This patent, awarded to Polk Audio, describes a very circumstantial loudspeaker plan that claims to be able to accomplish better stereo sound reproduction with “a realistic ambient field and a larger, and more unchangeable acoustic image.” This evidently aims at achieving something that Polk Audio actually patented nearly 40 years ago, with its first Stereo Dimensional Array (SDA) loudspeaker, and for which the fresh updated patent now provides a fresh improved version. This article was originally published in Voice Coil, February 2019.
Head-Related Transfer Function Equalization and Transducer Aiming of Stereo Dimensional Array (SDA) Loudspeakers
Patent/Publication Number: US20180317034A1
Inventors: Scott Orth (Baltimore, MD); Stuart W. Lumsden (Baltimore, MD)
Assignee: Polk Audio, LLC (Vista, CA)
Filed: April 27, 2018
Current CPC Class: H04S 1/002 20130101
Granted/Published: November 1, 2018
Number of Claims: 15
Number of Drawings: 10
Abstract from Patent
An enhanced Stereo Dimensional Array (“SDA”) Loudspeaker strategy 250, preferably including a mirror image pair of loudspeaker enclosures 280L and 280R configurable by a user or installer as a left-channel SDA loudspeaker and a right channel SDA loudspeaker uses a fresh driver array aiming configuration and an enhanced Head Shadow filter SDA signal processing strategy and method to accomplish a amazingly effective psycho-acoustically expanded image breadth by inter-aural crosstalk cancellation. The signal processing strategy and method supply an enhanced method for cancellation of apparent sources of inter-aural crosstalk, as compared to the commonly owned Polk SDA (Prior art) method.
Independent Claims
1. A sound reproduction strategy having a left channel output and a right channel output, apparatus for reproducing sound having an expanded and more unchangeable acoustic field and acoustic image, comprising: (a) a first loudspeaker strategy enclosure or tower 280L disposed in a first loudspeaker strategy enclosure location spaced from a listening location, the listening location being a place in a space for accommodating a listener’s head having a right ear location and a left ear location spaced along an ear axis, said first loudspeaker strategy enclosure having a multi-faceted or multi-planar front baffle surface comprising a first front baffle surface or facet which is angled rearward to recede at a selected (e.g., 10° to 30°, preferably 15°) angle from a vertical plane aligned with the talker axis on the left side, and a second front baffle surface or facet which is angled rearward to recede at a selected (e.g., 15°) angle from a vertical plane aligned with the talker axis on the right side, where the first and second baffle surfaces specify loudspeaker driver supporting and aiming structures aligned along substantially vertical planes; (b) wherein the first baffle facet carries and aims a first midrange driver having a midrange driver acoustic center and a first tweeter driver having a tweeter driver acoustic center which is preferably substantially vertically aligned with said first midrange driver acoustic center; (c) wherein the second baffle facet carries and aims a second midrange driver and a second tweeter, wherein said second midrange driver has its acoustic center spaced laterally from said first midrange driver by a selected distance D.sub.W (e.g., about 6” to 6.5”), and wherein said second tweeter driver has a tweeter driver acoustic center which is preferably substantially vertically aligned with said second midrange driver acoustic center and spaced laterally from said first tweeter driver by said selected distance D.sub.W (e.g., about6” to 6.5”); (d) said first loudspeaker strategy enclosure or tower 280L having external terminals for Main (+) and (-) signal inputs, and an SDA signal input/output terminal; and (e) first enclosure signal processing circuitry including a crossover with input terminals for said Main (+) connection, said main (-) connection and said SDA In connection and said SDA Out connection, wherein said crossover is configured to make (i) a “main” tweeter signal (ii) a “main” midrange signal, (iii) a “Head Shadow Filter” compensated SDA dimensional effect tweeter signal, and a “Head Shadow Filter” compensated SDA dimensional effect midrange signal; (f) wherein said signal processing circuitry communicates said SDA dimensional effect tweeter signal and said SDA dimensional effect midrange signal to an SDA dimensional effect radiating array including said first tweeter and said first midrange which are aimed by said first front baffle distant from the listening position.
