodipoles and surround ambience speakers driven by a hall impulse response Ambiovolver. Most 5.1 movie and music DVDs or music SACDs reproduce exceptionally well this way especially when compared with the ITU 5.1 standard speaker arrangement. Panorambiophonics, described below, uses four channel coding/media such as Dolby, DTS, SACD, or DVD-A to deliver an easily localizable 360-degree direct sound stage as in movies, or, for concerts, a very wide front stage that, if in a hall, automatically includes horizontal 360 degree hall ambience. A four channel recording mic, the Panorambiophone, has been designed to make such recordings. Only four speakers (two Ambiodipoles) are used in Panorambiophonic reproduction to reproduce all horizontal plane direct sound and horizontal hall ambience with full circle normal binaural physiology localization. Where the direct sound recording has been made in a dry or small studio, it is possible to enhance the reproduction of these front and rear direct sound fields by adding ambience surround speakers driven by an hall Ambiovolver as in standard Ambiophonics.
Periambiophonics adds a third elevated Ambiopole to Panorambiophonics to provide for a full direct sound stage in all dimensions including some height. The elevated Ambiodipole can be used for direct sound reproduction or ambience. In the latter case this allows a concert-hall direct sound performances to be recreated in a home with just three speaker pairs and no surrounds. Using three direct sound Ambiodipoles allows movies, virtual reality, games and soundscapes to sound more like the live experience. Furthermore, Periambiophonics can combine six-channel Periambiophone recording, and the front, rear, and elevated Ambiodipoles, with an Ambiovolver to add virtually any desired number of surround speakers so as to deliver physiological verisimilitude of a concert hall experience that also includes rear or overhead direct sound sources to a home listener via standard DVD/SACD media. Clearly, both Panorambiophonics and Periambiophonics are well suited to capture, create and reproduce 3D electronic music or virtual reality projects. This paper reviews the theory, techniques, and features, of the hardware and software required to make these various kinds of Pan/Peri/Ambiophonic recordings and to reproduce these as well as stereo CDs and the various multichannel surround media.
Review of Basic Ambiophonics (Fig. 1)
The simplest form of Ambiophonics is meant for the playback of ordinary stereo CDs, LPs, SACDs, MP3s, cassettes, stereo TV, etc. In stereo, the front stage is created between the speakers, in Ambiophonics the stage is created from the speakers outward and so can be much wider. The Ambiodipole speaker pair form an angle to the listener of from twenty to thirty degrees. Listeners can sit anywhere along the line between the speakers and can stand or recline, turns their heads, lean etc.
The scattered surround speakers are fed hall ambience signals calculated for both the left and right channels by a computer which we call an Ambiovolver. The Ambiovolver has stored within it the impulse responses of some of the great halls, churches, and auditoriums of the world and more such hall signatures are being accumulated all the time. One simply selects the hall best suited to the recording or the actual hall where the recording was made. The Ambiovolver is told the location of all the surround speakers in the room and it then generates the appropriate reflections and feeds them to a surround speaker that can then mimic a concert hall wall. In this way the levels, frequency responses, and the directionalities of the reverberant field are maintained. I have driven up to 24 surround speakers this way and, while clearly overkill, the results are gratifying. This is in contrast to normal 5.1 practice where recorded hall ambience whether from front, rear, overhead or the side is lumped together and launched from just two surround speakers. The attached references describe Ambiovolver design, hall impulse response measuring procedures and hall acoustic properties. It is often desirable to keep the listening room early reflection characteristics under control. Absorptive panels are quite effective. However, since the direct sound speakers are so close together and aimed forward, they are easier to position than for stereo or 5.1. Bad room acoustics are actually less of an issue in Ambiophonics than in stereo. The early reflections and the late reverberant generated by an Ambiovolver normally swamp the listening room acoustics.
Two Channel Ambiophonic Recording
While for many people, with large CD or LP collections, basic Ambiophonics will sound as good as they wish, others will find enjoyment in the improvement that can be achieved by making recordings specifically meant to be played back over Ambiophonic systems. The Ambiophone recording microphone assembly was designed to make this feasible. Basically it is a head shaped ball with two omnidirectional microphones mounted flush where the ear canals would be. The microphone is baffled. That is, it faces forward and is shielded from sound originating from overhead, the rear, or the extreme sides.
Figure 2. An Ambiophone. Two more mics are in
the head behind the baffle to pick up the rear stage.
