Patent Publication Number: US-7912233-B2

Title: Speaker system for musical instruments

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present invention relates to Japan Patent Application 2005-271432, filed Sep. 20, 2005 including the specification, drawings, claims and abstract, is incorporated herein by reference in its entirety and from which a priority filing date is claimed. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to speaker systems and methods, and in specific embodiments, to speaker systems for musical instruments that detect displacement of the voice coil, and process such displacement information in feedback to alleviate non-linear distortion, in which the user has control over the amount of motion feedback applied. 
     2. Related Art 
     In speaker systems for audio use, motional feedback (hereinafter referred to as “MFB”) is known in which the displacement of the voice coil of a speaker or the displacement of a sensor cap and the like is detected. The detected displacement is used as a difference value with the input signal as negative feedback, and the difference value is amplified by a power amplifier to drive a speaker. 
     It is known that the non-linear movement of the speaker is vastly improved by means of this MFB processing. In Japanese Laid-Open Patent Application Publication (Kokai) Number H 10-276492, which is incorporated herein by reference in its entirety, an MFB speaker system is disclosed in which the output of a filter to which the displacement that has been detected and the sound signal are supplied is averaged, and an abnormal sound is avoided, even in those cases where the center of the voice coil oscillation displacement has shifted. 
     However, in the MFB speaker systems of the past, although it is possible to improve the non-linear speaker movement, such systems have been impractical for musical instruments, because it is not possible to actively set the characteristics such as the timbre and the like of the musical tones that are produced. 
     In prior speakers for musical instruments that produce many low tones such as a bass guitar and the like, or musical instruments that have a wide tonal range like a piano, when the low tones are reproduced, the amplitude of the mechanical oscillation of the voice coil and the cone paper of the speaker is great. As a result, because non-linear distortion is produced and high amplitude sounds hit a peak, it has not been possible to satisfactorily carry out dynamic expression. Because of this, the performer may tend to feel that he or she must perform more forcefully in order to make the musical tones that have been produced by one&#39;s own performance better. 
     SUMMARY OF THE DISCLOSURE 
     Therefore, a speaker system for a musical instrument according to one embodiment of the present invention is furnished with an input terminal to which an electrical signal may be input. A preamp is connected to alter the frequency characteristics of the electrical signal that has been input to the input terminal. A power amplifier is connected to amplify the electrical signal and to drive a speaker. Feedback means detects the displacement of the speaker and feeds back the signal that has been detected to the power amplifier. The power amplifier amplifies the electrical signal in conformance with the output of the preamp and the feedback signal that has been fed back by the feedback means. Motional feedback in the speaker system of a musical instrument, can provide the advantageous result that the non-linear distortion that is output from the speaker is low such that it is possible to more faithfully reproduce and output the dynamic expression of the performance by the performer. 
     A speaker system for a musical instrument according to a further embodiment is furnished with feedback amount setting means that sets the amount of the feedback signal that is fed back by the feedback means as desired. The power amplifier amplifies the electrical signal in conformance with the output of the preamp and the feedback signal, the feedback amount of which has been set by the feedback amount setting means. Accordingly, the amount of feedback can be set as desired by the feedback amount setting means and it is possible to set the timbre that the performer intends. 
     A speaker system for a musical instrument according to a further embodiment is one in which the speaker is furnished with a cylindrical shaped voice coil that has a reflecting plate in the center. A light source that radiates light toward the reflecting plate, and a photoreceptor element receives the light that has been reflected by the reflecting plate. Accordingly, it is possible to accurately detect the displacement due to the oscillation of the voice coil using an optical format. 