6. In a stereophonic sound reproduction strategy having a left channel output and a right channel output, an improved apparatus for reproducing sound having a realistic ambient field and a larger, more unchangeable acoustic image, comprising: a right main talker and a left main talker disposed respectively at right and left main talker locations spaced apart along a talker axis, with a listening location located mostly along a listening axis perpendicular to the talker axis and intersecting the talker axis at a point midway between the right and left main talker locations; means coupling the right and left channel outputs, respectively, to said right and left main speakers; a right sub-speaker positioned on the talker axis at a right sub-speaker location spaced a predetermined distance from the right main talker location and further from the listening axis than said right main talker location; a left sub-speaker positioned on the talker axis at a left sub-speaker location spaced a predetermined distance from the right main talker location and further from the listening axis than said left main talker location; means connected to the right and left channel outputs for developing a left channel minus right channel signal and a right channel minus left channel signal; means coupling said left channel minus right channel signal to said left sub-speaker and said right channel minus left channel signal to said right sub-speaker; whereby sound reproduced by said apparatus as perceived by a listener located mostly along the listening axis has a realistic acoustic field and enhanced acoustic image; the improvement comprising: said left main talker is aimed toward the listening position at a selected main driver aiming angle from a line parallel to said listening axis, said selected main driver aiming angle being between 10° and 30° and wherein said left sub talker is aimed distant from the listening position at a selected sub driver aiming angle from a line parallel to said listening axis which is substantially equal in magnitude to said main driver aiming angle.
Reviewer Comments
One of the most crucial developments in multi-channel reproduction, on a formal basis, was revealed in the 1930s as Alan Blumlein defined a fresh recording and playback strategy for loudspeakers that was the basis for much of the work going forward. The first ideas were covered in large Britain Patent 394,325, “Improvements in and relating to Sound-transmission, Sound-recording, and Sound-reproducing Systems,” by Alan Dower Blumlein, June 14, 1933, assigned to Musical Industries, Ltd.
With the long-play albums as the standardized recording transportation format yet coming to pass in the late 1950s, stereo was yet realized as a popular commercial format. As awesome as the two-channel stereo was, compared to the standard, single loudspeaker mono reproduction of the day, it was besides immediately clear to experts at that time that stereo, erstwhile reproduced in a smaller, second venue listening room, was inactive a fundamentally limited format. Among the many shortcomings recognized at the time, was that of the full sound field being contained between the 2 loudspeakers, and also, that center image, mono signals reproduced through 2 loudspeakers, resulted in tonal coloration due to anomalies in the midrange amplitude consequence caused by acoustic crosstalk interference.
Within moments after two-channel stereo was becoming accepted into the commercial audio world, work began on how to grow the soundstage beyond that of the physical placement of the left and right loudspeaker and address the format’s another shortcomings. Many of the approaches incorporated various controlled radiation pattern techniques to reflect a condition of the sound off the sidewalls. These techniques could increase spaciousness, but at the expense of focused origin imaging and the addition of even greater errors in timbre.
Blumlein’s patents for two-channel systems for loudspeaker playback besides included configurations for two-channel recordings optimized for playback over headphones, by way of utilizing 2 microphones placed in the ear positions on a dummy head, a recording playback process that would later become known as “binaural.” In the early 1960s, Bell Laboratories started from the reflection that with binaural recordings—when played back over headphones — a convincingly accurate, substantially 360° horizontal representation of an first sound field could be reproduced, peculiarly if the dummy recording head included a reasonable facsimile of the full human head form, including an accurate formation of the ear/pinna shapes.
Anyone who has heard a well-captured binaural recording with peculiar effects (e.g., the sound of a bumble bee flying around one’s head) played back through headphones, will most frequently find the recreation to be rather convincing. But, erstwhile the same binaural recording is played through 2 loudspeakers positioned in front of a listener, the effect is reduced to the dominant images being restricted to placements between the 2 loudspeakers, with an even smaller lateral sound field than with most stereo recordings.
There is, of course, a simple and apparent reason for this difference in spatial reproduction. erstwhile 1 is listening to any singular, live sonic sound origin (e.g., a violin), there are 2 arrivals delivered to the listener, 1 to each ear from the violin. With headphones, this relation of 1 spatial origin and 1 arrival at each ear is maintained, with the right ear talker isolated from the left ear, and the left ear talker isolated from the right ear. The headphones substantially keep the amplitude and the timing differentials between the 2 ears to accomplish accurate horizontal directional cues and sustaining perceived origin position over at least 180°. (For the purposes of simplification, we are not including head movement and pinna effects that establish vertical location and front back hemispheric differentiation.)
When reproducing a single violin sound origin through a pair of loudspeakers, there are 4 arrivals (see Figure 1): 2 from the left talker (one to the left ear and 1 to the right ear) and 2 from the right talker (one to the right ear and 1 to the left ear). This reception of 2 additional acoustical crosstalk signals restricts the width the sound field to a span between the 2 loudspeaker (assuming no additional reflection or phase enhancements) and besides causes artificial amplitude anomalies for monophonic signals due to the interference effects of the additional pair of crosstalk signals.
