1. Field of the Invention
This invention relates generally to the reproduction of sound in multi-channel systems generically known as “surround-sound” systems and more specifically to the application of psychoacoustic principles in the design of a loudspeaker system for reproducing a surround sound experience from loudspeakers located only in front of the listener.
2. Background Art
One problem which is common to all methods of producing phantom rear located sound images from a sound source or sources located in front of the listener is that the phantom source illusion tends to collapse as the listener turns or moves the listener's head even slightly. This problem arises from fundamental differences in the way sound at the listener's two ears changes as the head turns in relation to the location of a sound source in front of the listener as compared to a rear located sound source. For example, for a front located sound source not on the median plane (i.e., the vertical plane equidistant from the listener's two ears.), the interaural time delay ITD for sound arriving at the listener's ear nearest the sound source relative to the arrival time of the sound at the ear farthest from the sound source will decrease as the listener turns toward the sound source. Similarly, the interaural level difference ILD between the listener's two ears will also decrease as the listener turns toward the sound source. Both the ITD and ILD will be zero when the listener faces directly toward the sound source. In comparison, for a sound source located behind the listener, the ITD will increase as the listener turns towards the sound source and will reach a maximum when the listener has turned such that the sound source is located directly to one side, 90 degrees from the median plane. In general, the ILD will also increase as the listener turns towards a rear located sound source and will reach a maximum when the sound source is located directly to one side. However, as is well known, the behavior of the ILD at individual frequencies is complex and may not follow this general rule.
A second problem arises specifically in systems which use variations of a difference signal to create phantom rear located sound images such as is described in parent application Ser. No. 10/692,692. Difference signals are formed by subtracting one audio signal from a second audio signal. Referring to
An additional problem is encountered in systems using passive methods to develop a difference signal by modifying and combining the speaker level output of two or more amplifier channels. Such a system is disclosed in U.S. Pat. Nos. 4,683,505 and 4,759,066 to Polk, et al. As is disclosed in these patents, amplifier channels which do not share a common ground may be damaged if DC current flows are permitted from one channel to the other. However, the methods proposed for isolation of the channels involve costly transformers which may also degrade performance.
In accordance with one embodiment of the present invention, in a system with main and sub-speakers, such as disclosed in parent application Ser. No. 10/692,692, a modified and inverted left surround signal is added to the signals driving the right main speaker and a modified and inverted right surround signal is added to the signals driving the left main speaker for the purpose of creating more credible rear located phantom sound images.
In accordance with another embodiment of the present invention, in a system using variations of a difference signal to create phantom rear located sound images, such as disclosed in parent application Ser. No. 10/692,692, the components of the difference signal are modified by introducing a time delay to one of the components for the purpose of preventing the components of the difference signal from substantially canceling each other.
In accordance with another embodiment of the present invention, in a system using variations of a difference signal to create phantom rear located sound images, such as disclosed in parent application Ser. No. 10/692,692, the components of the difference signal are modified by altering the relative level and frequency response of the components for the purpose of preventing the components of the difference signal from substantially canceling each other.
In accordance with yet another embodiment of the present invention, in a system using variations of the difference signal to create phantom sound images cost effective means are provided to isolate individual amplifier channels so as to block potentially damaging current flows without degrading performance.
a is a diagram showing the signal combinations of a first embodiment of the present invention.
a is a chart of the relative magnitude and frequency response of various signals developed in the third embodiment of the invention.
Preferred embodiments of the present invention are now described with reference to the figures where like reference characters/numbers indicate identical or functionally similar elements. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the invention.