     A speaker system for a musical instrument according to yet a further embodiment is furnished with level detection means that detects the output level of the preamp. The feedback amount setting means sets the amount of the feedback in conformance with the level that has been detected by the level detection means. For example, in those cases where the output level of the preamp is high, even if the amount of the feedback is made large, if there is no margin in the power amplifier performance, this will, on the contrary, be the cause of the generation of the electrical distortion of the preamp. Accordingly, in this case, the amount of feedback when the output level of the preamp is high, may be made small and the amount of feedback may be made large when the output level of the preamp is low. On the other hand, in those cases where there is a margin in the power amplifier performance, by making the amount of the feedback large when the output level of the preamp is high, it is possible to broaden the dynamic range and to expand the breadth of performance expression. 
     A speaker system for a musical instrument according to yet a further embodiment is furnished with a volume control operator that sets the volume of the audio that is output by the speaker as desired. The feedback amount setting means sets the amount of the feedback in conformance with the amount of operation that has been set by the volume control operator. For example, the settings can be made such that in those cases where the output level has been set high using the volume control, the amount of the feedback is small and in those cases where the output level has been set low using the volume control, the amount of the feedback is large. 
     A speaker system for a musical instrument according to yet a further embodiment is one in which the preamp is furnished with an equalizer operator that sets each of the levels of a plurality of frequency bands as desired. The feedback amount setting means sets the amount of the feedback in conformance with the amount of operation that has been set by the equalizer operator. For example, the settings can be made such that in those cases where it has been set so that the level of the low register is high using the equalizer operator, the amount of the feedback is small and in those cases where it has been set so that the level of the low register is low using the equalizer operator, the amount of the feedback is large. 
     A speaker system for a musical instrument according to yet a further embodiment is furnished with a low pass filter through which the low frequency component of the output of the preamp passes. The level detection means detects the output level of the low pass filter. Accordingly, by detecting the output level of the low pass filter, it is possible to set the amount of the feedback in conformance with the level of the low-pitched sounds for which the effect of the MFB processing is particularly great. 
     A speaker system for a musical instrument according to yet a further embodiment is furnished with sense adjusting means that adjusts the value of the output level of the preamp that has been detected by the level detection means as desired. The feedback amount setting means sets the amount of the feedback in conformance with the value to which the value of the level that has been detected by the level detection means, as adjusted by the sense adjusting means. Accordingly, it is possible to set the percentage of the amount of the feedback that is set in conformance with the value of the output level of the preamp (the sensitivity) as desired by means of the sense adjusting means and a feedback level can be obtained that conforms to the output level of the preamp that the performer intends. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram that shows an electrical configuration of a speaker system for a musical instrument according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of an operating panel for a speaker system according to an embodiment of the present invention; 
         FIG. 3  is a cross-section drawing of a speaker of a speaker system according to an embodiment of the present invention; 
         FIG. 4  is a block diagram that shows an electrical configuration of a preamp section and a feedback section of a speaker system according to an embodiment of the present invention; 
         FIG. 5  is a graph that shows I/O functions; and 
         FIG. 6  are drawings that show characteristics of a musical tone that changes in accordance with feedback amount, where  6 ( a ) is a graph that shows frequency characteristics and  6 ( b ) is a graph that shows an output waveform in a case where a sine wave has been input. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a speaker system  1  of an embodiment of the present invention. Components of the speaker system  1  are described with reference to functional blocks and not necessarily discrete hardware elements. The functions may be implemented using one or more of hardware, software, and firmware. In addition, more than one function, or different parts of functions, may be combined in a given hardware, software, or firmware implementation. 
     The speaker system  1  may be a speaker system for a musical instrument, and comprises: an input terminal  2 , a preamplifier unit  10  (preamp unit), a feedback unit  20 , a power amplifier unit  30 , and a speaker section  40 . However, the speaker system may be used in other non-musical applications as well. An electric signal may be applied to a input terminal  51 , which then enters the preamp unit  10 . The electric signal may be generated from a musical instrument, such as an electronic piano, electronic keyboard, electric guitar, electric bass, or the like. The signal source might also be a pre-recorded signal stored on a compact disc, tape, computer hard drive, flash memory, or other storage medium. 
