METHOD AND SYSTEM FOR MANAGING THE LOW FREQUENCY CONTENT IN A LOUDSPEAKER SYSTEM

Abstract

The present invention relates to a method for managing the low frequency content obtained by a loudspeaker system comprising a plurality of loudspeaker devices, such as a surround sound loudspeaker system, wherein each individual loudspeaker device has a known response as a function of frequency under anechoic conditions comprising a phase response, the method comprising the steps of providing a plurality of loudspeaker devices (1, 2, 3, 4) and for each of said plurality of loudspeaker devices (1, 2, 3, 4) providing the corresponding phase response as a function of frequency obtained under anechoic conditions and for each of the individual loudspeaker devices (1, 2, 3, 4) inserting a filter device (5, 6, 7, 8) in the signal chain to the corresponding loudspeaker device (1, 2, 3, 4), where the individual filter device (5, 6, 7, 8) is configured such that the resulting phase response of each individual loudspeaker device (1, 2, 3, 4) under anechoic conditions is substantially the same for all of the loudspeaker devices (1, 2, 3, 4). The invention further relates to a loudspeaker system implementing the method of the invention.

Claims

1. A method for managing the low frequency content obtained by a loudspeaker system comprising a plurality of loudspeaker devices, wherein each individual loudspeaker device has a known response as a function of frequency under anechoic conditions comprising a phase response, the method comprising the steps of: providing a plurality of loudspeaker devices (1, 2, 3, 4); for each of said plurality of loudspeaker devices (1, 2, 3, 4) providing the corresponding phase response as a function of frequency obtained under anechoic conditions; for each of the individual loudspeaker devices (1, 2, 3, 4) inserting a filter device (5, 6, 7, 8) in a signal chain to the corresponding loudspeaker device (1, 2, 3, 4), wherein the individual filter device (5, 6, 7, 8) is configured such that the resulting phase response of each individual loudspeaker device (1, 2, 3, 4) under anechoic conditions is substantially the same for all of the loudspeaker devices (1, 2, 3, 4).

2. The method according to claim 1, the method comprising the additional steps of: determining distances (L.sub.1, L.sub.2, L.sub.3, L.sub.4) from a given listening point (P) to each individual loudspeaker device (1, 2, 3, 4); if the loudspeaker devices (1, 2, 3, 4) are located at different distances (L.sub.1, L.sub.2, L.sub.3, L.sub.4) from said given listening point (P), introducing an additional individual phase compensation in the respective signal chains to compensate for said different distances (L.sub.1, L.sub.2, L.sub.3, L.sub.4).

3. The method according to claim 2, wherein said additional individual phase compensation is zero for the loudspeaker device (1) that is at a greatest distance (L.sub.1) from a listening point (P), such that the sound signals emitted by the remaining loudspeaker devices (2, 3, 4) are delayed corresponding to a difference between the distance (L.sub.2, L.sub.3, L.sub.4) between the loudspeaker device (1) at the greatest distance (L.sub.1) from the listening point (P) and the respective distances (L.sub.2, L.sub.3, L.sub.4) between each individual of the remaining loudspeaker devices (2, 3, 4) from the listening point (P).

4. The method according to claim 1, wherein said filter devices (5, 6, 7, 8) are all-pass filters.

5. The method according to claim 1, wherein said filter devices (5, 6, 7, 8) are II R filters.

6. The method according to claim 1, wherein said filter devices (5, 6, 7, 8) have a passband that is limited to a low-frequency region, preferably below 1000 Hz, more preferably below 500 Hz.

7. The method according to claim 1, including providing a database containing filter coefficients or filter parameters or a corresponding magnitude and the phase response of the loudspeaker devices measured anechoically for each of a plurality of loudspeaker devices.

8. The method according to claim 7, wherein said database is accessible via the internet or other communication networks, which may be wired or wireless.

9. A loudspeaker system comprising a plurality of loudspeakers devices, wherein each individual loudspeaker device has a known response as a function of frequency under anechoic conditions, comprising a phase response, wherein the system comprises: a plurality of loudspeaker devices (1, 2, 3, 4); for each of said loudspeaker devices (1, 2, 3, 4) a filter device (5, 6, 7, 8) capable of providing phase compensation of a input signal for each specific loudspeaker device (1, 2, 3, 4) and providing a filtered output signal (13, 14, 15, 16) either directly or via further signal processing means, such as filters, equalizers or amplifiers, to the specific loudspeaker device (1, 2, 3, 4); means for providing information about the necessary phase compensation of the input signal for each specific loudspeaker device (1, 2, 3, 4); means for adjusting the phase response of each filter (5, 6, 7, 8) according to said information.