As described in the parent application, four audio signal inputs, for example only and not by way of limitation, corresponding to signal channels of a surround sound system are provided. It is understood that these may be any four audio input signals. However, for purposes of clarity and consistency these signals will be referred to herein as left surround signal LS; left front signal LF; right front signal RF; and right surround signal RS. Left and right loudspeaker enclosures, LSE and RSE are also provided. Left loudspeaker enclosure LSE contains at least one left main speaker LMS and at least one left sub-speaker LSS. Right loudspeaker enclosure RSE contains at least one right main speaker RMS and at least one right sub-speaker RSS. As is well known by those skilled in the art unmodified audio signals reproduced by a pair of loudspeakers, such as in a typical stereo audio system, are perceived by a listener sitting in front of the speakers as originating from a range of sound locations between the two loudspeakers. Therefore, sounds produced only by main left and right loudspeakers LMS and RMS are perceived by a listener located at principle listening location LL as originating from a range of sound locations approximately between and bounded by the actual locations of left and right main loudspeakers LMS and RMS.
As shown in
Referring again to
Front-to-back filters 1 and 2 modify the surround signals LS and RS such that, at the listener's ears and over a certain frequency range, they will approximate the frequency response of sound signals as if they originated from the rear of the listener, even though they are being projected from the front of the listener. This modification is explained in parent application Ser. No. 10/692,293.
After passing through front-to-back filter 1, left surround signal LS passes through an inverter 5 and a low pass filter 11. It then passes through an adder 10, in which it is combined with right surround signal RS, which has passed through front-to-back filter 2 and low pass filter 8 such that the resulting combined signal is composed of a modified left surround signal LS′ subtracted from a modified right surround signal RS′. The combined signal is then transmitted to right sub-speaker RSS, located in right speaker enclosure RSE. The improvement of this application is shown in
In accordance with this first embodiment,
In this first embodiment, left sub-speaker LSS and right sub-speaker RSS are positioned relative to left main speaker LMS and right main speaker RMS and to the listener according to the teachings of U.S. Pat. Nos. 4,489,432; 4,497,064; 4,569,074 and 4,630,298 for the purpose of canceling IAC and producing a realistic acoustic field extending beyond the loudspeaker locations. As shown in prior art FIG. 1, and discussed in the above-referenced U.S. patents, the left and right sub-speakers LSS and RSS may be located on a common speaker axis with left and right main speakers LMS and RMS. However, as also discussed in the above-referenced U.S. Pat. No. 4,497,064, the sub-speakers may be placed in any location that produces the correct time delay relative to the respective main speakers for sounds aiming at the listener's ears. As shown in
Referring back to the addition of attenuators 13 and 14 and mixers 15 and 16 to the embodiment shown in
It has been found experimentally that the introduction of a suitable relative delay between the two surround signals, RS and LS, produces phantom rear sound images which are more credible for the listener, particularly in the area directly behind the listener. This relative delay may be accomplished by delaying just one of the two surround signals, RS or LS, or by introducing delays of different magnitudes to the two surround channels, RS and LS, such that there is a relative delay between the two surround signals. In addition it has been found experimentally that phantom rear sound images intended to be perceived as moving behind the listener from one side to the other are perceived as moving in a more continuous fashion with an appropriate delay applied to either one, but not both, of the two surround signals. Further, it has been found experimentally that a delay in the range of 0.5 ms to 2.5 ms produces the best results. In a specific embodiment of the present invention the delay 17 of
Front to back filters 101 and 102 perform the same function described in parent application Ser. No. 10/692,692, for the signals reproduced by the left and right main speakers LMS and RMS. Front to back filter 103, performs the same function for the signals reproduced by the left and right sub-speakers LSS and RSS. Circuit components C4, C5, C6, C7, L1 and L2 perform various of the filtering functions also described in parent application Ser. No. 10/692,692. Circuit components C1, C2, C3, R1 and R2 have been added in such a way as to change the relative level and frequency response of the two signal components comprising the difference signals applied to left and right sub-speakers LSS and RSS such that the signals components will not cancel each other and such that the difference signal applied to the sub-speakers will always be substantially non-zero.