Figure 1: Prior Art—The basic problem of acoustic crosstalk is illustrated by additional crosstalk signals L and R.
In 1962, 2 Bell Laboratories researchers, Bishnu Atal and Manfred Schroeder, began to address this two-loudspeaker acoustic-crosstalk issue. They filed a patent on their investigation into spatialized two-channel systems and were granted US Patent 3,236,949, “Apparent Sound origin Translator.” The researchers were among the earliest to lay the groundwork for the concept of employing crosstalk cancellation processing.
In the late 1960s, Volker Mellert and Peter Damaske extended and refined the first work of Atal and Schroeder and were able to optimize a binaural processing strategy for loudspeakers that was rather effective at 360° image placement, including front to back discrimination. They even addressed reproduction in the median plane, a precursor to the current work of Dolby’s ATMOS system. Much of the improvement was due to refinement in the dummy head construction details and crosstalk cancellation processing (See: Peter Damaske & Volker Mellert [1969] “Ein Verfahren zur richtungstreuen Schallabbildung des oberen Halbraumes über zwei Lautsprecher,” published in Acustica 22: 153-162, and Peter Damaske [1971] “Head-Related Two-Channel Stereophony with Loudspeaker Reproduction” published in The diary of the Acoustical Society of America 50 [4]: 1109-1115)
As more investigation was published on this topic, various companies started to look into how to commercialize this concept. 1 of the first of these commercial products was JVC’s BN-5 “Bi-phonic Processor” (1978-1982), developed primarily for reproducing binaural recordings through loudspeakers by way of crosstalk cancellation, but besides having adaptive settings for usage with conventional stereo program material. The strategy was based on many JVC patents, including US Patent 4,118,599, “Stereophonic Sound Reproduction System,” by inventors Makoto Iwahara and Toshinori Mori.
The simplified expression of the process JVC applied is that of adding 2 additional signals to the left and right channel outputs. The left channel input signal was inverted in phase, reduced in level, delayed by an amount corresponding to somewhat little than an ear spacing width, and then mixed into the right channel output. The same process was performed on the right channel input and mixed into the left channel output. The thought was to substantially destruct any crosstalk from the left loudspeaker to the right ear, and vice versa for a listener seated equidistant from the 2 loudspeakers.
But, as 1 can see, each crosstalk cancellation signal for 1 ear besides has a condition that crosstalks back to the another ear, theoretically requiring an infinite series of amplitude reduced, recursive crosstalk cancellations to accomplish equivalence with isolated binaural headphones erstwhile utilizing loudspeakers. Fortunately, as with many corrective systems, improvements diminish importantly after just a fewer recursions. On all these systems, additional frequency shaping and level shifting is required to optimize performance.
Even in these early efforts, the spatial impressions were rather dramatic, and for program material carefully recorded on a well-designed dummy head, the image recreations were comparatively faithful to the first origin placements. But, on much of the program material, tonal coloration was easy detected. Popularizing a version of the JVC-type acoustic crosstalk processor with additional capability, Robert W. Carver developed the Carver Sonic Hologram Generator, which alternatively of being primarily optimized for the reproduction of idealized binaural recordings, was repurposed to playback conventional stereo recordings.
Carver’s first patent on Sonic Holography was US Patent 4,218,585, “Dimensional sound producing apparatus and method.” Much of the novelty in the Carver signal processor was focused on multiple-delayed and amplitude-shaped versions of the out-of-phase crosstalk cancellation signal being summed into the primary channel signals to make a greater tolerance of head position and angle of the loudspeaker to the listener. Additional refinements were implemented in an effort to minimize tonal colorations in these devices.
However, before the electronic processor versions of these systems were introduced in the US, Makoto Iwahara of JVC was already developing a passive acoustic crosstalk cancellation capability into the driver configuration of the loudspeaker itself. As disclosed in US Patent 4,199,658, “Binaural Sound Reproduction System,” wherein a second set of drivers spaced horizontally to the outside of the primary drivers, with a center-to-center spacing corresponding to the left-to-right ear spacing distance of a listener seated on a centerline position in front of the left and right loudspeakers. In the JVC implementation, the outer crosstalk correction driver of the right channel loudspeaker was connected to the left channel amplifier output in reverse phase, with a low-pass shaping filter in the crossover. As compared to the electronic processor approach, there is no hold required due to the inherent hold due to the horizontal driver spacing of a head width. Unfortunately, there doesn’t appear to be any products developed by JVC with this loudspeaker-based crosstalk cancellation configuration.
