An audio stereo processing method, device and system

SWEDEN (12) PATENT (13) C2 (id 527 062 (19) SE (51)

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International class ⁷

H04S 1/00, 5/00

PATENT AND REGISTRATION (45) (41) (22) (24) (62) (86) (86) (83)

Patent filed Application widely available The patent application was submitted on expiration date

National application number International filing date Filing date for European patent application Deposit of microorganism

2005-12-13

2005-01-22

2003-07-21

2003-07-21 (21) Patent Application Number Q302110-2

Application received as:

Swedish patent application completed international patent application with number □ converted European patent application with number (30) Priority information (73) (72) (74) (54) (56) (57)

LICENSEE Embracing Sound Experience AB, Kungsgatan 37 111 56 Stockholm SE

INVENTOR Fredrik Gunnarsson, Huddinge SE

OMBUD Ehrner &amp; Delmar Patentbyrå AB

NAME Stereo sound processing method, device and system

CALLED PUBLICATIONS:

WO 98/36614, WO 01/39547

SUMMARY:

The present invention relates to a method and apparatus for processing and reproducing a stereo audio signal. The method produces a left output signal for transmission to a left speaker of a pair of speakers which is, or is equivalent to, the sum of the sum signal (M) and the difference input (S), the method further produces a right output signal for transmission to a right speaker of said pair, which is of, or is equivalent to, the sum of the sum signal (M) and the difference signal (S) phase offset by 180 °. The method is further characterized in that at least part of the difference signal or the sum signal (M) is phase shifted approximately 45'-135 ° with respect to the second signal before or during the production of the left and right outputs. The invention further relates to such a stereo sound reproduction system.

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The numbers in parentheses indicate the INID code.

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SUMMARY

The present invention relates to a method and apparatus for processing and reproducing a stereo audio signal. The method produces a left output signal for transmission to a left speaker of a pair of speakers which is, or is equivalent to, the sum of the sum signal (M) and the difference input (S), the method further produces a right output signal for transmission to a right speaker of said pair, which is of, or is equivalent to, the sum of the sum signal (M) and the difference signal (S) phase offset by 180 °. The method is further characterized in that at least part of the difference input or the sum signal (M) is phase shifted approximately 45 ° -135 ° with respect to the second signal before or during the production of the left and right outputs. The invention further relates to such a stereo sound reproduction system.

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FIELD OF THE INVENTION

The present invention relates to a method, apparatus and system for processing a stereo audio signal, and in particular, the present invention relates to a method, apparatus and system for processing a stereo audio signal according to the preamble of claims 1, 14 and 27, respectively.

BACKGROUND OF THE INVENTION

There are a large number of methods and systems that are intended for faithful reproduction of the sound experienced by a listener at the recording site. The system that comes closest to actually moving the listener to the recording location, ie. to convey an impression of the true location of the various sources at the origin event, consists of the binaural recording method and the binaural reproduction method (headphones). This method has the disadvantage that the sound is interpreted by auditory channels both at the recording stage and at the reproduction stage, and in the worst case also by two stick (outer ear) sets on its way to the listener's brain where the sound information is to be interpreted. There are solutions that use a simplified recording method which, instead of a replica of a head, comprises a head sized foam ball with microphone elements on each side of the ball. This is a compromise to gain sound quality at the expense of loss of location distinction between front, rear and height. All other ways than the binaural method of recording and reproducing sound are the creation of an imaginary sound image that is truly subjective. This is the case for both the recording step and the playback step.

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Unlike the prior art methods, the purpose of the rendering step is to transmit only the electrical differences to the listener's hearing system with minimal loss of information or information supplement. The location where the stereo sound image is created then consists of the recording and / or mixing step. The stereo image can be portrayed as a true, but still subjective, interpretation of the sound experienced by a listener in a concert hall, or as an illusion of an imaginary event that has never physically occurred, or a combination of the two.