10. The loudspeaker system according to claim 9, wherein said means for providing information about the necessary phase compensation is a database provided in the system.

11. The loudspeaker system according to claim 9, wherein said means for providing information about the necessary phase compensation is one or more remotely located databases and wherein the system comprises communication means configured to obtain information about the necessary phase compensation from the one or more remotely located databases.

12. The loudspeaker system according to claim 9, wherein said filters (5, 6, 7, 8) are all-pass filters.

13. The loudspeaker system according to claim 9, wherein said filters (5, 6, 7, 8) are IIR filters.

14. The loudspeaker system according to claim 9, wherein said filters (5, 6, 7, 8) have a passband that is limited to a low-frequency region, preferably below 1000 Hz, more preferably below 500 Hz.

15. The loudspeaker system according to claim 9, wherein said filter devices (5, 6, 7, 8) comprise a first filter unit (5, 6, 7, 8) configured to compensate for differences in phase response of the different loudspeaker devices (1, 2, 3, 4) under anechoic conditions and a second filter unit (5, 6, 7, 8) configured to compensate for said differences in respective distances (L1, L2, L3, L4) between the individual loudspeaker devices and a given listening point (P).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Further benefits and advantages of the present invention will become apparent after reading the detailed description of non-limiting exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein

[0040] FIG. 1a shows a schematical block diagram of a system used to implement the method according to the invention;

[0041] FIG. 1b shows a possible configuration of a filter device for use in the present invention comprising first and second filter units;

[0042] FIG. 2 shows the magnitude response as a function of frequency for a subwoofer device, a full band loudspeaker device with a lowpass filter inserted before the input terminals of the loudspeaker device and the resulting (summed) magnitude response of the two devices measured at a given position relative to the devices;

[0043] FIG. 3 shows the phase responses corresponding to the three magnitude responses shown in FIG. 1;

[0044] FIG. 4 shows an example of all-pass filter phase responses used to correct the respective phase responses shown in FIG. 3 of the subwoofer device and the full band, LP filtered loudspeaker device;

[0045] FIG. 5 shows the resulting phase responses with the phase corrections shown in FIG. 4 for the subwoofer device, the full band, LP filtered loudspeaker device and for the resulting (summed) response;

[0046] FIG. 6 shows the loudspeaker magnitude (amplitude) responses with phase correction for the subwoofer device, the full band, LP filtered loudspeaker device and the resulting (summed) combination of the two devices;

[0047] FIG. 7 shows a flow chart of an embodiment of the method according to the invention; and

[0048] FIG. 8 shows a flow chart of an alternative embodiment of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0049] With reference to FIG. 1a there is shown a non-limiting example of a set-up of loudspeaker devices in a listening room comprising a preferred listening point P (sometimes referred to as the sweet spot) at which a listener expects to obtain the best possible sound quality provided by this 30 set-up of loudspeaker devices. The set-up shown in FIG. 1a comprises four loudspeaker devices 1, 2, 3, 4 placed at locations P1, P2, P3, P4 at distances L.sub.1, L.sub.2, L.sub.3, L.sub.4 respectively from the listening point P.

[0050] The loudspeaker devices 1, 2, 3, 4 have different phase characteristics Phase1, Phase2, Phase3, Phase 4 respectively, measured under standard conditions in an anechoic room.

[0051] In the signal path from each of the respective input terminals 9, 10, 11, 12 to the corresponding loudspeaker device 1, 2, 3, 4 there is inserted a phase correcting filter device, 5, 6, 7, 8 by means of which the acoustic output signals provided by the individual loudspeaker devices can be corrected (equalized).

[0052] In the exemplary set-up shown in FIG. 1a, the loudspeaker devices are placed at different distances L.sub.1, L.sub.2, L.sub.3, L.sub.4 from the listening point P and these differences lead to corresponding frequency-dependent phase differences between the signals arriving at the listening point P from each individual loudspeaker device. Thus, the total phase difference of the signals from each individual loudspeaker at the listening point P will consist of the differences between the individual phase characteristics of the individual loudspeakers (measured under standard conditions in an anechoic room as mentioned above) and the phase differences caused by the propagation of the sound waves over the different distances from the respective loudspeaker devices to the listening point P. In a special case, where all of these distances are the same, there will of course be no phase differences caused by the sound traveling from the individual loudspeaker devices to the listening point P, and hence no corrections for such differences will have to be made.