By way of example and not of limitation, the positive right surround signal RS passes through capacitor C5 and inductor L2 and is then applied to the negative terminal of the left sub-speaker LSS, causing the left sub-speaker to reproduce an inverted and modified version of the right surround signal RS′″. After passing through the left sub-speaker LSS the modified right surround signal is divided between two paths. A portion of the signal current flows through resistor R1 and a portion through capacitor C1, back to the negative terminal of the right surround signal RS source. The remaining portion of the right surround signal current flows through front-to-back filter 103 and capacitor C3. It is then divided again between two paths with a portion of the current flowing through resistor R2 and capacitor C2 back to the negative terminal of the left surround signal LS source. The remaining portion of the right surround signal current is applied to the positive terminal of the right sub-speaker RSS, and then flows through inductor L1 and capacitor C4 back to the positive terminal of the left surround signal LS source. As is well known, multi-channel audio amplifiers typically share a common ground between all channels. For the purposes of this analysis the negative terminals of all the signal inputs may, therefore, be considered connected. It is also well understood that the output circuitry of most audio amplifiers appears as a short circuit to any externally applied signal. For example, in this embodiment of the present invention, some portion of the right surround signal RS is applied to the positive terminal of the left surround signal LS source where it will flow as if directly connected to the negative terminal which may, in turn, be considered as directly connected to the negative terminal of the right surround signal RS source. As can be seen by inspection, the remaining portion of the right surround signal RS applied to the positive terminal of the right sub-speaker RSS, causes right sub-speaker RSS to reproduce an in-phase version of the right surround signal RS″, which will be reduced in level and have a different frequency response as compared to the inverted version of the right surround signal RS′″ being reproduced by the left sub-speaker LSS. A similar analysis may be made for the current flows originating from the left surround signal LS.
a shows the relative magnitude and frequency response of the modified versions of the right surround signal RS′, RS″ and inverted RS′″ as reproduced by the right main speaker RMS, right sub-speaker RSS, and left sub-speaker LSS, respectively. The signals originating from the left surround signal LS are LS′, LS″ and LS′″ and will have the same magnitude and frequency response as shown for signals RS′, RS″ and RS′″ respectively and will be reproduced by the left main speaker LMS, left sub-speaker LSS, and right sub-speaker RSS, respectively. As shown in
In addition, front to back filters, 101, 102 and 103 are composed of an inductor, a capacitor and a resistor connected in parallel wherein the component values are approximately:
This is a substantial improvement over the prior art methods disclosed in U.S. Pat. Nos. 4,638,505 and 4,759,066 to Polk. Both of these patents describe methods for isolating the amplifier channels and optimizing bass response of the sub-speakers. In FIG. 2 of the '066 patent, an arrangement showing the sub-speakers partially connected in series is shown with channel isolation and bass response optimization accomplished using a single transformer. This arrangement has been used successfully in a consumer product, the Polk Audio SRT home theater speaker system. However, the transformer required for the system to operate properly without significant signal degradation was very costly and weighed over 10 lbs. Since that time, additional research has shown that bass response optimization for the sub-speakers is not necessary for the achievement of stable phantom sound images either front or rear located. In the present invention and as shown in
Referring to
For the purpose of illustration but not of limitation, the various embodiments of the present invention have been discussed primarily as having either four input signals comprising two front channels and two rear channels, or as having just two front channel input signals. It will be immediately apparent to anyone skilled in the art that the methods of the present invention may applied to any single pair of input signals for the purpose of creating phantom sound images. Various unmodified input signals and corresponding drive units for reproducing them may also be added to any of the embodiments without departing from the spirit of the invention. By way of example and not of limitation, a front center channel signal, such as commonly found in surround sound systems, and speaker for reproducing same could be added to the embodiment shown in
This application is a continuation-in-part of application Ser. No. 10/692,692, filed Oct. 27, 2003, the entirety of which is incorporated herein by reference.
Number | Date | Country | |
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Parent | 10692692 | Oct 2003 | US |
Child | 11147447 | Jun 2005 | US |