Abstract:
The present invention relates to a method and a circuit for testing a tweeter. The tweeter is part of a loudspeaker system. The method includes the steps of: applying a high-frequency voltage signal to one terminal of the tweeter, whereby the high-frequency voltage signal is generated by first electronic means. The method also includes applying a constant voltage signal to the other terminal of the tweeter, whereby the constant voltage signal is generated by second electronic means. The method includes measuring a current (I load ) that flows through the tweeter into the second electronic means and determining a connect/disconnect state of the tweeter from the value of the current.

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to a method and a circuit for testing a high-frequency sound reproducing loudspeaker being part of a loudspeaker system. 
     DESCRIPTION OF THE RELATED ART 
     The output stages of loudspeaker systems, which are installed for instance on board motor vehicles, usually feature either a low frequency sound reproducing loudspeaker and a medium-frequency sound reproducing loudspeaker or a single medium-low sound frequency reproducing loudspeaker, which are generally directly connected to the amplifiers of such output stages. 
     An additional loudspeaker is usually provided, for reproducing high audio frequencies (also referred to hereinafter as “tweeter”), which is connected to the amplifiers of such output stages via a capacitor, as well as to the other loudspeakers. 
     Particularly, the operation of such loudspeaker systems is checked when they are installed in the vehicle. 
     Prior art diagnostic methods and circuits are known to be able to only ascertain the connect/disconnect state of the low and/or mid-frequency sound reproducing loudspeaker, because such loudspeaker is directly connected to the outputs of the output stage amplifiers. 
     A tweeter connected to the output stages via a capacitor cannot be tested using the methods and circuits developed for low and/or mid-frequency sound loudspeakers. 
     In view of obviating such drawbacks, it is known to use a circuit that implements a test during which an AC signal (typically an ultrasonic sine wave, e.g. at a frequency above 20 KHz) is transmitted to the tweeter and the current flowing in the tweeter is checked for its amplitude, to determine whether the tweeter is connected. 
     In recent times, Class D switching amplifiers are being increasingly used, also in the automotive field, and provide a much greater efficiency than Class AB amplifiers. 
     With reference to  FIG. 1 , there is shown a possible configuration of a bridge-type Class D switching amplifier  1  installed in a motor vehicle, which can drive a loudspeaker system  1 A. 
     The bridge-type switching amplifier  1  is schematically composed of a left arm  2  and a right arm  3 , each being coupled to a terminal of the loudspeaker system  1 A via pass-band filters  5  and  6 . 
     The left arm  2  has a first input  2 A, a second input  2 A′ and an output  2 C, the latter being in feedback relationship with the second input via a feedback line  2 B, and the right arm  3  also has a first input  3 A, a second input  3 A′ and an output  3 C, the latter being in feedback relationship with said second input  3 A′ via a feedback line  3 B. 
     As shown in  FIG. 1 , each of the left arm  2  and the right arm  3  has a feedback arrangement thanks to a feedback line  2 B and  3 B at a point  2 C and  3 C of the circuit  1 , upstream from the low-pass filter  5 ,  6 . 
     The loudspeaker system  1 A is embodied by a load  4 , as shown in  FIG. 2 , which can consist, for example, of a combination of a low frequency loudspeaker  4 A (woofer) and a high-frequency loudspeaker  4 B (tweeter). 
     As is shown, the tweeter  4 B is coupled to the woofer  4 A via a filter  4 C which can filter the high frequencies of the signal delivered by the amplifier  1 . 
     Each of the low-pass filters  5  and  6  includes an inductor L 1 , L 2  in series with a capacitor C 1 , C 2 . 
     Particularly, the inductor L 1  is connected on one side to the output  2 C of the left arm  2  of the amplifier, which output also acts as a virtual ground, and on the other side to the capacitor C 1  and to a terminal  4 D of the load  4 ; the capacitor C 1  in turn having a terminal connected to the ground. 
     The same applies to the low-pass filter  6 , in which the inductor L 2  is connected on one side to the output  3 C of the right arm  3  of the amplifier, which output also acts as a virtual ground, and on the other side to the capacitor C 2  and to a terminal  4 E of the load  4 ; the capacitor C 2  in turn having a terminal connected to the ground. 
     During operation of the amplifier  1 , the voltage at the output terminals  2 C and  3 C is a modulated square wave which is low-pass filtered by the filters  5  and  6  before being transmitted to the load  4 , so that the audio component to be reproduced by the load can be extracted from the square wave signal. 
     If low-pass filtering were not provided, there might be electromagnetic compatibility problems (electromagnetic interference, EMI) and an unnecessary high power would be dissipated, thereby causing damages to the load. 
