Abstract:
In a device having a sound tube and a receiver, a test signal is generated to apply to the receiver. An outgoing acoustic wave is created at the receiver from the test signal. A reflected acoustic wave is received at the receiver, wherein the reflected acoustic wave is delayed in time from the outgoing acoustic wave. A difference in time is measured between the outgoing acoustic wave and the reflected acoustic wave. The difference in time is used to estimate a length of the sound tube.

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is related to the following U.S. applications commonly owned together with this application by Motorola, Inc.: 
     Ser. No. 10/33/284 T. B. filed Dec. 27, 2002, titled “Adaptive Equalizer for Variable Length Sound Tubes Utilizing an Acoustic Pressure Response Measurement” by Willis; and Ser. No. 10/33/281 T. B. filed Dec. 27, 2002, titled “Adaptive Equalizer for Variable Length Sound Tubes Utilizing an Electrical Impedance Measurement” by Willis. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an adaptive equalizer for variable length sound tubes utilizing an acoustical time of flight measurement. 
     BACKGROUND OF THE INVENTION 
     Some users of earpiece accessories have a strong preference for certain types of sound delivery systems over others (e.g., an accessory sound delivery tube that terminates in the ear canal versus an intraconcha (inside the bowl of the ear) device). The equalization used to control acoustical standing waves in the earpiece tubing has been shown to have a key role in lowering the threshold of feedback in the earpiece as well as improving the quality of the audio presented to the user. The standing wave frequencies are a function of the combined length of the sound tube and accessory tube; and therefore, the equalization must be tuned to the particular delivery system used. For example, an accessory that terminates near the entrance to the ear canal results in a total tube length different than one that is inserted into the ear canal. This requires a realignment of the equalization in order to maintain feedback suppression and optimum sound quality. 
     Thus, there exists a need for being able to recognize the length of the accessory sound tube that is connected to the earpiece in order to select the proper settings for the equalizer. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which: 
     FIG. 1 illustrates a simple block diagram of an earpiece for the acoustic pressure response technique in accordance with the present invention; 
     FIG. 2 illustrates a graph of sound pressure level produced by a receiver for various tubing lengths in accordance with the present invention; 
     FIG. 3 illustrates a simple block diagram of an earpiece for the electrical impedance and time of flight measurement techniques in accordance with the present invention; and 
     FIG. 4 illustrates a graph of the electrical impedance of a receiver for various tubing lengths in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements. For clarity, the present invention defines the sound tube as the section of tubing inside the earpiece, and the accessory as the section of interchangeable tubing outside the earpiece. 
     The present invention proposes a solution for selecting the proper setting for an equalizer (realigning the equalization) based on the length of tubing coupled to a device (e.g., an earpiece). The term “tubing” will refer to the total length of tubing (sound and/or accessory tubes) coupled to the device. Typically, the sound tube is at least partially internal to the device. Thus, the sound tube may have an adjustable length or may have an interchangeable accessory attached thereto. As noted above, realigning the equalization based on the length of tubing maintains feedback suppression and optimizes sound quality. The present invention focuses on three techniques for estimating the total length of tubing coupled to the device: an acoustic pressure response technique, an electrical impedance response technique, and a time of flight measurement technique. Once the total length of tubing is estimated, the present invention is able to select the proper settings for the equalizer. 
     Let us first discuss the acoustic pressure response technique. FIG. 1 illustrates an example of a simple block diagram of an earpiece  100 . As shown, when a receive audio signal  102  enters the earpiece  100 , it is processed by a digital signal processor (“DSP”)  104 ; the DSP  104  comprises an equalizer  106  for equalization of the receive audio signal  102  to compensate for the acoustical response of a receiver  108 , sound and/or accessory tubes  110 . The receive audio signal  102  is converted into an analog electrical signal  112  by a digital-to-analog (“D/A”) converter  114 . The analog electrical signal  112  is fed into an amplifier  116 , and the output of the amplifier  116  is coupled to the receiver  108 . The receiver  108  converts the analog electrical signal  112  to an acoustical output  118 . The sound and/or accessory tubes  110  are coupled to the receiver  108  to deliver the acoustical output  118  to the user&#39;s ear canal  120 . Typical responses of the acoustic pressure delivered to the user&#39;s ear canal  120  for varying lengths of tubing (i.e., the combination of both the sound tube and the accessory)  110  are illustrated in FIG.  2 . 
     The send audio path consists of a microphone  122 , which picks up the user&#39;s voice  124 . The output of the microphone  122  is fed into an analog-to-digital (“A/D”) converter  126 , which sends a digitized signal  128  to the DSP  104 . The digitized signal  128  is passed to the send output  130 . 
     Acoustical coupling exists between the receive and send audio paths due to acoustic leakage  132  resulting from sound leaking out of the user&#39;s ear  120  and finding its way into the microphone  122 . The present invention takes advantage of the acoustic leakage  132  by using the microphone  122  in the earpiece  100  to measure the acoustic pressure response of the sound and/or accessory tubes  110  when a test signal (not shown) is played through the receiver  108 . The present invention estimates the total length of the tubing (i.e., the sound tube and/or the accessory)  10  by the frequency of at least one peak in the acoustic pressure response and the sound speed in the tubing  110 . The present invention assumes that the sound speed in the tubing  110  is non-dispersive. 
