HK1075167B - Method for improving spatial perception in virtual surround - Google Patents

Description

Method for improving spatial sense in virtual surround

Technical Field

The present invention relates to audio signal processing. More particularly, the present invention relates to improving the spatial perception of a multi-channel sound source when reproduced by two loudspeakers.

Background

Multichannel sound reproduction systems such as Dolby Pro Logic or Dolby Digital (Dolby, Dolby Pro Logic and Dolby Digital are trademarks of Dolby laboratories franchise) require, for example, five loudspeakers placed at specific positions and at specific angles. This can be costly and space consuming. It is desirable to have surround sound without a rear speaker to save cost and space. However, conventionally, the front speaker provides only a front sound image.

Techniques are known for processing multiple channels representing sound from many directions, combining them into two signals for reproduction on headphones, while preserving the apparent multiple directions. Using headphone reproduction, the left signal enters the left ear and the right signal enters the right ear, without crosstalk. The sound may sound from the side of the listener as well as from the front, or in some cases from the back.

Considering each of the multi-channel inputs as representing sound from a particular direction, such processing for headphones typically involves applying at least suitable HRTFs (head-related transfer functions) to each input to simulate paths from its desired apparent direction to both ears, so that the headphone listener perceives each channel as coming from the desired direction. Such headphone processors, which provide two outputs in response to more than two inputs, are known by various names, such as "multi-axis binaural control" processors, "multi-channel binaural synthesizers," "headphone virtual surround" processors, and so forth. In addition to applying directional HRTFs, some headphone processors also provide processing such as adding simulated reflections and/or simulated environments to one or more channels. All of these processors, whether using only directional HRTFs or adding processing such as artificial reflections and/or ambience, are referred to herein as "headphone processors". Some examples of headphone processors include the headphone processors described in published international application WO99/14983 (assigned to the united states) and us patents 5,371,799, 5,809,149 and 6,195,434B 1. Each of the applications and patents mentioned are hereby incorporated by reference in their entirety.

Conventional two-channel stereo material is intended to be reproduced on two loudspeakers. Each ear of the listener receives sound from two speakers, of course, with different path lengths and frequency responses. In other words, there is acoustic crosstalk. Typically, all sound so reproduced sounds in the space between the loudspeakers.

It is also known to modify the signal before it is supplied to both loudspeakers to at least partially cancel acoustic crosstalk. This allows the apparent position of the sound to be well outside the space between the loudspeakers and is the basis for the "virtual surround" process. The sound entering the ear from both speakers is similar to that provided by the headphones in terms of crosstalk cancellation, i.e. no crosstalk. Crosstalk cancellers (sometimes referred to as "spatializers" or "panoramic processors") are well known in the art, at least beginning with U.S. patent No. 3,236,949(Atal and Schroeder), which is incorporated herein by reference in its entirety. A computer software implemented acoustic crossfeed canceller using very low personal computer processing resources is disclosed in U.S. patent application 08/819,582 to Davis et al, filed 3, 14, 1997, which is incorporated herein by reference in its entirety.

As is known, signals representing multiple channels, including sound originally originating outside the space between the loudspeakers, may be processed as such for reproduction on headphones and then fed through an acoustic crosstalk canceller to two front loudspeakers arranged in a conventional stereo configuration, for example at the side of a computer monitor or television picture tube. This combination of headphone processing and crosstalk cancellation allows the apparent location of a sound source to be on the side, or in some cases the rear, using only a pair of front speakers.

Fig. 1 is a schematic block diagram illustrating a prior art arrangement in which a plurality of channels (each channel representing a direction, e.g. left front, center front, right front, left surround and right surround) of a multi-channel audio source, such as a five-channel audio source, are provided to a headphone processor 2. The two outputs of the headphone processor are supplied to a crosstalk canceller 4, which also has two outputs. One output of the crosstalk canceller is supplied to a first loudspeaker 6 and the other output is supplied to a second loudspeaker 8.

The combination of headphone processing and crosstalk cancellation feeding a pair of loudspeakers is better than a crosstalk canceller alone, because the processing for headphone reproduction introduces additional directional cues by introducing directional HRTFs (crosstalk cancellers may only include "one ear to another" HRTFs) and, in some headphone processors, analog multiple sound paths (including reflections) between apparent sound image locations (outside the loudspeakers) and the listener's ears. Thus, using combined headphone processing and crosstalk cancellation, the virtual sound image may sound not only to the side of the listener's head, but also from further back.