     In order to determine whether the tweeter  4 D is actually connected to the terminals  4 D and  4 E, also with reference to  FIG. 1 , an electronic current-reading device  7  is provided, allowing measurement of the amplitude of the current I load  circulating in the tweeter  4 B. 
     In this configuration, the test for determining whether the tweeter  4 D of the loudspeaker system  1 A is actually connected to the terminals  4 D and  4 E, according to a specific method, is performed by applying a test voltage VinAC varying in frequency, e.g. at a frequency above 20 KHz, to each input terminal  2 A and  3 A of the arms  2  and  3  of the amplifier. 
     Particularly, a voltage +VinAC may be applied to the input  2 A, which voltage is replicated (at least ideally) by the feedback  2 B, to the terminal  4 D of the load  4 , and a voltage −VinAC may be applied to the input  3 A, i.e. a voltage opposite in phase to the voltage applied to the input  2 A, which is replicated (at least ideally) by the feedback  3 B to the terminal  4 E of the load  4 . 
     Nevertheless, the presence of the low-pass filters  5  and  6  causes problems in reading the proper current in the load  4 : the low-pass filters  5  and  6  at the frequencies of the variable test signal ±VinAC, of about 20 KHz, do not correspond to an infinite load, but a current I outamp  flows in such load  4 , and adds to the load current I load . 
     Thus, the current detection device  7  detects both the I load  current flowing into the load  4  and the current circulating in the capacitor C 2  (or the capacitor C 1  if the detection device  7  is coupled to the left arm  2  of the amplifier  1 ). 
     This may affect accuracy or make the method as described above for detecting the load  4  totally ineffective. 
     Also, with further reference to  FIGS. 3 and 4 , there are shown the results of two simulations of the circuit as shown in  FIG. 1 , in which the x axis indicates time in msec, and the y axis indicates current in amperes, when the load  4  is simulated as an impedance having a resistance value of 4Ω (see  FIG. 4 ). 
     In both simulations, L 1  and L 2  are assumed to be 20 μH and C 1 , C 2  are assumed to be 2 μF and Vout=4Vpeak (i.e. the potential difference between the points  4 D and  4 E when a sinusoidal peak voltage of +2V/−2V is applied to the input terminals  2 A and  3 A respectively). 
     Particularly, it can be noted that both the load current I load  and the current I outamp  flowing through the low-pass filter  6  into the left arm  3  flow into the load  4 , because the frequencies at which the variable test signal −Vin is applied do not correspond to an infinite load. 
     It should be noted that, for clarity, the simulations of  FIGS. 3 and 4  do not account for the current associated with the output square wave, typically of a relatively low value, and reduced to a negligible value by other techniques, which are well known to those of ordinary skill in the art and will not be described herein. 
     Still with reference to such  FIGS. 3 and 4 , the results of such simulations show that the current I load  that flows into the load  4  and the current I outamp  that flows in the right arm  3  can assume the following values: 
     if the load  4  is simulated by a 10 KΩ resistance (see  FIG. 3 ), corresponding to a situation in which such load  4  is an open circuit, the current I outamp  is in a range of peak values from −2 A to +2 A, whereas the current I load  that flows into the load is substantially zero; 
     if the load  4  is simulated by a 4Ω resistance (see  FIG. 4 ), corresponding to a situation in which such load  4  is a normal load (i.e. a normal loudspeaker combination), the current I outamp  is in a range of peak current values from about −1 A to +1 A, whereas the current I load  that flows into the load  4  is also in a range of peak current values from about −1 A to +1 A. 
     Apparently, no accurate detection is possible if the load  4  is simulated by a 10 KΩ resistance (see  FIG. 3 ) because, while the load current I load  has a negligible or zero value, the current I outamp  is very high, of about 2 A, due to the current that flows in the output filter  5 . 
     In other words, the device  7  reads a current value that cannot be used to determine whether the load  4  is actually disconnected. 
     BRIEF SUMMARY 
     Therefore, a need is strongly felt of checking the connect/disconnect state of a tweeter, to facilitate maintenance and/or testing. 
     In other words, a need is felt of checking for a disconnected terminal of a loudspeaker connected to the outputs via a capacitor. 
     One embodiment obviates the above mentioned problems of prior art testing methods and circuits. 
     One embodiment is a method for testing a tweeter being part of a loudspeaker system as defined by the features of claim  1 . 
     One embodiment is a circuit for testing a tweeter being part of a loudspeaker system as defined by the features of claim  7 . 