     The preamp unit  10  adjusts frequency characteristics and level of the electric signal applied to it from the input terminal  51 , and generates an output signal that is fed into the feedback unit  20 . The output of the preamp unit  10  and the output of the sensor  45  ( FIG. 3 ) that detects displacement of the voice coil  41  ( FIG. 3 ) of the speaker  40 , are input to the feedback unit  20 . The feedback unit  20  generates an output signal that is fed into the power amplifier unit  30 . The power amplifier unit  30  carries out power amplification of the output signal generated by the feedback unit  20 . This amplified signal drives the speaker section  40 . 
       FIG. 2  shows a schematic diagram of an operating panel  50  of the speaker system  1  of an embodiment of the present invention. The operating panel  50  comprises the input terminal  51 , a bass adjustment knob  52 , a mid-range adjustment knob  53 , a treble adjustment knob  54 , a motional feedback level adjustment knob (MFB knob)  55 , a dynamics sense knob  56 , and a volume control knob  57 . The bass  52 , mid-range  53 , and treble  54  adjustment knobs adjust the frequency characteristics of the input electric signal. The MFB knob  55  adjusts the amount of feedback of the sensor  45  output. The dynamics sense knob  56  adjusts the output level of the preamp unit  10  in those cases in which the output of the preamp unit  10  is detected and the amount of feedback of the output of the sensor  45  is changed in accordance with that level. The volume control knob  57  adjusts the volume of the speaker system  1 . 
     In an embodiment of the present invention, the input terminal  51  may be an input jack or socket that is configured to accept an output plug from a musical instrument. The plug can be freely connected to and disconnected from the input terminal  51 . The bass adjustment knob  52 , mid-range adjustment knob  53 , and treble adjustment knob  54 , respectively adjust parameters within the equalizer  12  (see  FIG. 4 ) that control the bass, mid-range, and treble portions of the frequency spectrum of the input electric signal. All of the knobs  52 ,  53 ,  54 ,  55 ,  56 ,  57  are fastened to the shaft of a rotating-type variable resistor. Adjusting the resistance of each variable resistor by turning the knobs affects the signal each is designated to control. 
       FIG. 3  illustrates a schematic diagram depicting the cross section of the speaker section  40 . The speaker section  40  may be a cone speaker that comprises a voice coil  41 , cone paper  42 , a magnet  43 , a reflecting plate  44 , a sensor  45 , a center cap  46 , a suspension module  47 , and a frame  48 . The voice coil  41  may have a cylindrical shape, and may be arranged so that it oscillates along an axis parallel to the length of the cylinder (from left to right in  FIG. 3 ). Oscillating current flow through the voice coil  41  wire creates a magnetic field around the voice coil  41  wire that alternates in direction. This alternating magnetic field induced by the voice coil  41  reacts with the magnetic field formed by the permanent magnet  43 , causing the entire voice coil  41  to oscillate at the same frequencies present in the signal flowing through the voice coil  41 . 
     The cone paper  42  is fastened to the voice coil  41 , causing it to oscillate with the voice coil  41 . As the cone paper  42  oscillates, it disrupts molecules in the medium surrounding it (typically air, but may also be liquid in alternate embodiments), creating sound waves and musical tones in the surrounding medium. The frame  48  forms the outer periphery of the speaker section  40  and acts as a support structure for the speaker section  40 . The suspension module  47  connects the cone paper  42  and voice coil  41  to the frame  48  and helps keep the cone paper  42  and the voice coil  41  centered with respect to the frame  48  along an axis perpendicular to the oscillation by the voice coil  41 . The center cap  46  is positioned over the center of the cone paper  42  to cover the voice coil  41 . 