The microphone is placed first to fifth row center depending on taste. The perspective one hears during reproduction is the same as if one were at the mic position during the recording session. The usual considerations of hall radius or ratios of direct to reverberant sound do not apply here since the mic is baffled. Since all hall ambience will be generated from this or other hall impulse responses, it is not necessary to actually record hall reverb during the recording session. The Ambiophone must also collect horizontal frontal or proscenium ambience since this indirect sound should emerge from the Ambiodipole with the direct stage sound. The head shape of the Ambiophone provides the Interaural Level Difference for sounds from the stage sides. The Ambiodipole, being centered in front of the home listener, does not provide this and the rule is there must be a head shadow in the system somewhere. The Ambiophone captures both correct ILD and ITD compared to coincident microphone techniques, spaced omnis, spot mic mixing, etc. The Schoeps KFM-6 turns out to be a good match for an Ambiophone, if baffled during use.
Figure 3. Two Channel Ambiophonic System
Where classical music, reproduced in the home environment is concerned, two channel Ambiophonic recording and reproduction should satisfy even the most golden-eared audiophiles. Ambiophonics appears to entirely swamp the digital sampling differences between 2 channel media such as CD, SACD and DVD-A. It would be an interesting double blind project to see if the different media can actually be distinguished when Ambiophonic conditions prevail during both recording and playback.
Ambio Playback of 5.1 Discs (Fig. 4)
Home theater surround movies or music recordings can be played back Ambiophonically rather than stereophonically in a manner analogous to the playback of CDs and LPs, discussed above. The psychoacoustic disadvantages of the LCR reproduction scheme are reviewed in Appendix B. The left and right frontal 5.1 channels can be fed directly to a crosstalk canceller and thence to an Ambiodipole. An Ambiodipole also functions as a center speaker it is an easy matter to tell the player to mix the center channel signal into the left and right frontal pair. In Ambiophonics, a center speaker is never required and is even detrimental. Certainly, it is easier to set up the front part of a home theater system using just two center speakers 20 or 30 degrees apart so as not to stand in front of the TV screen than setting up three speakers that must be equidistant and spaced symmetrically. Also, for home TV, viewers like to be centered so the major supposed advantage of the LCR arrangement seems of limited value.
Figure 4. Ambiophonic Playback of 5.1 Material
The two rear surround channels go to a second crosstalk canceller and a rear Ambiodipole. For many movies this arrangement produces a rear stage with excellent localization up to 180 degrees in the rear. Of course, this applies only to movies that were recorded in stereo in the rear, not just dual mono or fabricated sound effects, or ambience. Music DVDs often include real hall ambience captured during the performance and again, if not mono, can provide an ambient field spread across the rear. While not ideal, since ceiling rear and frontal ambience comes from this horizontal rear arc, this effect is better than the standard ITU plus/minus 110-degree arrangement whose properties are discussed in the Appendix. Better yet, for much 5.1 music, where there are no instruments or vocals in the rear, the rear surround channels can well be ignored and the more natural ambience generated by the Ambiovolver can be used in its placed now free from the constraints of the ITU surround speaker position mandate. It is also possible to use both the Ambiovolver and the rear Ambiodipole simultaneously. For those who want applause coming from the rear, this arrangement works well for both live music and movie sound effects.
If a four channel medium such as SACD, DVD, or DTS-CD is available, then it is possible to record direct sound sources over 360-degrees in the horizontal plane. A special microphone assembly which we have called a Panorambiophone (or Panambiophone and Panambio for short) is used to capture signals appropriate for reproduction via one front Ambiodipole and one rear Ambiodipole. Front and rear Ambiodipoles do merge seamlessly. We have already demonstrated to hundreds of visitors that the combination of the Panambiophone and the two Ambiodipoles does indeed allow normal binaural localization over the full circle including the 90-degree positions at the extreme sides.
Figure 5. Panorambiophonics Delivers Full Circle Localization via 4 Channels and Speakers
The Panambiophone consists of two Ambiophones placed one behind the other but still both facing in the forward direction. The two head shaped balls must be placed nose to baffle to nape since if there is too much separation the differential delay of slightly off 90-degree direct sound sources will cause comb filtering when reproduced. A baffle between the two Ambiophones insures that the front stage is picked up mainly by the front half head and the rear stage is mainly heard by the rear half head. For concert music both half heads should be protected from overhead reflections. The Panambiophone, like the Ambiophone is placed at the best seat in the house or at the center of the sonic action. In reproduction, two crosstalk cancellers feed two Ambiodipoles and listeners should sit, stand, or recline on the line between them for best effect. Off-line seating still yields interesting front/back localization but the exact angles are unpredictable. During symphonic recording, the rear Ambiophone picks up the rear half horizontal hall ambience while the front Ambiophone automatically captures the frontal half of direct and ambient sound. Thus, one can have a reasonable you-are-there 360 degree ambience sound experience with just four speakers. This methodology should be compared with the random difficulties encountered using other ambience pickup microphone arrangements such as IRT, Fukada, ORTF, Williams, Decca, etc.