     Thanks to the present invention, a testing method and a testing circuit can be provided for more accurately determining whether a tweeter being part of a loudspeaker system is connected to the output stage of an amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The features and advantages of the invention will appear from the following detailed description of one practical embodiment, which is illustrated without limitation in the annexed drawings, in which: 
         FIG. 1  shows a possible circuit configuration of an output stage with a Class D switching amplifier when a load is connected to the terminals, according to the prior art, 
         FIG. 2  shows a schematic view of the load of  FIG. 1 , i.e. a possible circuit implementation of a loudspeaker system, according to the prior art; 
         FIGS. 3 and 4  show the results of simulations of the circuit as shown in  FIG. 1 ; 
         FIG. 5  shows a possible circuit implementation of the present invention; 
         FIGS. 6 and 7  show the results of simulations of the circuit as shown in  FIG. 5 ; 
         FIG. 8  shows a further possible circuit implementation of the present invention; 
         FIGS. 9 and 10  show the results of simulations of the circuit as shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 5 to 10 , in which the elements described above are designated by identical reference numerals, the circuit for testing a tweeter  4   b  being part of the load  4  is shown to comprise: 
     a first electronic circuit  8  for generating a voltage signal VinAC to be applied to a first terminal, such as the terminal  4 D, of the load  4 ; 
     a second electronic circuit  9  for generating a constant voltage signal VinDC to be applied to a second terminal, such as the terminal  4 E, of the load  4 ; 
     the current detection device  7  connected to the left arm  2  of said amplifier  1 , depending on where said second electronic means  9  are connected. 
     Particularly, as namely shown in  FIG. 5 : 
     the first electronic circuit  8  for generating a voltage signal VinAC includes a voltage generator  8 A that can preferably generate a sinusoidal voltage signal having a frequency above 20 KHz, which is coupled to the input terminal  2 A of the left arm  2 , 
     the second electronic circuit  9  for generating a voltage signal VinDC includes a voltage generator  9 A that can preferably generate a constant voltage signal which is coupled, for example, to the input terminal  3 A of the right arm  3  of the bridge-type switching amplifier. 
     In this configuration, the current detection device  7  is connected to the right arm  3  of the bridge-type switching amplifier  1 . Particularly, this current detection device  7  is connected to the output terminal  3 C of the right arm  3 , i.e. in the virtual ground point. 
     In an advantageous configuration, the voltage generator  9 A is preferably embodied by a grounding element, so that the input terminal  3 A of the right arm  3  of the amplifier  1  is at a constant zero value. 
     Advantageously, the test voltage signal to be applied to the input terminals  2 A,  3 A of the bridge-type switching amplifier and hence to the terminals  4 D,  4 E of the load  4 , is only present on one the input terminals, and hence on one of the outputs  2 C,  3 C. 
     In other words, the bridge-type switching amplifier  1  is controlled in a differential manner, i.e. voltage is applied to one input terminal, whereas the other terminal is grounded. 
     Particularly, the voltage VinAC is applied to the terminal  2 A, whereas the input terminal  3 A is grounded, which means that VinAC is present at the terminal  4 D and the terminal  4 E is grounded. 
     It shall be noted that the circuit configuration as shown in  FIG. 5  (although this also applies to the configuration of  FIG. 8 ) may be implemented by providing a dual arrangement of the first and second electronic circuits  8  and  9 . In other words, the first electronic circuit  8  generates the voltage signal VinAC to be applied to the terminal  4 E of the load  4  whereas the second electronic circuit  9  generates the constant voltage signal VinDC to be applied to the terminal  4 D of the load  4 , where the current detection device  7  is connected with the second electronic circuit  9 . 
     Referring now to the simulations of the circuit of  FIG. 5 , whose results are shown in  FIGS. 6 and 7 , and to allow comparison of such results with those of  FIGS. 3 and 4 , a voltage VinAC that corresponds to twice the voltage Vin (VinAC=2*Vin) is applied to the input terminal  2 A, by the generator  8 A, and grounding is applied to the input terminal  3 A by the generator  9 A, assuming that L 1 , L 2  are 20 μH and that C 1 , C 2  are 2 μF, so that such simulations show that the current I load  that flows into the load  4  and the current I outamp  that flows in the right arm  3  can assume the following values: 
     if the load  4  is simulated by an impedance having a resistive value of 10 KΩ (see  FIG. 6 ), corresponding to a situation in which such load  4  is an open circuit, the current I outamp  is lower than 40 mA and in a range of peak values from −30 mA to +30 mA, whereas the current I load  that flows into the load is nearly zero; 
     if the load  4  is simulated by an impedance having a resistive value of 4Ω (see  FIG. 4 ), corresponding to a situation in which such load  4  is a normal load (i.e. a normal loudspeaker combination), the current I outamp  is in a range of peak current values from about −3 A to +3 A, whereas the current I load  that flows into the load  4  is also in a range of peak current values from about −0.8 A to +0.8 A. 