Most of today's reproduction systems are based on a pair of separate speakers, and true reproduction of the electric stereo signal, both in terms of relative intensity between the sound waves perceived by the listener's ears and the time difference between them, can at best only be perceived at a single position in the relationship with the speakers. These methods often tend to incorrectly translate the electrical stereo information, depending on the preferences of the separate speakers and how the speakers are positioned relative to the listener. Thus, there is a need for an audio reproduction system that provides identical reproduction of the stereo audio image regardless of the arrangement of the speakers, and the quality of the speakers.

A system that solves this problem is described in the patent application

WOO1 / 39548, assigned to the Applicant of the present invention, which describes a method of processing and reproducing an input stereo audio input. A difference signal is divided into a first and a second intermediate signal, where the first intermediate signal is equal to the difference signal and the second intermediate signal is equal to the first intermediate signal phase shifted 180 °, a sum signal is attenuated by a factor α which compensates for imperfections in

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527 062 the balance between sum and difference signals occurring in the playback stage, the attenuated sum signal is added to both the first and second intermediate signals to form a stereo audio output, and the stereo signal is sent to a stereo audio playback system which comprises a pair of speaker units located in proximity of each other. The system described in WO01 / 39548 enables a stereo audio signal to be reproduced with a high original fidelity with great consistency in the perceived stereo image regardless of system quality.

However, a problem with such a system with nearby speaker units is that at high frequencies, exceeding 1-5 kHz, the original fidelity of the perceived stereo power is completely reduced or lost.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of processing a stereo audio signal which solves the above problems. This object is achieved by a method as defined in the characterizing part of claim 1.

Another object of the present invention is to provide a device for processing a stereo audio signal which solves the above problems. This object is achieved with a device as defined in the characterizing part of claim 14.

Another object of the present invention is to provide a system for processing a stereo audio signal which solves the above-mentioned problems. This object is achieved by a system as defined in the characterizing part of claim 27.

According to the present invention, a left output signal is transmitted for transmission to a left speaker in a pair of speakers which is, or is equivalent to, the sum of a

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527 062 sum signal M and a difference signal S, wherein at least a portion of the difference signal S or sum signal M is phase shifted approximately 45 ° -135 ° with respect to the second signal, and where a right output signal for transmission to a right speaker of said pair is formed, or is equivalent to, the sum of the sum signal M and a 180 ° phase-shifted difference signal S, wherein at least part of the difference input S or the sum signal M is phase shifted approximately 45 ° -135 ° with respect to the second signal.

This has the advantage that the phase difference introduced by the present invention into the stereo signal converts incoming level difference into phase difference between the stereo channels. This phase difference will be converted to a level difference when the stereo signal is played through a pair of speakers. Level difference, unlike phase difference, constitutes strong directional information for shorter wavelengths, and consequently, the phase shift introduced by the present invention will significantly improve the original fidelity of the perceived stereo effect.

The sum signal M can be attenuated by a factor α and / or the difference input S can be amplified by a factor β in producing the left output and the right output. This has the advantage that a stereo audio signal consisting of level difference for long wavelengths and phase difference for short wavelengths can be obtained, which signal will be played by a pair of speakers such as phase difference for low frequencies, which is strong directional information for low frequencies, and level difference for high frequencies, which , as stated above, provides strong directional information for high frequencies.

The inputs of the present invention may be a left input L and a right input R, in which case

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527 The 062 sum signal M is produced as the sum of the left input L and the right input R, and the difference input is produced as the difference between the left input L and the right input R. This has the advantage that a conventional stereo signal can be used as input signals in the present invention.

The speaker elements may be located close to one another, in particular, the pair of speaker elements may consist of a pair of identical speaker elements which are acoustically insulated from each other, and located within less than a quarter of a wavelength of the shortest wavelength emitted by the elements, or, if the shortest wavelength emitted by the elements. is less than 68 cm, less than 17 cm. This has the advantage that the present invention is very well suited for use in a method and system as described in WOO1 / 39548.

