12-09-2014, 02:09 PM
Ambiophonic Principles for the Recording and
Reproduction of Surround Sound for Music
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ABSTRACT
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.
INTRODUCTION
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, cross-talk cancellation is usually required, for canceling the spurious signals that go to the “wrong” ear.
THE AMBIOPHONICS METHOD
The goal of the Ambiophonics reproduction method is to create a realistic listening experience starting from existing 2-channel or even 5.1 recordings. Fortunately, the recordings themselves are not usually predistorted by the stereo reproduction process. That is, the recordings do not contain crosstalk and do not know that they will be listened to via a stereo speaker triangle that engenders crosstalk, requires phantom imaging rather than binaural localization, generates comb filtering and introduces pinna/HRTF angle errors. Normal recordings typically include very limited “3D surround” information. Of course, the missing information must be recreated in some way: this is done by means of convolution with suitable room impulse responses.
The method can be basically explained as the superposition of two simultaneous periphonic reproduction systems: cross-talk cancelled reproduction over a pair of closely-spaced loudspeakers (as is usually done for binaural loudspeaker reproduction), and approximate wavefront reconstruction with an Ambisonics array, being fed with reconstructed hall ambience signals derived from the left and right direct sound disc channels convolved with a set of weakly-correlated real hall impulse responses.
CONCLUSION
This paper presented the theoretical background and the practical implementation of the Ambiophonics surround system. The system can be seen as the superposition of two already established surround techniques (Stereo Dipole and Ambisonics): each of them is employed for what it does best.
The superposition of the two cooperating systems produces significant advantages, which are clearly outlined by the theoretical analysis, and were confirmed by listening tests.
Although Ambiophonics until now has been a quite expensive system suited only for audiophiles in the high-end, its modern implementation in the form of freeware software for low-cost PCs opens its use to the vast majority of music lovers, provided that they take care of allocating a suitable, well treated listening environment.