Abstract:
The invention concerns a method ( 300 ) and system ( 100 ) for controlling audio output. The method includes the steps of inputting ( 312 ) an audio signal and a voltage level signal, measuring ( 314 ) the audio signal and the voltage level signal, mapping ( 316 ) the audio signal against at least one table ( 134 ) of predetermined corresponding gain targets ( 138 ) and selecting ( 318 ) at least one gain target for the audio signal. The mapping step and the selecting step are based at least in part on the measurement of the voltage level signal and the measurement of the audio signal. The method also includes the step of applying ( 320 ) the gain target to the audio signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     BACKGROUND 
     1. Technical Field 
     This invention relates in general to handheld devices and more particularly, to audio systems for such devices. 
     2. Description of the Related Art 
     Many handheld devices, such as personal digital assistants (PDA), mobile telecommunications units and gaming units, use batteries to power their electronics. In most of these devices, one or more speakers are included to broadcast voices or digitally generated tones, such as musical notes. 
     In general, a power amplifier is used to drive the speakers in these devices. The batteries in these devices supply voltage directly to the power amplifier. As a result, the output of the power amplifier is dependent on the voltage supplied by the batteries. Because of this relationship, as the charge on the batteries begins to drop, the output of the power amplifier is affected. Notably, however, the signal input to the power amplifier, which has a much lower voltage level than that of the output of the power amplifier, is literally independent of the battery voltage swings. 
     Accordingly, when the battery voltage drops, the output signal from the power amplifier may be clipped. This clipping will produce low quality audio, and the user of the device may hear a distorted signal when it is broadcast from the speaker. Thus, there is a need for a system and method for maintaining the quality of audio generated by a device that outputs audio when the power supply of that device begins to deteriorate. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a method for controlling audio output. The method includes the steps of inputting an audio signal and a voltage level signal, measuring the audio signal and the voltage level signal, mapping the audio signal against at least one table of predetermined corresponding gain targets, selecting at least one gain target for the audio signal and applying the gain targets to the audio signal. The mapping step and the selecting step are based at least in part on the measurement of the voltage level signal and the measurement of the audio signal. 
     In one arrangement, at least a portion of the predetermined corresponding gain targets can describe at least one transfer function that can compress the audio signal, which can reduce clipping of the audio signal. In addition, as the value of the audio signal increases as determined during the measuring step, the selection of the gain targets can be such that the compression of the audio signal is increased. 
     In another arrangement, the mapping step can further include mapping the audio signal against one of a plurality of tables of predetermined corresponding gain targets. At least a portion of the predetermined corresponding gain targets for a first table can describe at least one transfer function that can compress the audio signal, and the predetermined corresponding gain targets for each successive table can describe at least one transfer function that can compress the audio signal to a greater degree than transfer functions described by the corresponding gain targets of a previous table. Each of the tables can correspond to one of a plurality of predetermined ranges of the voltage level signal. Also, as the voltage level signal decreases and falls from a first predetermined range to a second predetermined range, the table against which the audio signal is mapped can contain predetermined corresponding gain targets that can describe at least one transfer function that can result in an increase in the compression of the audio signal as compared to the transfer functions described by the corresponding gain targets of a previous table. 
     The inputting, measuring, mapping, selecting and applying steps can be performed in a communication device. The communication device can have a power amplifier coupled to a speaker, and the method can further include the step of outputting the audio signal to the power amplifier. Also, the method can further include the step of digitizing the voltage level signal prior to the inputting step. In another arrangement, when the audio signal is measured to be below a predetermined threshold, the predetermined corresponding gain targets selected from the table describe a transfer function of unity. 
     The present invention also concerns a system for controlling audio output. The system includes a feedback loop and a compressor. The feedback loop inputs a voltage level signal to the compressor, and the compressor samples an audio signal. The compressor can measure the audio signal and the voltage level signal, map the audio signal against at least one table of predetermined corresponding gain targets, select at least one gain target for the audio signal and apply the gain targets to the audio signal. The compressor maps the audio signal and selects the gain targets based at least in part on the measurement of the audio signal and the measurement of the voltage level signal. The system also includes suitable software and circuitry to carry out the processes described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
         FIG. 1  illustrates a system for controlling audio output in accordance with the inventive arrangements; 
         FIG. 2  illustrates in detail a component of the system of  FIG. 1  in accordance with the inventive arrangements; 
         FIG. 3  illustrates a method for controlling audio output in accordance with the inventive arrangements; 
         FIG. 4  illustrates a graph of a compression curve in accordance with the inventive arrangements; 
         FIG. 5  illustrates a graph of a plurality of compression curves in accordance with the inventive arrangements. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. 