The phase shift can be performed such that the entire difference input S or the sum signal M is phase shifted approximately 45 ° -135 °, preferably 90 °. This can advantageously be done by digital signal processing, e.g. using a Hilbert transform. Alternatively, the phase shift can be accomplished by means of a phase dependent filter, such as an analog allpass filter. This has the advantage that a cheaper solution can be obtained for cost-sensitive applications and / or applications where the processing time is critical. The summing signal M can be delayed by a time corresponding to the delay of the phase shifting means. This can facilitate obtaining a desired phase relationship between the difference input S and the sum signal M.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram illustrating a known system for processing stereo signals;

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Fig. 2 is a block diagram illustrating a first embodiment of the present invention;

Fig. 3 is a block diagram illustrating a second embodiment of the present invention; and

Fig. 4 shows an example of the frequency characteristics of an all-pass filter in the embodiment shown in Fig. 3.

DESCRIPTION OF A PREFERRED EMBODIMENT

Fig. 1 illustrates the principle of operation of a prior art device for processing a stereo audio signal. The stereo audio input consists of a left stereo signal L and a right stereo signal R. The L and R signals are used to obtain a sum signal M and a difference signal S, corresponding to the sum of the left L and right R stereo signals, and the difference between the left L and the right signal. R the stereo signals. Stereo signals L_Oout, which is to be transmitted to a left audio reproduction unit, such as a loudspeaker, is the sum of the difference signal, S, and the sum signal multiplied by a attenuation factor a, while the stereo signal Rout to be transmitted to a right audio reproduction unit is the sum of the inverted difference signal. S, and the sum signal multiplied by an attenuation factor a.

The system described in Fig. 1 allows an electrical stereo audio signal to be reproduced with high original fidelity with great consistency in the perceived stereo image regardless of the quality of the system. However, as indicated above, the system in Fig. 1 suffers from the original fidelity of the perceived stereo effect decreasing or disappearing completely at frequencies exceeding 1-5 kHz.

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This is due to the fact that level difference in L_Oout and R_Ooutput resulting from respective addition and subtraction of the S signal is converted to phase difference upon playback through the speaker elements. This phase difference constitutes strong directional information for low frequencies and results in excellent stereo resolution for these lower frequencies.

However, due to the characteristics of the human ear, at high frequencies, the ability to detect phase difference between two signals received by the left and right ear disappears.

The reason for this is the phase lock of the auditory nerve which tends to fire at a particular phase for a stimulating low frequency tone (&lt;4-5 kHz), with a nail burst per cycle for frequencies below about 1000 Hz. The intervals between the nails tend to occur at integer multiples of the tone period.

At high frequency tones (&gt; 4 - 5 kHz), phase locking becomes weaker and then disappears because the capacitance of internal hair cells prevents them from changing voltage sufficiently quickly. The lack of phase locking over 4-5 kHz confirms that the system of Fig. 1 provides weak directional information for sounds containing only short wavelengths and containing only level difference between the stereo channels.

The present invention seeks to solve the above problems with a device as illustrated in Figure 2.

The device of Fig. 2 is similar to that of Fig. 1 with the difference that in Fig. 2 an extra unit has been added. As in Fig. 1, a sum signal M is obtained by summing the left L and right R stereo signals, and a difference signal is obtained by subtracting the right stereo signal R from the left stereo signal L. The difference signal S is then phase shifted 90 ° before the formation of the stereo signals L_0Ut and the Root Stereo signal L₀ot is then obtained by calculating the sum of

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527 062 the phase-shifted difference signal S and the sum signal Μ multiplied by a attenuation factor a, while the stereo signal Root is obtained by subtracting the phase-shifted difference signal S from the sum signal M multiplied by an attenuation factor a. the sum signal M multiplied by an attenuation factor a. Inverting the difference signal is equivalent to ignoring it or phase shifting it 180 °.

The attenuation factor α can typically be -6 dB to -12 dB. However, in a general case, the attenuation factor α can be adjusted to optimize the stereo effect perceived by the listener, and allowed to vary in a range from -3 dB to - 15 dB.