A fewer years after the Carver Sonic Hologram Generator popularized the concept utilizing active processing, Matthew Polk at Polk Audio filed for its version of a passive, loudspeaker-based acoustic crosstalk cancellation system, and was granted a patent — US Patent 4,489,432, “Method and apparatus for reproducing sound having a realistic ambient field and acoustic image.”
One of the primary differences between the Polk Audio crosstalk cancelling loudspeaker and the first JVC version is that alternatively of simply reversing the phase and reducing the level of the additional crosstalk drivers, the Polk strategy connects the left and right (L-R) crosstalk drivers in series with each another and between the affirmative terminals of the left and right amplifier channels. This method derives an L-R signal to the left talker crosstalk driver and R-L signal to the right talker crosstalk driver, which inherently provides the phase reversed cancellation signal and appropriate level shifting needed for effective acoustic crosstalk cancellation and image expansion while maintaining a substantially timbre neutral and spatially central anchored mono signal.
While all of these crosstalk systems, going back to the first work of Schroeder and Damaske in the early 1970s, tend to operate on the same organizing principles, they besides tend to win or neglect based on how well they are implemented in terms of the fine details. As with the early binaural recording work, it was found that the dummy heads with artificial pinna had to be carefully matched to the real elements of a human listener, even including a correct torso formation. In the same vein, the later systems, erstwhile attempting to correct for head-related crosstalk, must have the cancellation correction signal coming straight from the left loudspeaker, perfectly match the level and frequency consequence form with ideally matching opposing phase to that of the crosstalk signal coming around the face of the listener. In many of the systems, a simple low-pass filter was used, and others deployed shaping networks to better match the real-world crosstalk response.
Nearly 40 years after Polk Audio’s first patent filing on its Stereo Dimensional Array (SDA) loudspeaker, in the patent herein under review, it is now providing a fresh improved version addressing the “Head Related Transfer Function” issue, not by just implementing additional filtering, but primarily by reforming the front of the loudspeaker enclosure form to more closely mimic the impact of acoustic waves “cross-talking” around a human face.
As can be seen in the first Polk SDA loudspeaker, as shown in Figure 2, the frontal baffle surface is substantially flat. The “outer” crosstalk cancellation driver had a direct, un-diffracted pathway to the ear of the listener, whereas the unwanted crosstalk signal that it was attempting to inversely emulate from the opposing loudspeaker must traverse around the face to the other ear, changing frequency response. With this format, the correction signal will only be able to partially cancel the crosstalk signal, since they are not effectively matched.
Figure 2: Prior Art—A loudspeaker with both the primary and additional crosstalk cancellation drivers mounted on a flat baffle.
In the fresh system, as can be seen in Figure 3 and Figure 4, the primary and the crosstalk correction drivers are angled back specified that a virtual “nose” is formed on the front of the enclosure, causing a diffracted amplitude consequence that more effectively mimics that of the crosstalk waveform moving across the listener’s face.
Figure 3: The loudspeaker invention showing inward angled primary drivers and outward angled crosstalk cancellation drivers and protruding barrier between the two.
Figure 4: This is simply a frontal view of 2 versions of the loudspeaker invention.
The effect is further refined by the angled drivers being designed specified that the crosstalk driver’s off-axis consequence forms an additional origin toward the optimal response. Last, a final amplitude trimming network is placed in the crosstalk arm of the crossover network to complete the task of maximizing the match to the perfect head-related transfer function, allowing the strategy to have a much more complete and precise crosstalk cancellation result (see Figure 5).
Figure 5: This is an SPL vs. frequency game for an HRTF curve (or head shadow) mark consequence curve developed as part of the present invention for a crosstalk-cancelling loudspeaker.
By having the acoustical crosstalk cancellation being realized in the formation of the loudspeaker structure alternatively of with an active signal processor, there are a number of advantages. erstwhile the distance between the primary and the crosstalk drivers corresponds to an actual left to right ear spacing, the delays are inherently adapted as the listener sits closer or further from the loudspeakers or if the distance between the left and right loudspeakers is changed. Also, with these same-use model variations, with the fresh Polk baffle formation, the amplitude changes in the head-related transfer function are besides tracked more accurately without adjustment of the signal. In the active processor approach, any of these changes in listener-loudspeaker relation ideally requires calibrated, adaptive changes in phase and amplitude to keep effective cancellation and minimize tonal coloration.
Based on the limitations of the prior art and gauging the parameters that have the top effect on crosstalk strategy performance, the refinements disclosed in the fresh Polk Audio patent, decently implemented, will most likely supply useful improvements in the application of crosstalk cancellation systems to loudspeakers. VC
This article was originally published in Voice Coil, February 2019
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