LU100989B1

LU100989B1 - Method for improving sound reproduction of low frequencies and apparatus therefore

Info

Publication number: LU100989B1
Application number: LU100989A
Authority: LU
Inventors: Dieter Leckschat; Christian Epe; Oliver Kreuer
Original assignee: Hochschule Duesseldorf
Priority date: 2018-11-12
Filing date: 2018-11-12
Publication date: 2020-05-12

Abstract

The invention relates to a Method for improving sound reproduction of low frequencies, comprising the steps of receiving an input for sound to be reproduced, applying the input to a linkwitz compensator, having a pair of poles, whereby the parameter set of the linkwitz compensator is adaptable such that the pair of poles is shifted in frequency depending on at least one property of the signal which is applied at the input/towards a sound reproducing device, thereby generating a compensated input, ? providing the compensated input towards a sound reproducing device. The invention also relates to an apparatus corresponding to the method.

Description

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -1- LU100989 Method for improving sound reproduction of low frequencies and apparatus therefore -— The invention relates to a Method for improving sound reproduction of low frequencies and an apparatus therefore.

Background Sound is typically reproduced by sound reproducing devices such as loudspeakers.

Any kind of sound reproducing device provides a certain operating range.

This operating range may be limited e.g. in a manner that for a constant input level a like output is achieved.

Like output may encompass subject to a definition differences in power of 0,5 … 2 ... 6 dB.

This operating range may also be defined in terms of impedance including variations thereof.

Itis known that loudspeakers when having a closed cabinet or speaker enclosure may be understood as a high-pass filter of 2™ order.

Such a high-pass filter may be described by a Quality factor (also known as Q-factor) and a cutoff frequency f. or when described as complex angular frequency w, = 2xfç . The cutoff frequency thereby represents a frequency where the amplitude of a signal applied at the input is weakened by about 3dB at the output.

Such filtering device may also be described within s-plane as a transfer function.

Typically, the transfer function of 2" order high-pass may be described as AQ. p10 1858

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -2- LU100989 2 Ss H(s) = ——— s°+ Ts + w? A transfer-function may also be converted to a different representation known as pole-zero representation. A 2™ order high-pass transfer-function comprises a pair of poles Py, P which are complex conjugates, seefigurel.

The frequency range of a typical (closed) loud speaker is dependent on the size. le. as a rule of thumb the larger a loudspeaker is the lower frequencies are within the operating range.

It has been proposed in the past to use a so called Linkwitz transformer also known as Linkwitz equalizer or Linkwitz compensator. Such filter designs are described e.g. on http://www linkwitzlab.com/filters.htm#9 (site layout as provided by https://web.archive.org/web/20181020095205/http://www.linkwitzlab.com/filters.htm) which is explicitly incorporated by reference.

By usage of this technique, the (immediate/amplified) input of the sound reproducing device is subjected to a filtering. This filtering is such that a transfer function is provided comprising a pair of zeros Ny, N,. The pairs of zeros N,, N are such that they closely match the poles of the 2" order high- pass transfer function of the sound reproducing device. Thereby the high-pass behavior of the sound reproducing device may be compensated. Normally the Linkwitz Transformer provides for settable new poles Ps, P, defining a new designable high-pass behavior.

To illustrate this, we assume a loudspeaker having a cutoff frequency of 60 Hz and a Q-factor of 2. By use of the filter one may tune the loudspeaker to an enclosure having a cutoff frequency of 20 Hz 12 LAA. AE

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -3- LU100989 and a - free of ringing -Q-factor of 0,7. That is, the operating range is expanded to frequencies below the original cutoff frequency of 60 Hz. This is also illustrated in figure 3. There, the frequency response of a typical loudspeaker is shown in the graph a), the frequency response of a linkwitz compensator is shown in graph b) and the target frequency response resulting of the combination is shown in graph c). That is, by usage of the filter a smaller enclosure may be used. This relates to reduced costs as well as to reduced space requirements.

Nevertheless, it is to be noted that this filtering which may be beneficial for low signal levels leads also to some disadvantages. A first disadvantage is that in the lower frequency regions the adapted electrical behavior does also require higher signal levels to be provided by an amplifier. This may result in a situation where the amplifier may no longer be able to operate linear and thereby may lead to distortions in the reproduced sound or may even damage the amplifier. Another disadvantage is that the loudspeaker itself should be able to produce a certain diaphragm displacement. However, the displacement is limited. Again, the reproduced sound may be distorted or the loudspeaker itself may even be damaged. Problem it therefore exists a need for improved methods and apparatuses allowing for improved sound reproduction while decreasing potential harms for loudspeakers and/or amplifiers. AR. SAAT TS

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -4- LU100989 Brief summary of the invention To overcome one or more problems in the art, the invention proposes a method according to claim 1 as well as an apparatus according to claim 5. Further advantageous embodiments are subject to the dependent claims as well as the detailed description and the accompanying figures.

