Electrical audio signal processing systems and devices – Binaural and stereophonic – Stereo speaker arrangement
Reexamination Certificate
1999-11-12
2003-10-14
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Stereo speaker arrangement
C381S099000
Reexamination Certificate
active
06633648
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the fields of audio signal reproduction and audio signal processing, and more particularly to a system for increasing the area over which a satisfactory audio illusion is created and maintained, relative to prior art audio reproduction systems. The method may employ a multi-way loudspeaker pair with drivers operating over diverse frequency ranges arrayed generally in a horizontal dimension (for a normally-oriented head of a listener) and with higher-frequency drivers generally closer together and displaced more towards the center of the listening space than lower-frequency drivers, and specially-adapted signal processing components for audio imaging to create or maintain desirable audio imaging.
BACKGROUND OF THE INVENTION
The history of stereophonic sound (“stereophony” or, more commonly and colloquially, “stereo”) includes a number of methods of recording sounds and another number of methods for playing those recorded signals back to a listener or listeners. While it has always been an accepted idea that a listener should be “transported” to another acoustical space, such as the acoustic space occupied by an audience member at a live concert or a more synthetic, more conceptual, space for many modern popular recordings in which there was no actual performance in front of a live audience, the methods used for this “transporting” have largely failed in that goal. A reason for the failure has been that no systematic, rigorous method was usually applied in designing the various systems, designers and recording personnel frequently instead relying primarily upon largely unscientific principles and serendipity to achieve their goals. That billions of commercial recordings have been sold and broadcast is more a statement of the appeal of the content of the recordings than the ability to transport the listener into another space. However, workers such as Schroeder and Atal, and Cooper and Bauck, have devised playback systems which employ signal processing methods which are firmly footed in engineering science and based on the concept that particular signals will be placed in and around the ears of one or more listeners so that it becomes the task of the producer of the program material to provide a version of the “desired” signals.
These latter-day methods, though currently far in the minority of systems and recordings purchased by consumers to date, can be extraordinarily effective in transporting the listener to another, believable, acoustic space, when properly designed. Perhaps an indication of the failure of traditional systems to perform as hoped, and of the success of the latter-day systems, is that the newer systems are often called “3D audio,” “3D sound,” and the like (“3D” meaning three-dimensional), and the vernacular use of “stereophonic” often refers to the earlier systems. A simple translation of “stereophonic” from its Greek-root components means “of or relating to three-dimensional sound.” Thus, with the advent of practical implementations of the latter-day systems, the audio community found it necessary to coin a new phrase, thus “3D audio” and the like.
In keeping with current usage, we will use the current term, 3D audio, to refer to the latter-day systems. These systems typically employ some kind of circuitry or algorithm which compensates for the fact that sound emanating from each of two loudspeakers impinges on both ears of a listener, so that, for example, sound radiating from a left-placed loudspeaker of a pair of loudspeakers travels to the left ear of a listener, but also travels to the right ear of a listener, this latter sound being called crosstalk. The transmission from each loudspeaker to each ear can be anticipated by designing the circuitry or algorithm, from knowledge of so-called head-related transfer functions (HRTFs), so that when the circuit or algorithm, taken together with at least two loudspeakers, all as a unit, can separately and distinctly control the sounds at the ears of one or more listeners. It is also possible to correct for frequency response aberrations caused by the diffraction of the listener's head so that a natural timbre is perceived by the listener.
It is known in the art, especially in the patents of Cooper and Bauck, that improved performance can be achieved by deliberately modifying the filters comprising the crosstalk cancelling circuitry or algorithms or related circuitry or algorithms from their strict specifications from HRTFs. For example, it may be necessary in some cases to modify the filters in such a way as to make them stable or otherwise realizable. Other modifications are known in the art, such as using HRTFs measured from a model mannequin head rather than the listener's own head, the use of minimum phase transfer functions, the use of simplified head models such as smoothed HRTFS, spheres, or two points in free space (for ears), and the use of delays to convert noncausal filters into causal filters. Some deviations from the full HRTF specifications may be quite extreme, for instance, following the HRTF specification up to only some 600 Hz and allowing factors other than the most precise imaging to specify the response above 600 Hz. Any such modification, while deviating from the strict specification of the listener's own HRTFs, may be considered to be advantageous, either for the sake of performance or economies or both. Also, such modifications may result in less than perfect cancellation of crosstalk and/or less than perfect correction of timbre. Nonetheless, we will refer to all such devices as crosstalk cancellers. Crosstalk cancellers are the heart of most 3D audio systems, allowing predetermined control of signals at the ears of the listener or listeners, thus removing many elements of luck from the playback experience. It is therefore an object of the invention that any crosstalk canceller with any of the several described modifications or other modifications may be used either explicitly or implicitly as the imaging component of the invention.
One application of crosstalk cancellation is in playing back recordings made with an acoustical mannequin, a dummy head with microphones placed in its ear canals or thereabouts. Such a recording-playback system results in the most realistic impression of being transported to another space.
Another application of crosstalk cancellers is as part of an imaging circuit or algorithm, a so-called speaker-spreader or layout reformatter such as described by Schroeder and Atal, and Cooper and Bauck. In this application, the listener can receive the impression that, for example, a pair of loudspeakers which is placed on the sides of a television receiver cabinet, much too close for perceiving any readily noticeable amount of stage width, appear to be farther apart, with well-defined sounds apparently emanating from points in space where there are no actual loudspeakers, a “virtual loudspeaker” impression. In this application, it is most common for the input signals to be any kind of ordinary stereo; the input signals may also be provided by a home theater or multichannel television audio decoder, providing five or more channels of audio signals.
Still another application of the principle of crosstalk cancellation is in the creation of interactively-controlled sound sources (and their reflections in an acoustic environment, if desired) such as would exist in computer-based or game-console-based games, when the sounds for those games are presented to the player or players over loudspeakers.
So it is seen that a crosstalk canceller is a basic component of controlling signals at the ears of a listener, usable with either binaurally recorded programs or with any kind of traditional stereo programs, for the general enhancement thereof.
Playback systems which do not effectively use a crosstalk canceller are also sometimes known as 3D. Such systems can create the impression that sound is arriving from points in space where there are no actual loudspeakers, but rather than provide the impression that there
Isen Forester W.
Pendleton Brian Tyrone
Welsh & Katz Ltd.
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