[0053] The individual phase correcting filter devices 5, 6, 7, 8 can be configured in many different ways. One possible configuration would be as shown in FIG. 1b, wherein each of the individual filter devices 5, 6, 7, 8 comprises a series connection of a first filter unit 5, 6, 7, 8 configured to compensate for differences in phase response of the different loudspeaker devices 1, 2, 3, 4 under anechoic conditions and a second filter unit 5, 6, 7, 8 configured to compensate for the differences in the distances L.sub.1, L.sub.2, L.sub.3, L.sub.4 between the individual loudspeaker devices and the given listening point P. The transfer functions of these two filters are H1i and H2i respectively. It should however be understood that the configuration shown in FIG. 1b merely constitutes a simple, non-limiting configuration of a correcting filter device that can be used in the present invention.

[0054] When using multiple loudspeakers in the same frequency range to play back correlated signals, the phase response of each loudspeaker has an influence on the resulting response. This is illustrated by a non-limiting example with reference to FIGS. 2 through 6. In FIGS. 2 through 6 two different loudspeaker devices (a subwoofer and a lowpass-filtered full band loudspeaker) are placed at the same distance from the listening point P.

[0055] With reference to FIG. 2 there is shown as a non-limiting example the frequency response 17 of a subwoofer, the frequency response 18 of a lowpass filtered full band loudspeaker, and the resulting summed frequency response 19 of the two loudspeaker devices in an anechoic space in a listening position at the same distance from each of the loudspeakers.

[0056] Even though the individual responses of each loudspeaker are well controlled, the summed frequency response is very uneven. Further, at certain frequencies, the resulting level is decreased by using two loudspeakers rather than one. The reason for this is that the phase responses of the two loudspeaker devices are not aligned.

[0057] With reference to FIG. 3 there are shown the phase responses of each of the two loudspeaker devices. The phase response of the subwoofer is indicated by 20, the phase response of the lowpass filtered full band loudspeaker device is indicated by 21 and the summed phase response is indicated by 22.

[0058] The phase responses of each loudspeaker device are clearly different, and the result is that the resulting amplitude response becomes uneven as shown by 19 in FIG. 2. Additionally, the resulting phase response 22 becomes irregular.

[0059] In order to make the resulting phase and amplitude response more even, all-pass phase correction filters are inserted between each signal input and each individual loudspeaker device. The all-pass filters are in this embodiment of the method according to the invention designed as IIR filters with a common phase target for each loudspeaker device.

[0060] With reference to FIG. 4 there is shown an example of all-pass filter phase responses 23, 24 used to correct the respective phase responses shown in FIG. 3 of the subwoofer device and the full band, LP filtered loudspeaker device. Response 23 is for the subwoofer and response 24 is for the full band loudspeaker device.

[0061] The filters can be implemented as IIR all-pass filters which implies that the filters have a flat magnitude response.

[0062] With reference to FIG. 5 there are shown the phase responses of the individual loudspeaker devices including the phase correction filters. Response 25 is for the subwoofer, response 26 is for the full band loudspeaker device and response 27 is the summed response.

[0063] Applying the phase correction all-pass filters makes the phase responses of the two loudspeaker devices similar and therefore also the summed phase response is the same.

[0064] With reference to FIG. 6 there are shown the amplitude responses of the loudspeaker devices with the phase correction filters inserted in the signals path to the respective loudspeaker device, again in an anechoic space and at a listening position at equal distance from the individual loudspeakers. Reference numeral 28 indicates the amplitude response of the subwoofer, 29 indicates the amplitude response for the lowpass filtered full band loudspeaker device and 30 indicates the summed amplitude responses of the subwoofer and the full band loudspeaker device.

[0065] It is observed that the individual loudspeaker amplitude responses are not affected whereas the summed amplitude response is much more even and has increased in level at all frequencies compared with the amplitude response with reference numeral 19 in FIG. 2.

[0066] The introduction of IIR all-pass filters in the signal chain will of course introduce a larger phase shift of the resulting response relative to the input, which is also apparent in previous phase plots. To overcome this problem a FIR phase equalization filter may be introduced to the global bass signal.

[0067] If the loudspeaker devices 1, 2, 3, 4 are placed at different distances from the listening point P it is required to know the distances L.sub.1, L.sub.2, L.sub.3, L.sub.4 from each loudspeaker device 1, 2, 3, 4 to the listening point P and to take the propagation delay of the sound waves into account in order to maintain accuracy of the phase equalisation according to the invention These distances can according to an embodiment of the invention be measured and entered (manually or automatically) into a sound processing unit that implements the method of the invention during the system setup procedure. Compensating for these differences in the distances between the individual loudspeaker devices and the listening point P will be generally important although especially important in the high end of the low frequency region.