     The reflecting plate  44  may be affixed to the side of the voice coil  41  closest to the center cap  46  (as shown in  FIG. 3 ). The reflective side of the reflecting plate  44  should face in the direction away from the center cap  46  and towards the light sensor  45 , located near the back of the speaker section  40 . Because the reflecting plate  44  is affixed to the voice coil  41 , it oscillates in tune with the voice coil  41 , cone paper  42 , and center cap  46 . Generally the reflecting plate  44  may be affixed to any component of the speaker section  40  that oscillates in rhythm with the voice coil  41 . In prior systems and methods, the reflecting plate  44  has been fastened to the center cap  46 . However, doing so may change the sound pressure characteristics of the center cap, causing undesirable distortion. 
     In an embodiment of the present invention, the center portion of the cylindrical shaped magnet  43  has a cylindrical shaped void that is concentric with the magnet  43 . In other embodiments, the magnet  43  may have other polygonal shapes in combination with a center void that may be cylindrical or any other polygonal shape. The light sensor  45  is furnished with a light source that radiates light through the void in the center of the magnet  43 . The light reflects off the reflecting plate  44  and is sensed by the photoreceptor element located on the light sensor  45 . The light source radiates light toward the reflecting plate  44  at all times while the power of the speaker system  1  is turned on. The photoreceptor element comprises a phototransistor or the like, which generates an electric signal according to an amount of light reflected by the reflecting plate  44  onto the phototransistor. The shorter the distance between the sensor  45  and the reflecting plate  44 , the higher the voltage generated by the photoreceptor element. The light sensor  45  is fastened to the frame  48  (not shown in  FIG. 3 ). 
       FIG. 4  shows a more detailed block diagram of some of the components that comprise the preamp unit  10  and feedback unit  20  of the speaker system  1 . The components are described with reference to functional blocks and not necessarily discrete hardware elements. The functions may be implemented using one or more of hardware, software, and firmware. In addition, more than one function, or different parts of functions, may be combined in a given hardware, software, or firmware implementation. 
     Referring to  FIG. 4 , the preamp unit  10  comprises a head amplifier (head amp)  11 , an equalizer  12 , and a volume control  13 . The head amp  11  amplifies the electric signal that has been applied to the input terminal  51 . The equalizer  12  adjusts the frequency characteristics of the now amplified electric signal in accordance with the settings of the bass adjustment knob  52 , mid-range adjustment knob  53 , and treble adjustment knob  54  (see  FIG. 2 ). The volume control  13  may be a variable resistor that that controls the amplitude of the electric signal in accordance with the settings of the volume adjustment knob  57 . 
     The feedback unit  20  comprises a low-pass filter  21 , a level detector  22 , an input/output function adjustment section (I/O adjustment section)  23 , a dynamics sense adjustment control  29 , a head amplifier (head amp)  24 , a filter  25 , a motional feedback level adjusting control unit (MFB control)  28 , a voltage controlled amplifier (VCA)  26 , and a differential amplifier  27 . 
     The output of the preamp unit  10  is applied to the positive terminal of the differential amplifier  27 , and to the input of the low-pass filter  21 . The low-pass filter  21  allows low frequency components of the input signal to pass through and be applied to the input of the level detector  22 . The frequency characteristics, such as cut-off frequency, of the low-pass filter  21  are set by the low register. As an example, the low-pass filter may have a cut-off frequency of 100 Hz. The level detector  22  carries out full wave rectification of the signal applied to its input, and acquires the absolute value of the signal. The resulting signal represents an envelope of the amplitude of the electric signal that was applied at the input of the low-pass filter  21 . The I/O adjustment section  23  takes in input signal values and scales them to generate output signal values in accordance with one or more of the various curves shown in  FIG. 5 . 
       FIG. 5  shows a plurality of conversion curves that the I/O adjustment section  23  may use when scaling input signal values. Generally, conversion curves “a” and “b” have a positive slope so that output signal values increase as input signal values increase. Specifically, conversion curve “a” has a curved shape such that the rate of increase in the output signal values are small when input signal values are small, and the rate of increase in the output signal values are large when the input signal values are large. Conversion curve “b” has curved shape such that the rate of increase in the output signal values are large when input signal values are small, and the rate of increase in the output signal values are small when input signal values are large. 