     Referring to  FIG. 1 , a system  100  for controlling audio output is shown. The system  100  can include a compressor  110 , a feedback loop  112 , a digital-to-analog (D/A) converter  114 , a power amplifier  116  and a speaker  118 . The feedback loop  112  may contain an analog-to-digital (A/D) converter  120 . Furthermore, an input  122  can be coupled to the compressor  110 , and the compressor  110  can be coupled to the D/A converter  114 . The D/A converter  114  can be coupled to the power amplifier  116 , which can be coupled to the speaker  118 . A voltage supply +V can be coupled to the power amplifier  116 , and the feedback loop  112  can couple the voltage supply +V to the compressor  110  through the A/D converter  120 . As such, a voltage level signal can be fed to the compressor  110  through the feedback loop  112 . In one arrangement, each of the components listed above can be part of a communications device  124 , such as a mobile phone or a personal digital assistant. Of course, the invention is not limited in this regard, as each of these components can be part of any other suitable device. Moreover, those of ordinary skill in the art will appreciate that the system  100  is not limited to the components shown. 
     The input  122  can input, for example, digital audio signals to the compressor  110 , and, as will be explained later, the compressor  110  can modify the audio signals. The compressor  110  can then transfer the audio signals to the D/A converter  114 , which can convert the audio signals to an analog signal and forward them to the power amplifier  116 . The power amplifier  116  can drive the speaker  118 , and the speaker  118  can broadcast the audio signals. 
     In one arrangement, the A/D converter  120  can digitize the voltage level signal from the voltage supply +V, and the compressor  110 , through the feedback loop  112 , can monitor or measure this voltage level signal. Additionally, the compressor  110  can measure the input audio signals. Based on these measurements, the compressor  110  can modify the audio signals. For example, if the compressor  110  determines that the voltage level signal, and hence the voltage supply +V, drops to a certain threshold, the compressor  110  can compress the audio signal, which can help prevent the audio signal from clipping. This process will be explained below in detail. 
     Referring to  FIG. 2 , some of the internal components of the compressor  110  are shown. As an example, the compressor  110  can include an absolute value component  126 , a comparator  128 , memory  130  and a gain component  132 . At least one table  134  can be programmed into the memory  130 , and the comparator  128  can have access to each of these tables  134 . In one arrangement, each table  134  can include predetermined input measurements  136  and predetermined gain targets  138  that correspond to the predetermined input measurements  136 . The input measurements  136  can be predetermined signal levels of, for example, an audio signal. Further, the corresponding gain targets  138  can describe at least one transfer function that, when applied to the input audio signal, can modify the signal, such as by compressing it. The invention can include any suitable number of tables  134 , even just one. In one particular arrangement, each table  134  can be associated with or correspond to one of a plurality of predetermined ranges  140  of the voltage level signal. Any suitable number of predetermined ranges  140  can be employed in the invention. 
     Referring to  FIG. 3 , a method  300  of controlling audio output is illustrated. To explain the method  300 , reference will be made to the components illustrated in  FIGS. 1 and 2 , although it must be noted that the method  300  can be practiced in any other suitable system. At step  310 , the method  300  can begin, and at step  312 , an audio signal, such as a digital audio signal, and a voltage level signal can be input. As an example and referring to  FIGS. 1 and 2 , an audio signal can be input into the comparator  128  of the compressor  110  through the input  122 . In one arrangement, the absolute value component  126  can calculate the absolute value of the audio signal and transfer it to the comparator  128 , as shown in  FIG. 2 . Additionally, as illustrated in  FIGS. 1 and 2 , a voltage level signal can be input to the comparator  128  of the compressor  110  through the feedback loop  112 . As such, the comparator  128  is able to monitor the supply voltage +V that is being supplied to the power amplifier  116 . In one arrangement, the A/D converter  120  can digitize the voltage level signal prior to the voltage level signal being fed to the comparator  128 . 
     Referring back to the method  300  of  FIG. 3 , the audio signal and the voltage level signal can be measured, as shown at step  314 . As an example, referring once again to  FIGS. 1 and 2 , the comparator  128  can measure the input audio signal received from the input  122  and the voltage level signal received from the feedback loop  112 . Referring back to  FIG. 3 , at step  316 , the audio signal can be mapped against at least one table of predetermined corresponding gain targets, and at step  318 , at least one corresponding gain target can be selected. In one particular arrangement, the table against which the audio signal is mapped and the gain targets that are selected can be based at least in part on the measurement of the voltage level signal and the measurement of the audio signal. 
     As an example, referring to  FIG. 2 , after the comparator  128  has measured the voltage level signal, the comparator  128  can compare or map the audio signal against one of the tables  134 . This mapping step can include the comparator  128  selecting a table  134  that corresponds to the predetermined range  140  in which the voltage level signal falls. For example, if the voltage level signal, which can be referred to as V LS , is less than a predetermined value of V 0 , then comparator  128  can map the audio signal against the table  134  designated by the reference numeral  1 . As another example, if the V LS  is higher than the predetermined value V 1  but less than or equal to the predetermined value V 2 , the comparator  128  can map the audio signal against the table  134  designated by the reference numeral  3 . 
     When the comparator  128  maps the audio signal against a particular table  134 , the comparator  128  can compare the values of the input audio signal with the predetermined input measurements  136 . Based on this comparison, the comparator  128  can then select the gain targets  138  that correspond to the input measurements  136  that match the values of the audio signal. 