     In order to estimate the total length of the tubing in accordance with the present invention, a test signal is needed. The test signal may comprise a sequential set of stepped tones. Generation of the test tones can be simplified in the DSP  104  by using square waves close to the highest frequency peak in the acoustic pressure response. Since this peak is near the upper limit of the earpiece passband, the square wave harmonics will fall well outside the passband and be attenuated. The algorithm then steps through a number of frequencies surrounding the peak. From this data, a curve fit of the acoustic pressure response over a limited frequency band surrounding the highest frequency peak can be constructed. The maximum of the curve fit will be at a resonance frequency of the sound tube. This frequency is then used to tune the equalizer response to the attached accessory. 
     Using the lengths of the sound and/or accessory tubes  110 , an equalizer is selected or designed. For example, if a lookup table is used, the length of the sound and/or accessory tubes  110  will identify which accessory has been attached to the earpiece  100 . The appropriate equalizer for that accessory can be loaded into the DSP  104  to optimize audio quality and earpiece stability. Alternatively, the lengths of the sound and/or accessory tubes  110  may be used to design an equalizer; preferably, a model of the acoustic pressure response, in which the tubing length is an adjustable parameter, is used to define an inverse equalizer to compensate for the responses for the receiver  108  and sound and/or accessory tubes  110 . 
     Let us now move the discussion to the electrical impedance response solution in accordance with the present invention. A similar method to the acoustic pressure response technique as described above can be done using the electrical impedance of the receiver. The electrical impedance response of the receiver is strongly influenced by the acoustical loading of the standing waves in the tubing. As illustrated in FIG. 3, measuring the electrical impedance response requires the addition of a resistor  300  in series with the receiver  108 , and a double pole, double throw switch  302  to bypass the resistor  300  during normal operation of the earpiece  100  and to select the input to the A/D converter  126  in order to measure the voltage across the receiver  108 ; alternatively, current could also be measured through the receiver. 
     With the switch  302  in test mode, such that the resistor  300  is not being bypassed and the A/D converter  126  is measuring the voltage across the receiver  108 , the voltage measured across the receiver  108  is in proportion to the electrical impedance response. An example of the electrical impedance response of a balanced armature receiver is illustrated in FIG.  4 . Although the inductive and capacitive elements in the electro-acoustic model of the receiver prevents the minimums and maximums in the electrical impedance response from coinciding with the standing wave frequencies of the tubing, the minimums and maximums in the electrical impedance response illustrated in FIG. 4 shift in accordance with the standing wave frequencies in FIG.  2 . Using the square wave technique described in the acoustic pressure response method, shifts in the resonant frequencies of the tubing can be determined from the electrical impedance response of the receiver. FIG. 4 illustrates that the electrical impedance response is much more sensitive to the standing waves than the acoustic pressure response. 
     The total length of the sound and/or accessory tubes  110  is estimated by the shift in the frequency of at least one peak in the electrical impedance response. This requires that the tubing length is known a priori for one corresponding peak frequency in the electrical impedance response. Using the length of the sound tube and accessory together, an equalizer is selected or designed as described above. 
     The third technique, the time of flight measurement, estimates the length of the sound and/or accessory tubing by measuring the time required for an acoustical pulse produced by the receiver  108  to propagate down the length of the tubing, reflect at the open end of the accessory, and return to the receiver  108 . The length of the tubing is one half the time of flight to the end of the tubing and back to the receiver multiplied by the speed of sound in the tubing, where the speed of sound is constant over the length of the tubing. A more complicated equation results if the sound speed varies along the length of the tubing. 
     The hardware configuration required for the time of flight measurement is the same as for the electrical impedance response, however, the receiver  108  is used as both an acoustical source and receiving device. The resistor  300  is necessary in this case to prevent the returning pulse from being shorted out by the low output impedance of the amplifier  116 . The acoustical pulse must be shorter than the time required for the pulse to travel down the length of the tubing, reflect at the open end of the accessory, and return to the receiver  108 . 
     In operation, using the time of flight technique, the DSP  104  generates a test pulse  302 . The test pulse  302  exits the D/A converter  114  and goes through the series resistor  300 . At this point, the A/D converter  126  detects the outgoing test pulse. The receiver  108 , which is in parallel with the AID converter  126 , receives the test pulse at the same time as the AID converter  126 , and generates an acoustic pulse  304  that travels down the length of the tubing  110 , reflects  308  at the open end  306  of the accessory and travels back to the receiver  108 . A voltage develops across the series resistor  300  produced by the receiver  108  that now acts as a dynamic microphone. The A/D converter  126  detects the reflected pulse  308 . The difference in time between the outgoing test pulse  302  and its reflection  308  is used to calculate the length of the tubing. 
     While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.