Brief summary of the figures In the following detailed description reference will be made towards the figures, in which Fig. 1 shows a pole-zero plot of a conventional loudspeaker, Fig. 2 shows a pole-zero plot of a conventional linkwitz equalizer, Fig. 3 shows frequency responses of a sound reproducing device, a conventional linkwitz equalizer and a resulting frequency response when applying said linkwitz equalizer to said sound reproducing device Fig.4 an exemplary control arrangement according to embodiments of the invention, Fig. 5 another exemplary control arrangement according to embodiments of the invention and Fig. 6 yet another exemplary control arrangement according to embodiments of the invention.

Detailed description The present disclosure describes preferred embodiments with reference to the Figures, in which like reference signs represent the same or similar elements.

Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure,

| Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -5- LU100989 or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The described features, structures, method steps or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the description, numerous specific details are recited to provide a thorough understanding of embodiments of the invention.

Le., unless indicated as alternative only any feature of an embodiment may also be utilized in another embodiment. In addition, even though at some occurrences certain features will be described with reference to a single entity, such a description is for illustrative purpose only and actual implementations of the invention may also comprise one or more of these entities. |.e. usage of singular also encompasses plural entities unless indicated.

Definitions comprising values are not to be understood as limiting. Rather, any value may be understood as also pertaining to a range of tolerance including +/- 1% up to +/- 10 %.

References to standards, specifications, norms and the like are to be understood as a reference to standards, specifications, norms being valid at the date of application and/or — in case of claiming priority — being valid at the date of the prior application. it is emphasized that any mentioning of such standards, specifications, norms does not exclude applicability to subsequent or replacing standards, specifications, norms.

In the following it is assumed that the linkwitz compensator may be provided by analogue / discrete elements or it may be provided by a digital arrangement. E.g. a linkwitz compensator may be embodied by a digital biquad filter. A digital biquad filter is a second-order recursive linear filter, @ AA 555

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -6- LU100989 containing two poles and two zeros. "Biquad" is an abbreviation of "biquadratic”, which refers to the fact that in the z domain, its transfer function is the ratio of two quadratic functions. ba + bi 27} + by, 77? H(z) = 21-27 ap + 4,27} +a,z? The invention is based on the idea that a filter may be adapted dynamically. Hence, unlike in prior art no fixed compensator will be used which is calculated once when being implemented, the filter will be adapted as needed. Le. depending on one or more boundary conditions imposed by a complete sound reproducing system or elements thereof, the filter is dynamically adapted such that the respective element is not harmed.

Le. in a method for improving sound reproduction of low frequencies according to the invention, in a first step an input for sound to be reproduced is received.

This received input is applied to an adaptable linkwitz compensator. The linkwitz compensator itself may be analogue or digital. The linkwitz compensator provides for a pair of poles P; and P, as well as compensating zeros Nj, N,. The pairs of zeros Ny, N; are such that they closely match the poles of the 2" order high-pass transfer function of the sound reproducing device. Thereby the high-pass behavior of the sound reproducing device may be compensated.

The parameter set of the linkwitz compensator is adaptable such that the pair of poles is shifted in frequency depending on at least one property of the signal which is applied at the input/towards a sound reproducing device, thereby generating a compensated input.

ARAL ATE

Our ref.; HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -7- LU100989 That is — as can be seen in figures 4 to 6, the transfer function Huy of the linkwitz compensator is adapted on one or more parameter derived from the input or output generated. In figure 4 it is assumed that one may be able to derive control parameters on the input side of the linkwitz compensator which may be used to adapt the poles Ps, P,. This may be achieved in the formula given above by adapting the values of a; and/or a; and/or a,. Typically two of these values are adapted. In figure 5 it is assumed that one may be able to derive control parameters on the output side of the linkwitz compensator which may be used to adapt the poles Ps, P,. This may be achieved in the formula given above by adapting the values of a, and/or a; and/or a,. Typically two of these values are adapted. However, as shown in figure 6, parameters derived by a power amplifier, shown by its transfer function Hes, May also be used alternatively or in addition. That is, one may actually use actual power data to remain within the operating range of the amplifier. Additionally, or alternatively as shown in figure 6 also displacement parameters of a loudspeaker representing a sound reproducing device and being shown in figures 4-6 by its transfer function Hy may also be used alternatively or in addition. That is, one may actually use displacement data to remain within the operating range of the sound reproducing device. Once the input is equalized / compensated, the compensated input may be provided towards a sound reproducing device.