     Conversion curves “c” and “d” generally have a negative slope so that output signal values decrease as input signal values increase. Specifically, conversion curve “c” has a curved shape such that the rate of decrease in the output signal values are small when input signal values are small, and the rate of decrease in the output signal values are large when the input signal values are large. Conversion curve “d” has curved shape such that the rate of decrease in the output signal values are large when input signal values are small, and the rate of decrease in the output signal values are small when input signal values are large. 
     The output of the I/O adjustment section  23  is then applied to the dynamics sense adjustment control  29  which may be a variable resistor that controls the amplitude of the electric signal at that point in accordance with the settings of the dynamics sense knob  56 . The output of the dynamics sense adjustment control  29  is then applied to the control terminal of the VCA  26 . 
     The photoreceptive element of the light sensor  45  generates an electric signal that represents the displacement as a function of time of the voice coil  41  and attached cone paper  42  and center cap  46 . This signal is amplified by the head amp  24  and then filtered by a secondary differentiation filter  25 . The filter  25  output signal represents the acceleration as a function of time of the voice coil  41 , and attached cone paper  42  and center cap  46 . This quantity is useful because the sound pressure characteristics of the cone paper  42  and center cap  46  (and thus tones produced by the cone speaker) are proportional to the acceleration of the voice coil  41 , and attached cone paper  42  and center cap  46 . 
     The output of the filter  25  is then applied to the MFB control  28 , which may be a variable resistor that controls the amplitude of the electric signal at that point, in accordance with the settings of the MFB knob  55 . Thus the MFB control  28  controls the level of the feedback signal that is applied to the differential amplifier  27 . The output of the MFB control  28  is then applied to the input terminal of the VCA  26 . The VCA  26  is an amplifier whose amplification gain can be varied based on the voltage level supplied to its control terminal. The output of the VCA  26  is applied to the negative terminal of the differential amplifier  27 . The differential amplifier  27  amplifies the signal difference between its positive and negative terminals, and outputs the result to the power amplifier unit  30 . The power amplifier unit  30  amplifies the signal and applies it to the speaker section  40 . 
       FIG. 6(   a ) illustrates the frequency response of the speaker system  1  with varied levels of motional feedback signal applied by the MFB level adjusting control  28 . The graph depicts sound pressure (vertical axis) produced by the speaker section  40  as a function of frequency (horizontal axis). The solid line represents the frequency response of the speaker system  1  when the motional feedback amount is zero; the dashed line represents the frequency response of the speaker system  1  when the motional feedback amount is small; and the long and short dashed line represents the frequency response of the speaker system  1  when the motional feedback amount is great. As the amount of motional feedback is increased, the sound pressure of the speaker section  40  increases for lower frequencies, giving the speaker system  1  a flatter, wider frequency response. 
     Although it is not shown in  FIG. 6(   a ), when motional feedback is applied, the sound pressure at mid-range and high frequency areas of the frequency response (where the dashed line and long and short dashed line are flat) is slightly lower then when the amount of motional feedback is zero. However, an amplifier may be used at the output signal of the preamplifier unit  10  to gain up the signal in proportion to the amount of motional feedback that is to be applied. This ensures that the gain of the speaker system at mid-range and high frequencies when motional feedback is applied is comparable to the gain at those frequencies when motional feedback is not applied. 
       FIG. 6(   b ) shows the output of the speaker section  40  in the time domain with varied levels of motional feedback applied by the MFB level control  28 . The graph depicts sound pressure (vertical axis) produced by the speaker section  40  as a function of time (horizontal axis) for a 50 Hz sine wave signal applied to the input terminal  51 . The solid line represents the sound pressure as a function of time for the speaker system  1  when the motional feedback amount is zero; the dashed line represents the sound pressure as a function of time for the speaker system  1  when the motional feedback amount is small; and the long and short dashed line represents the sound pressure as a function of time for the speaker system  1  when the motional feedback amount is great. When the amount of motional feedback applied is zero, there is considerable distortion and compression of the 50 Hz sine wave. As motional feedback is applied, the amount of distortion is reduced, resulting in a fairly faithful reproduction of the sound wave when the amount of motional feedback is great. 