However, such a combined headphone processing and crosstalk cancellation scheme has some drawbacks. The front channels (left front, center front and right front) of the multi-channel audio source are intended to be reproduced over the loudspeakers and satisfactorily by the two loudspeakers reproducing the left front and right front channels and also providing a virtual or "phantom" center front image (assuming, of course, that the listener is properly positioned with respect to the two loudspeakers). Therefore, processing the front channel is not necessary and should be avoided (according to the "lowest processing" principle). Headphone processing of the front channels involves at least applying directional HRTFs that may result in, for example, a tone stain or a change in tone color. Other headphone processing techniques, such as the simulation of reflections or reverberation, may introduce other noticeable and unnecessary changes to the front channel signal or may produce artifacts. Crosstalk cancellation may also adversely affect the front channel. Crosstalk cancellation is most effective when the playback environment, i.e. the listening room, introduces little signal due to reflections. Thus, in practical "real listening room" applications, crosstalk cancellation is incomplete. Thus, even if the headphone processing of the front channel is transparent, subsequent crosstalk cancellation in prior art of the type shown in fig. 1 would likely impair the reproduced front channel sound.

According to the present invention, impairment of front channel reproduction is avoided while maintaining the advantages of improved surround channel reproduction by a pair of loudspeakers.

Disclosure of Invention

According to an aspect of the present invention, there is provided a method for improving the spatial perception of a plurality of channels when reproduced by two loudspeakers, each channel representing a direction, the method comprising:

applying channels representing directions other than a front direction to the loudspeakers via headphone and crosstalk cancellation processing, wherein the headphone processing comprises adding simulated reflections and/or simulated ambience to the channels representing directions other than a front direction; and

the channel representing the front direction is applied to the loudspeaker without headphone and crosstalk cancellation processing and without adding analog reflections and/or artificial ambience to the channel representing the front direction.

Drawings

Fig. 1 is a schematic block diagram illustrating a prior art arrangement in which multiple channels of a multi-channel audio source, such as a five-channel audio source (each channel representing a direction, e.g., left front, center front, right front, left surround and right surround), are provided to a pair of front speakers through a headphone processor and crosstalk canceller.

Fig. 2 is an idealized functional block diagram of an apparatus according to the present invention.

Detailed Description

Fig. 2 illustrates an idealized functional block diagram of an apparatus according to the present invention that receives multiple channels of a multi-channel audio source, such as a five-channel audio source (each channel representing a direction, such as left front (L), center front (C), right front (R), left surround (Ls), and right surround (Ls)), provides secondary channels (e.g., left surround and right surround) to a pair of front speakers through a headphone processor and crosstalk canceller, and provides primary channels (e.g., left, center, right) to the pair of front speakers without headphone processing or crosstalk cancellation processing.

The source of the multi-directional sound source provided to the device shown in fig. 2 is not critical and may be any suitable sound source, including for example a Dolby Pro Logic sound source, a Dolby Digital sound source, a Digital transducer Systems Corporation ("DTS") sound source ("DTS" is a trademark), a discrete sound source or some other sound source. Although the invention is described in connection with embodiments having three primary channels and two secondary channels, the invention is not so limited. For example, there may be only two primary channels, e.g., left and right, and/or there may be more than two secondary channels, e.g., five secondary channels (e.g., Left Front Surround (LFS), Left Rear Surround (LRS), Right Front Surround (RFS), Right Rear Surround (RRS), and Center Surround (CS)). The number of secondary channels is limited only by the complexity of the headphone processor and its ability to simulate placement of sound in a large number of directions.

As shown in fig. 2, a portion of the apparatus is a conventional prior art Dolby MP matrix encoder configured as a 3:2 encoder. The matrix encoder 10 receives three separate input signals: left front, center front and right front (L, C, R) and generates two final outputs, left sum right sum (Lt and Rt). The C input is divided equally and added to the L and R inputs after a 3dB reduction in level, thereby maintaining a constant sound power.

The left total (Lt) and right total (Rt) coded signals can be expressed as:

l + 0.707C; and

Rt＝R+0.707C，

where L is the left front input signal, R is the right front input signal, and C is the center front input signal. When the Lt-encoded signal is reproduced by the front speaker located at the left portion and the Lt-encoded signal is reproduced by the front speaker located at the right portion, a listener located at an appropriate position can perceive a virtual or "phantom" center channel sound image. The use of the center channel is not essential and may be omitted, in which case the L and R input signals may be coupled directly to the loudspeakers without any matrix being required to mix with the center channel. If an encoder matrix is used, it does not need to be mixed with the center channel at-3 dB, but some other mixing level may be used. In any case, according to the present invention, the main channels that are intended to be reproduced by the two front speakers (e.g., the left front, center front (if used), and right front channels) are not provided to the two speakers through the headphone processor and/or crosstalk canceller.

