|The Science of Domestic Concert Hall Design|
by Ralph Glasgal
Ambiophonic Principles for the Recording and Reproduction of Surround Sound for Music
Angelo Farina (1), Ralph Glasgal (2), Enrico Armelloni (1), Anders Torger (1)
(1) Industrial Eng.
Dept., Universitř di Parma, Via delle Scienze ˝ 43100 Parma - ITALY
Email: email@example.com Web:
This paper discusses the psychoacoustical background and the computational issues involved in the real-time implementation of a complete Ambiophonics reproduction system based on binaural technology. Ambiophonics, which requires only two media channels, evolved from previously known approaches such as the reproduction of binaural recordings over closely spaced loudspeakers through cross-talk cancellation, and the reconstruction of hall ambience by convolution from suitable impulse responses. The equations for the design of the digital filter coefficients are derived with regard to the many possible kinds of pre-existing recordings (binaural, sphere, ORTF, M/S), and their implementation on available hardware and software platforms are described. The authors suggest psychoacoustic explanations for the perceived audible performance, and describe the first results of a comparative listening test, evaluating the realism of three periphonic surround reproduction systems: Stereo Dipole, Ambisonics and Ambiophonics.
In recent years many different surround reproduction systems have been developed. Many of them, such as 5.1, are spatially limited and are not considered to be psychoacoustically valid methods for achieving a realistic reproduction of recorded music. Other paradigms that are capable, in principle, of complete periphony (reproduction of apparent acoustic sources everywhere in the space, over a complete sphere around the listener) have been proposed, but none of them has gained acceptance or become commercially available on the market, in part, because they were not compatible with the vast existing library of two channel LPs and CDs. In this paper only potentially complete periphonic systems are considered, with the goal of providing the mathematical description and the implementation details of one of these methods, termed Ambiophonics; it will be shown that this method, used only in reproduction or used in both recording and reproduction, makes use of physical principles and digital filtering techniques, which are also found separately in other periphonic surround methods, but are here coupled together in a consistent and psychoacoustically correct form.
Most periphonic methods fall in one of two broad categories, summarized here:
A) Binaural methods: the sound field is originally recorded with some sort of dummy head microphone, and reproduced by delivering the recorded signals unaltered to the entrances of the ear canals of the listener. B) Wavefield reconstruction methods: the system replicates the wavefronts, impinging on an array of microphones in the original space, through the use of coarse or dense arrays of loudspeakers during reproduction in a different space. The binaural methods have the advantage of requiring the recording and transmission of just two channels, and are thus compatible with traditional two-channel stereophony, a form of monodimensional reproduction (virtual sound sources located on a line). Possible methods of replicating the recorded acoustic pressure signals at the ears of the listener include headphone reproduction (with or without head tracking) and loudspeaker reproduction. In the latter case, crosstalk cancellation is usually required, for canceling the spurious signals that go to the "wrong" ear.
The common binaural methods are very sensitive to both the shape and directional characteristics of the original microphone employed for the recording and of the particular human head of the listener . If these two characteristics do not match well, the spatial illusion is poor and the reproduced sound field is judged unnatural. Furthermore, localization errors are common, particularly for sound sources near the median plane (front-back confusion, height uncertainty). High-end audiophile-level results can only be obtained when the recording mount is the actual human head of the listener, equipped with wearable binaural microphones. Thus a pure binaural technique is limited to the amateur recording of live music, and is obviously not applicable to the reproduction of the large catalogue of already existing recordings often made using a so-called stereo microphone which is usually inherently unsuitable for binaural reproduction especially via earphones. The wavefield reconstruction methods usually require many more than two channels for transmitting the spatial information: the most basic method, 1st-order Ambisonics , requires four channels, carrying the pressure signal and the three particle velocity Cartesian components, all recorded at a single point in space. Although an Ambisonics system can employ a much larger number of loudspeakers for reproduction, the wavefront reconstruction capabilities are somewhat limited, and the localization of sound sources is not robust since a 1storder system only samples the spherical space with 1st-order spherical harmonics .
Much more accurate wavefront reconstruction methods have been proposed: 2nd order Ambisonics requires the recording and transmission of 9 channels, and is thus already impractical. The Wave Field Synthesis method  goes up to some hundreds of channels, which prevent its application for recorded music distribution, being applicable primarily in the real-time recreation of performing spaces. These methods are substantially incompatible with mainstream sound recording and delivery, which nowadays is almost completely done in two-channel stereo.
The 5.1 discrete surround sound system, primarily intended for movies, is not periphonic (no height information). The addition of a center speaker does not materially alter its nature as a monodimensional system that relies on phantom imaging. The 5.1 reproduction arrangement also does little to enhance the realistic reproduction of existing two-channel material on LP or CD.