     In the embodiments of the speaker system  1  described above, detection of the mechanical oscillation of the speaker section  40  to provide and utilize a feedback signal, reduces low frequency signal distortion to produce musical tones having optimum timbre. 
     In an embodiment of the present invention, the amount of feedback applied by the MFB control  28  can be reduced when the signal output level of the preamp unit  10  is large (the volume control knob  57  setting is high) to not overdrive the power amplifier  30  and to help reduce distortion. 
     In the embodiments of the present invention described above, the preamp unit  10  and the feedback unit  20  may be comprised of analog circuit devices. In alternate embodiments of the present invention, the preamp unit  10  and the feedback unit  20  may be comprised of digital circuit components, or a combination of analog and digital circuit components. An analog to digital (A/D) converter may digitize the analog input signals applied to the input terminal  51  at a specific sampling rate, and the functions performed by the preamp unit  10  and feedback unit  20  may also be carried out using digital circuit components, such as but not limited to digital signal processors (DSPs) and field programmable gate arrays (FPGAs). 
     Thus, functions performed by the head amp  11 , equalizer  12 , volume control  13 , low-pass filter  21 , level detector  22 , I/O adjustment section  23 , and dynamics sense control  29 , may all be performed in the digital domain by use of digital components, such as DSPs and/or FPGAs. Also, an A/D may digitize the analog signal provided by the light sensor  45  before applying it to the head amp  24 . The functions performed by the head amp  24 , filter  25 , MFB control  28 , and VCA  26 , may also be performed using digital circuit components, such as one or more of DSPs and/or FPGAs. A digital to analog converter (D/A) may then be used to convert the digital signal output from the digital circuit components to an analog signal that may be amplified by the power amplifier  30 . 
     In the embodiments of the present invention described above, the feedback unit  20  includes a low-pass filter  21 , level detector  22 , I/O adjustment section  23 , dynamics sense control  29 , and VCA  26 . These components helped adjust the amount of motional feedback applied based on the signal output level from the preamp unit  10 . However, in other embodiments of the present invention, the feedback unit  20  may be only comprised of one head amp  24 , a filter  25 , a MFB control  28 , and a differential amplifier  27 . The output of the preamp unit  10  may be input to the positive terminal of the differential amplifier  27 . The output of sensor  45  may be applied to the head amp  24 ; the output of the head amp  24  may be applied to the filter  25 ; the output of the filter  25  may be applied to the MFB control  28 ; and the output of the MFB control  28  may be directly connected to the negative terminal of the differential amplifier  27 . 
     In other embodiments of the present invention, the I/O adjustment section  23  may be configured to scale the signal value detected by the sensor  45 , and applying that scaled value to the differential amplifier  27 . 
     Although in some embodiments the output signal level detected of the preamp unit  10  adjusts the amount of feedback applied, in other embodiments the level set at the volume control  13  by the volume control knob  57  may control the amount of feedback applied. In yet other embodiments, the amount of feedback applied may be based on the equalizer  12  settings controlled by the bass  52 , mid-range  53 , and treble control knobs  54 . 
     In yet other embodiments of the present invention, the reflecting plate  44  and the sensor  45  may be replaced by attaching a piezoelectric element to the voice coil  41  that senses the acceleration of the voice coil  41 . In such a configuration the filter  25  is not necessary. 
     The embodiments disclosed herein are to be considered in all respects as illustrative, and not restrictive of the invention. The present invention is in no way limited to the embodiments described above. Various modifications and changes may be made to the embodiments without departing from the spirit and scope of the invention. The scope of the invention is indicated by the attached claims, rather than the embodiments. Various modifications and changes that come within the meaning and range of equivalency of the claims are intended to be within the scope of the invention.