     Referring back to the method  300  of  FIG. 3 , the gain targets can be applied to the signal, as shown at step  320 , and the method  300  can end at step  322 . As an example and referring back to  FIG. 2 , once the corresponding gain targets  138  have been selected, the gain component  132  can apply the selected gain targets  138  to the audio signal, which can then be transmitted to the D/A converter  114 . 
     In one arrangement, at least a portion of the gain targets  138  in the table  134  against which the audio signal is mapped can describe at least one transfer function that compresses the audio signal. As such, when the comparator  128  detects a decrease in the supply voltage +V (see  FIG. 1 ) when it measures the voltage level signal, the compression can reduce the chances that the audio signal will be clipped as a result of the lowered supply voltage +V. 
     In one particular arrangement, for a particular table  134 , as the value of the input audio signal increases, corresponding gain targets  138  that describe at least one transfer function that provides greater compression can be selected. Conversely, as the value of the audio signal decreases, corresponding gain targets  138  that result in less or even no compression can be selected. An example of this process is illustrated in  FIG. 4 , which shows a graph of a compression curve that can be applied to an audio signal in which the graph can be based on the input audio values to, for example, the compressor  110  and the output audio values from, for example, the compressor  110 . Referring to a curve  410 , as the value of the input audio signal increases, the compression of the audio signal can increase. In one arrangement, if the value of the input audio signal remains at or below a predetermined threshold I T , the audio signal can undergo little or no compression. As an example, when the input audio signal is at or below I T , the corresponding gain targets  138  that the comparator  128  selects can describe a transfer function of one or unity. It is understood, however, that the invention is not limited in this regard, as compression of the audio signal may occur at any other suitable value or may not occur at all. 
     In another arrangement, referring back to  FIG. 2 , if a plurality of tables  134  are employed, the amount of compression can vary based on which particular table  134  is selected, i.e., against which table  134  the audio signal is mapped. For example, if the comparator  128  maps the audio signal against the table  134  represented by the reference numeral  2 , the amount of compression provided by this table  134  can be less than that provided by the selection of the gain targets  138  of the table  134  designated by the reference numeral  1 . The table  134  represented by the reference numeral  2  is associated with a higher voltage level signal than that of the table  134  represented by the reference numeral  1 . Thus, for a higher voltage level signal, the compression of the audio signal can be less based on the selection of a particular table  134 . Conversely, for a lower voltage level signal, the compression of the audio signal can be greater, as the table  134  against which the audio signal is mapped contains target gains  138  that result in greater compression of the audio signal. 
     This particular compression scheme can apply to all the tables  134  in the compressor  110  such that the gain targets  138  for a first table  134  describe at least one transfer function that compresses the audio signal, and the gain targets  134  for each successive table  134  can describe transfer functions that compress the audio signal to a greater degree than the transfer functions described by the gain targets  138  of previous tables  134 . An example of this scheme is illustrated in  FIG. 5 , which shows a graph of several compression curves that can be applied to audio signals. 
     In  FIG. 5 , curves  510 ,  512 ,  514  and  516  represent compression curves in which each curve can be associated with a particular table  134  and hence, a particular predetermined range  140  of the voltage level signal. For example, referring to  FIGS. 2 and 5 , the curve  510  can represent the compression curve to be applied to the audio signal when the audio signal has been mapped against the table  134  designated by the reference numeral  1 . This table  134  is associated with the predetermined range  140  in which V LS  is less than or equal to V 0 . This predetermined range  140  is the lowest one into which the V LS  may fall, and as such, a greater amount of compression may be needed to avoid clipping. 
     In contrast, if the V LS  is within the highest predetermined range  140 , in which the V LS  is higher than the predetermined value V n , then the audio signal can be mapped against the table  134  designated by the reference letter X. The gain targets  138  in this table  134  can describe transfer functions that result in less compression of the audio signal when compared to any of the previous tables  134 . Because the supply voltage +V is higher in this example, the chances of the audio signal being clipped are reduced, and less compression is needed. 
     Eventually, the V LS  may decrease and fall from a first predetermined range  140 , such as where the V LS  is greater than V n , to a second predetermined range, such as where V LS  is greater than V 1  but less than or equal to V 2 . In accordance with the inventive arrangements, the audio signal can be mapped against a table  134  associated with the second predetermined range  140  that will result in greater compression as compared to its mapping against the previous table  134  associated with the first predetermined range  140 . Thus, as the supply voltage +V gradually drops, greater compression of the audio signal can occur. In another arrangement, each of the curves  510 – 516  can include a predetermined threshold I T  in which no or little compression can occur when the level of the audio signal is low, as described above in reference to  FIG. 4 . 
     It is understood that the invention can include any suitable number of predetermined ranges  140 . Moreover, the invention is not limited to the level of compression illustrated in  FIG. 4  or  5 , as the audio signal can be modified in any other suitable manner. Those of ordinary skill in the art will appreciate that the invention is not limited to the processing of digital signals, as the compressor  110  (see  FIG. 1 ) can be a compressor that processes analog signals. Although the processes described above can be performed in a communication device having a speaker, it is understood that the invention can be employed in any other suitable device. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.