Still referring to fig. 2, the left surround (Ls) and right surround (Rs) auxiliary channel signals are provided to left surround (Ls) and right surround (Rs) inputs of the headphone processor 12. The headphone processor 12 has the characteristics described above. Such a headphone processor may also have inputs for left front (L), center front (C), and right front (R) signals, as shown in fig. 2; however, these inputs are not used. As explained above, there may be other auxiliary channel signals provided to the headphone processor 12, provided that the device is capable of processing more than two secondary channel inputs. The headphone processor 12 provides two output signals: a left earphone (Lh) and a right earphone (Rh). These outputs will provide the headphone listener with the perception that each secondary channel input is coming from the desired direction. The Lh and Rh output signals are not provided to headphones but are provided to a crosstalk canceller 14, which in turn provides crosstalk cancelled versions of the Lh and Rh signals, identified herein as left cancellation (Lc) and right cancellation (Rc). The Lc signal may additionally be combined with the Lt signal in the adder 16 to generate a left virtual (Lv) signal, and the Rc signal may additionally be combined with the Rt signal in the adder 18 to generate a right virtual (Rv) signal. The Lv signal may then be coupled to a suitable front left located speaker (not shown) and the Rv signal may then be coupled to a suitable front right located speaker (not shown). Reproduction of Lv and Rv signals by these speakers provides the perception of primary channel sound to a suitably positioned listener without the drawbacks of headphone processor and/or crosstalk canceller processing, while providing an improved phantom image of secondary channel sound.

It should be understood that the implementation of other variations and modifications of the invention and its various aspects will be apparent to those of ordinary skill in the art, and that the invention is not limited by the specific embodiments described.

The present invention and its various aspects may be implemented in hardware, or as software functions executed within a digital signal processor, a programmed general purpose digital computer, and/or a special purpose digital computer, or as a combination of hardware and software functions. The interface between the analog and digital signal streams may be implemented by functions within suitable hardware and/or software and/or firmware.

Claims (6)

1. A method for improving the spatial perception of a plurality of channels when reproduced by two loudspeakers, each channel representing a direction, the method comprising:

applying channels representing directions other than a front direction to the speakers via headphone processing and crosstalk cancellation processing, wherein the headphone processing includes adding simulated reflections and/or simulated ambience to the channels representing directions other than a front direction; and

applying the channels representing the front direction to the loudspeakers without headphone processing and crosstalk cancellation processing and without adding analog reflections and/or artificial ambience to the channels representing the front direction, wherein the process of applying the channels representing the front direction to the loudspeakers without headphone processing and crosstalk cancellation processing comprises encoding the channels representing the front direction to reduce the number of channels representing the front direction to two when more than two channels representing the front direction are present, wherein the encoding uses an N:2 matrix encoder.

2. The method of claim 1, wherein the headphone processing comprises adding both analog reflections and simulated environments.

3. The method of claim 1 or 2, wherein the headphone processing further comprises applying directional HRTFs to channels to be applied to the loudspeakers via headphone processing and crosstalk cancellation processing.

4. The method of claim 1, wherein the matrix encoder is a 3:2 matrix encoder.

5. An audio device for improving the spatial perception of a plurality of channels when reproduced by two loudspeakers, each channel representing a direction, the device comprising:

a channel processor receiving channels representing directions other than a front direction and providing two output signals, the channel processor including a headphone processor using directional HRTFs and a crosstalk canceller, wherein the headphone processor further includes an analog reflection and/or an artificial environment processor;

a first additive combiner receiving one of the outputs of the channel processors, receiving a channel representing a front direction according to its direction using a relative scale, wherein the channel representing the front direction is not added to a simulated reflection and/or simulated environment, and providing a signal for one of the loudspeakers;

a second additive combiner receiving the other output of the channel processor, receiving channels representing front directions according to their directions using relative proportions, and providing signals for the other of the loudspeakers, wherein the channels representing front directions are not added to the simulated reflection and/or simulated environment; and

an N:2 matrix encoder, wherein channels of the plurality of channels not coupled to a headphone processor are coupled to the additive combiner through the N:2 matrix encoder.

6. The apparatus of claim 5 wherein there are five channels L, C, R, Ls and Rs, said L, C, R channel representing a front direction, said Ls and Rs channels representing directions other than the front direction, said channel processor receiving said Ls and Rs signals, said L, C and R channels being applied to said first and second additive combiners in relative proportions such that all of the L channel is received by one of said combiners and none of the R channel is received by the other of said combiners, all of the R channel is received by the other of said combiners and none of the L channel is received by the other of said combiners, and the C channel is received by each of said combiners in substantially equal proportions.