ALMA AZ PS

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -8- LU100989 ft is noted that several data may be available as rated data. This rated data may be used for implementing a model allowing for adapting the parameters of the linkwitz compensator accordingly. However, the invention also allows for measured data to be taken into account. The method may be embodied in software. The invention is not limited to a method but may also be embodied in a like apparatus for improving sound reproduction of low frequencies. The apparatus according to the invention may comprise a digital sound processing entity adapted for receiving an input for sound to be reproduced, said digital sound processing entity being adapted for applying the input to a linkwitz compensator, having a pair of poles, whereby the parameter set of the linkwitz compensator is adaptable such that the pair of poles is shifted in frequency depending on at least one property of the signal which is applied at input / towards a sound reproducing device thereby generating a compensated input, said digital sound processing entity being adapted for providing the compensated input towards a sound reproducing device. Typically, a linkwitz compensator according to the invention may be provided by a programmable Digital Signal Processor (DSP) or an application-specific integrated circuit (ASIC) or a field programmable gate array (FPGA) implementing like circuitry.

Hence, by usage of the invention it is possible e.g. in case of high levels to shift the cut-off frequency relating to the poles P; and P, to higher frequencies to remain within a reasonable / stable operation avoiding to harm an element and/or avoiding severe nonlinear distortion. In embodiments of the invention the adaptation is a dynamical adaptation. NA. A4 05 À

Our ref. HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf | -9- LU100989 In a further embodiment of the invention the adaptation is dependent on a signal amplitude which may be provided by the input, see e.g. figure 4. In still a further embodiment of the invention the adaptation may alternatively or additionally dependent on a signal output amplitude, see e.g. figure 5. In yet another embodiment of the invention the adaptation is dependent on diaphragm displacement provided by said sound reproducing device, see figure 6.

The invention may also be embodied in a manner that an apparatus according to embodiments of the invention is used in a vehicle. Within a vehicle, sound reproducing devices are used for various reasons, including but not limited to reproducing sound originating from stream sources, radio/video, phone, entertainment, information as well as engine sounds.

The invention may also be embodied in a manner that an apparatus according to embodiments of the invention is used in an amplifier. Amplifiers may be sold separately to loudspeakers. Amplifiers may provide for complex sound processing. As such they often include Digital/Analogue Converters and DSPs having processing capabilities to implement pre-modeled (in view of loudspeaker capabilities) / capability based (in view of the available power levels by the amplifier) dynamic adaption within a dynamic linkwitz compensator arrangement according to the invention.

x The invention may also be embodied in a manner that an apparatus according to embodiments of the invention is used in a sound reinforcement system.

The invention may also be embodied in a manner that an apparatus according to embodiments of the invention is used in a loudspeaker. Nowadays loudspeakers including power amplifiers and being

M AA AEP

Our ref. HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf 100989 10- LU10 fed digitally are produced. Again, these loudspeakers are provided with processing capabilities. Since these systems are pre-known a predetermined model may be applied.

VAN NER EN

Claims

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -11- LU100989 Claims

1. Method for improving sound reproduction of low frequencies, comprising the steps of | ® receiving an input for sound to be reproduced, e applying the input to a linkwitz compensator, having a pair of poles, e whereby the parameter set of the linkwitz compensator is adaptable such that the pair of poles is shifted in frequency depending on at least one property of the signal which is applied at the input/towards a sound reproducing device, thereby generating a compensated input, # providing the compensated input towards a sound reproducing device.

2. Method according to claim 1, whereby the adaptation is a dynamical adaptation.

3. Method according to claim 1 or 2, whereby the adaptation is dependent on the maximum amplitude which may be provided by the input.

4. Method according to claim 1 or 2, whereby the adaptation is dependent on a maximum output amplitude.

5. Method according to one of the preceding claims, whereby the adaptation is dependent on diaphragm displacement provided by said sound reproducing device.

6. Apparatus for improving sound reproduction of low frequencies, comprising: ® a digital sound processing entity adapted for receiving an input for sound to be reproduced, ® said digital sound processing entity being adapted for applying the input to a linkwitz compensator, having a pair of poles, whereby the parameter set of the linkwitz compensator is adaptable such that the pair of poles is shifted in frequency depending on at least one property of the signal which is applied at input, /towards a sound reproducing device thereby generating a compensated input, ® said digital sound processing entity being adapted for providing the compensated input towards a sound reproducing device.

7. Apparatus according to claim 6, whereby the adaptation is a dynamical adaptation.

8. Apparatus according to claim 6 or 7, whereby the adaptation is dependent on the signal level which is applied at the input.

9. Apparatus according to claim 6 or 7, whereby the adaptation is dependent on a maximum output amplitude. MAS 3

Our ref.: HDU 47250 P LU Application No.: NN Applicant: Hochschule Düsseldorf -12- LU100989

10. Apparatus according to one of the preceding claims 6 to 9, whereby the adaptation is dependent on the diaphragm displacement provided by said sound reproducing device.

11. Use of an apparatus according to one of the preceding claims 6 to 10 within a vehicle.

12. Use of an apparatus according to one of the preceding claims 6 to 10 within an amplifier.

13. Use of an apparatus according to one of the preceding claims 6 to 10 within a loudspeaker controlling unit.

14. Use of an apparatus according to one of the preceding claims 6 to 10 within a loudspeaker. AL AN AS 5