     As shown by  FIG. 6 , the results of the simulations indicate that, with a 10 KΩ load  4 , an acceptable, although not perfect result can be achieved, because I outamp &lt;40 mA, whereas in the case of  FIG. 7 , in which the load  4  is 4Ω, the determination can lead to an error, because the current I outamp  is comparable to the value of the current that flows into the load I load . 
     In other words, once the current reading device  7  has completed its measurement process, it is possible to determine with a certain degree of certainty whether the load  4  is actually disconnected because I outamp &lt;40 mA, but it is not possible to determine with the same degree of certainty whether the load  4  is connected, because the value of the current I outamp  is comparable to the value of the current that flows into the load I load . 
     In certain cases, this can be a problem. 
     This occurs because, considering the specific circuit configuration as shown in  FIG. 5  and due to the frequencies of the test voltage VinAC, a certain amount of current may flow in the capacitor C 2  of the low-pass filter  6  thereby leading to an error in the detection of current I outamp . 
     Furthermore, such inaccuracy may be caused by a possible attenuation (overshoot) induced by the resonance frequency of the inductor L 2  of the low-pass filter  6 , which resonance frequency can cause the signal at the ends of the load  6  to be different from the signal that is set by the voltage generators  8 A and  9 A. 
     To obviate this problem, further referring to  FIG. 8 , in which the elements described above are designated by identical reference numerals, another circuit configuration  10  is provided for the bridge-type Class D switching amplifier, in which: 
     the left arm  2  includes a feedback line  2 B′ which is directly coupled to the terminal  4 D of the load  4 , 
     the right arm  3  includes a feedback line  3 B′ which is directly coupled to the terminal  4 E of the load  4 . 
     The advantage provided by the circuit configuration of  FIG. 8  is self-evident. 
     The voltage VinAC applied to the input terminal  2 A is transmitted nearly unchanged to the terminal  4 D of the load  4 , whereas the voltage VinDC applied to the input terminal  3 A is transmitted nearly unchanged to the terminal  4 E of the load  4 . 
     If a zero volt voltage VinDC is selected as an appropriate value, i.e. the input value  3 A is grounded, the terminal  4 E is also grounded because, thanks to the feedback line  3 B, the terminal  4 E acts as a virtual ground node. 
     In other words, the load  4  has the high-frequency voltage signal (frequency above 20 KHz) at the terminal  4 D and grounding at the other terminal  4 E, i.e. a potential difference corresponding to the voltage VinAC applied to the input terminal  2 A is provided in the load. 
     Referring now to the simulations of the circuit of  FIG. 8 , whose results are shown in  FIGS. 9 and 10 , and to allow comparison of such results with those of  FIGS. 3 and 4 , a voltage VinAC that corresponds to twice the voltage Vin is applied to the input terminal  2 A, by the generator  8 A, and grounding is applied to the input terminal  3 A by the generator  9 A, assuming that L 1 , L 2  are 20 μH and that C 1 , C 2  are 2 μF, so that such simulations show that the current I load  that flows into the load  4  and the current I outamp  that flows in the right arm  3  can assume the following values: 
     if the load  4  is simulated by a 10 KΩ resistance (see  FIG. 9 ), corresponding to a situation in which such load  4  is an open circuit, the current I outamp  and the current I load  are in a range of peak values of ±400 μA; 
     if the load  4  is simulated by a 4Ω resistance (see  FIG. 10 ), corresponding to a situation in which such load  4  is a normal load (i.e. a normal loudspeaker combination), the current I outamp  and the current I load  that flows into the load  4  are in a range of peak values of ±1 A. 
     In other words, the currents I outamp  and I load  coincide in either case, i.e. either when the load  4  is simulated by an impedance having a 10 kΩ resistance (see  FIG. 9 ) or when the load  4  is simulated by an impedance having a 4Ω resistance (see  FIG. 10 ), thereby eliminating any possible error. 
     Thus, the device  7  that reads the current flowing into the load  4  after measuring the amplitude of the current flowing into such load  4  determines whether the load is connected to the amplifier. 
     In other words, by applying a high-frequency voltage signal to the terminal  4 D of said load  4  and a constant voltage signal to the other terminal  4 E of said load  4 , it is possible to measure the current I load  that flows through said load  4  and determine a connect/disconnect state of said load  4  from the value of said current I load . 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.