If the Panambiophone is used in the concert hall during a live performance, the ambient field cannot be captured with precision because the overhead ambience is either mixed in with the horizontal components or excluded if the microphone is shielded from the ceiling. Likewise in circular direct sound recordings made in studios or on location, it is difficult to include a realistic ambient field as part of the four channel medium. Thus, there are good reasons to use an impulse response, an Ambiovolver, and surround speakers to enhance the performance of basic Panambio.
Figure 6. Panorambiophonics Including Both Circular Direct Sound and Hall Ambience
An example of this is Robin Miller's Panorambiophonic recording of an outdoor parade. The bands pass by in front and the crowd's shouts and claps come from behind and from the sides. But when you add impulse response derived reflections from the surrounding office buildings to the mix the scene becomes that much more vivid. Another example is Robin Miller's studio recording of country music with a boisterous audience present. The Ambiodipoles take care of all the direct, front and rear, tunes and shouts but the Ambiovolver transports the whole scene to Nashville.
Periambiophonics adds height to the Ambiophonic mix. This requires another pair of media channels but ADAT, DVD-A and DTS-ES are examples of commercially available systems capable of delivering sufficient data to create close to full upper hemisphere soundscapes. Another baffled Ambiophone head is used to capture an elevated front stage and this signal pair needs to be fed to a crosstalk canceller and an elevated Ambiodipole. If one has four pairs of media channels available, then the two front stages and the two rear stages can produce virtually anything desired. To date only frontal and rear elevated stage merging has been tested and Periambiophonics is still a work in progress. The real issue is whether there is a viable commercial home market for such a direct sound technology.
Figure 7. Periambiophonics Provides An Elevated Stage
Figure 7 also shows that convolved ambience can provide periphonic envelopment. As discussed above all the various Ambiophonic methods can employ the Ambiovolver to produce signals for surround speakers at any azimuth or elevation if the impulse response used has been taken in three dimensions.
Ambisonics + Ambiophonics
In the absence of a three ball Ambiophone or impulse response expertise, it is possible to make live recordings using a single Ambiophone facing forward to catch the stage and an Ambisonic WXYZ coincident microphone just behind it to only record hall ambience in all dimensions. The Ambisonic array must be baffled to prevent it from picking up frontal direct sound. Six media channels are also required to store this version of periphonic sound. Instead of an Ambiovolver an Ambisonic decoder is required to deliver the ambience to the surround speakers. Normally, Ambisonic surround speakers must be symmetrically arranged about the listening spot. However, when only ambient sound is being handled Ambisonically, the requirements are less stringent. Also the more speakers used the better the results.
Figure 8. Ambisonic Techniques Can Be Used to Provide Surround Ambience With
Since Ambisonic technology and some hardware and software has been available since the 70s, this route may be more attractive to researchers than the Ambiovolver approach. The advantage of the Ambiovolver however, is that hall ambience need not be recorded during the performance. There is also the complexity of the various Ambisonic four-capsule microphones and control units. Ambisonic techniques are also often used to capture hall impulse responses.
It took 25 years for stereophonics to seriously begin to replace monophonics. It is likely that a similar period will be required for Ambiophonics to replace or at least supplement stereophonics and its twin brother 5.1. But the development of digital signal processors and algorithms able to process digital audio in real time, without audible distortion or noise, has now made it feasible and practical for music/movie lovers to enjoy and recording engineers to deliver greater physiological verisimilitude in music and video recording. Recordings already made with the various varieties of Ambiophones can be demonstrated to all who are interested or doubting. Ambiophonics provides binaural realism and a normal stage perspective when reproduced via one or more Ambiopoles. Ambiovolver driven surround speakers easily provide surround ambience without requiring media bandwidth or recording session bother. Ambiophonic recordings should need no spot microphone support, panning algorithms, artificial reflections, or HRTF manipulation and consume just two media channels for classical music or four or six if rear and/or overhead sound stages are desired. Best of all, not only is Ambiophonic reproduction of existing CDs and LPs superior to stereo triangle reproduction but Ambiophonic surround reproduction of 5.1 DVDs and SACD is also psychoacoustically superior and easier to implement than the ITU 5.1 speaker arrangement. However, Ambiophonics is for domesticity and is not suitable for large group listening applications.
Major contributions to advancing this technology and making Ambiophonics a living reality have come from Angelo Farina, Anders Torger, Robin Miller, and Enrico Armelloni. They represent Italy, Sweden, and the USA. Their support has been unstinting.
 Ralph Glasgal.
Ambiophonics, 2nd Edition. www.ambiophonics.org