[0068] In an embodiment of the invention, the processing delay (or the differences between the processing delays) in the individual loudspeaker devices can also be compensated by respective filter devices inserted in the signal paths leading to the respective loudspeaker devices.

[0069] With reference to FIG. 7 there is shown a flow chart of an embodiment of the method according to the present invention, where the method starts at box 31 by providing the required number of loudspeaker devices to be used in a specific set-up in a room. It is however understood that even though the flow chart comprises a number of individual functional boxes 31 through 43 it may be advantageous in practical implementations of the method of the invention to combine some of the functions carried out in these boxes for instance in order to reduce the calculational requirements of algorithms that implement the method of the invention.

[0070] In box 32, the phase responses as a function of frequency obtained under anechoic conditions are provided for each individual loudspeaker device.

[0071] In box 33, the locations of each individual loudspeaker device in a specific room are chosen. According to the present invention there are no limitations to which locations within the room may be chosen (see for instance the set-up shown in FIG. 1a).

[0072] In box 34, a specific (preferred) listening position in the room is chosen. Again, according to the present invention, there are no limitations to which listening position within the room may be chosen (see for instance point P in the set-up shown in FIG. 1a).

[0073] In box 35, the distances between the listening point P and each individual loudspeaker device are determined.

[0074] If these distances are substantially equal, the method steps indicated in box 36a, 37, 42 and 43 are followed, whereas, if the distances are different, the method steps indicated in box 38, 39, 40, 41, 37, 42, 43 are followed. This decision point is indicated by box 36 in the flow chart. In a practical implementation, it must be defined what the term substantially the same means. One option is to define a threshold value of differences below which the differences are regarded as equal and above which they are regarded as different.

[0075] If it is determined that the distances between P and the individual loudspeakers are substantially equal, the correction filter H2i is made equal to zero for each individual loudspeaker device in box 36a. Then, the method continues to box 37, in which, for each individual of the loudspeaker devices there is determined a correction filter H1i (i designating the specific loudspeaker device) that makes the phase response of each individual loudspeaker device equal to a given (predefined) target phase response.

[0076] In the case where the distances between P and the individual loudspeaker devices are substantially equal, the resulting correction filters mentioned in box 42 are simply H1i and these resulting correction filters are in box 43 inserted in the signal path to each individual loudspeaker device. In this case, the correction filter H2i is made equal to zero for each individual loudspeaker device.

[0077] If the distances between P and each individual loudspeaker device are different, the method steps indicated in box 38, 39, 40, 41, 37, 42, 43 are followed.

[0078] In box 38, the loudspeaker device that is located at the greatest distance Lmax from the listening point P is determined. This is the loudspeaker device that does not require an additional phase correction H2i and is hence chosen as a reference loudspeaker device for determining the required additional phase corrections of the remaining loudspeaker devices.

[0079] In box 39, the differences between Lmax and the distances from the listening point P to the remaining loudspeaker devices are determined.

[0080] In box 40, the additional phase corrections that are required for compensating for the different distances from Lmax to the remaining loudspeaker devices are determined.

[0081] In box 41, there is for each of the remaining loudspeakers determined a correction filter H2i (i designating the specific loudspeaker device) that provides the additional phase correction that is necessary to compensate for the different distances between Lmax and the distance from P to each of the individual remaining loudspeaker devices.

[0082] Then, the method continues to box 37, in which the individual correction filters H1i are determined, as described above.

[0083] In box 42, for each of the loudspeaker devices a resulting phase correction filter is determined that is a combination of H1i and H2i and in box 43, these resulting compensation filters are inserted into the signal paths to the respective individual loudspeaker devices.

[0084] With reference to FIG. 8 there is shown an alternative embodiment of the method according to the invention. The difference between the embodiments shown in FIGS. 7 and 8 is that in the embodiment shown in FIG. 8, box 37 in which the individual correction filters H1i are determined is provided between boxes 32 and 33. This means that if it is determined in box 36 that the differences are substantially the same, the method continues directly to box 42 in which the correction filters H1i and H2i are combined, H2i in this case having been made equal to zero in box 36a, i.e. no additional phase corrections due to differences between the individual distances are required.

[0085] It is understood that although the method of the invention has been described by the two alternative embodiments shown in FIGS. 7 and 8, respectively, wherein the embodiments comprise a number of separate functional boxes it may be possible to implement some of these functional boxesand hence the corresponding method stepsdifferently, for instance by combining some of the functions of individual boxes or for some of the boxes to alter the order of the boxes in the flow charts.