The phase shift can be achieved by a digital signal processor, e.g. using a Hilbert transform. Digital signal processing has the advantage that a true 90 ° phase shift can be performed for all wavelengths and can be obtained with little or no amplitude change with frequency (use of analog circuits can result in a phase drift in the audible spectrum of 500 ° -700 ° or more, however with a relative phase difference of 90 ° between the sum signal M and the difference signal S). This type of phase shift is particularly suitable for systems in which digital signal processing already exists, and where the applications are not time-critical.

Furthermore, it may be desirable to include a delay circuit in the device, shown as 21 in Fig. 2, to delay the summation signal M with a time corresponding to the processing time of the phase shifting means. This facilitates maintaining the desired phase relationship between the difference input S and the sum signal M.

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Fig. 3 illustrates a second embodiment of the present invention. The second embodiment of the present invention provides a solution for applications where phase shifting is desirable but where the application is cost sensitive and / or processing time is time-critical, such as in professional recording studios. In the second embodiment, the sum signal M and the difference signal S are obtained as in FIG. 2, and the difference signal S is then changed with a unit 30 which includes a frequency dependent analog all-pass filter with a center frequency set well above the shortest audible wavelength. This means that the phase shift starts with only a few degrees at e.g. 500 Hz to reach + 90 ° at e.g. 10 kHz. The phase pass of the allpass filter is thus tailored to gradually transfer the phase difference of the stereo signal to level difference as the phase locking becomes weaker for higher frequencies. Since the phase passage of an analog filter can hardly be of a negative nature, the unit 30 further includes means for inverting the signal to obtain the desired result with a phase shift of -90 °. The phase shift should preferably be negative since the original L and input signals may otherwise be confused. An example of a phase pass for the all-pass filter is shown in Fig. 4. As can be seen in the figure, the phase shift starts at substantially 0 ° at low frequencies to reach 90 ° at high frequencies (eg 10 kHz). It is also possible to create the frequency-dependent phase shift with the help of digital signal processing, but with the extra cost this can incur.

The factor α in Fig. 3 can be made frequency dependent so that the factor is different for different driver circuits for, for example, different elements in a multi-way speaker configuration. The sum signal is then added to the phase shifted difference signal S to form

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527 062 a first output signal, and the phase-shifted difference signal S is then subtracted from the sum signal M to form the second output signal.

In general, the method described in the present invention can be used equivalent to arbitrary expressions which can be described as a linear transformation of the R and L signals or the M and S signals, but for convenience, the method has been exemplified using M and S signals. images and R and L images respectively. The method should therefore be interpreted as a method having an output equivalent to S_ps+ aM and Sp_S+ c &lt;M, where S_ps the S signal is phase-shifted by 90 °. As described, the M and S signals can be produced by an intermediate step in the process, but this need not be the case as long as the resulting output ratio is met.

In the above description, the phase shift has been described as 90 °. However, this phase shift can be any phase shift in a range between 45 ° -135 °. Further, in the above description, the phase shift has been performed on the difference signal S. It can, however, also be performed on the sum signal M.

Furthermore, in the above description, however, the analog all-pass filter can be exchanged for a digital filter that performs an identical filtering function as the above-described analog all-pass filter. In this case, it may also be desirable in this embodiment to include a delay circuit in the device, as shown as 21 in FIG.

Further, in the above description, the stereo signals may consist of an L and an R signal. However, the input signals may also exist

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527 062 of M and S signals, in which case the first addition and subtraction steps are omitted.

Furthermore, in the above description, the sum signal is attenuated by a factor a. However, it is of course possible to amplify the difference signal S by a factor β instead.

In the detailed description of the present invention, the phase shift has been performed on the difference input S. However, the phase shift can be performed equally well on the sum signal

M.

In so far as the present invention is subject to variations, modifications, and changes in detail, some of which are set forth herein, it is intended that anything described throughout this description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting manner.

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