Abstract:
A software-based equalization method corrects for effects of impedance interactions of a source device and an output device, on an aggregate frequency response, in audio playback of a system. The correction results in perceived sound remaining consistent from any source device and output device combination. The method does not require any hardware modifications and a cloud-based database of source devices and output devices provides corrections for a wide array of possible device pairings.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to correcting the frequency response of audio devices and in particular to correcting a frequency response which has been altered by interactions of impedances of an acoustic source device and output device combination. 
     Interaction between the impedance of an acoustic source device and an output device has been found to alter the frequency response in headphones, and other audio devices. The impedance interaction may prevent a consistent perception of sound when listening to any given output device connected to any given source device. 
     The impedance interaction problem is known in the audio industry. The known solutions propose changes to the electrical or acoustical design of either the source device or the output device. No known software-based impedance interaction problem solutions are available. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing a software-based equalization method which corrects for the effects in an aggregate frequency response of a system caused by impedance interactions of a source device and an output device. The correction results in perceived sound remaining consistent from any source device and output device combination. The method does not require any hardware modifications and a cloud-based or locally stored database of source devices and output devices provides corrections for a wide array of possible device pairings. 
     In accordance with one aspect of the invention, there is provided a software-based equalization method for correcting source device impedance and output device impedance interactions requiring no modification to hardware. 
     In accordance with another aspect of the invention, there is provided a software-based equalization method for correcting source device impedance and output device impedance interactions for a multiplicity of source and output device pairings. The method includes a library of source and output device pairings stored in a cloud-based or locally stored database and provided to uses as needed. 
     In accordance with still another aspect of the invention, there is provided a software-based equalization method for correcting source device impedance and output device impedance interactions. The method is applicable to any source device and output device pairing with impedances which cause perceived differences in the combined acoustic responses. 
     In accordance with yet another aspect of the invention, there is provided a software-based equalization method. The method includes the steps of measuring the complex impedance of the source device, measuring the complex impedance of the output device, determining the frequency response of the interaction between the source device and the output device based on the measured impedances, generating an inverse equalization of the frequency response, and applying the inverse equalization to the audio signal in the source device, or in the audio stream being played through the source device. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  shows an audio system including a source device and an output device according to the present invention. 
         FIG. 2  shows the variation of impedance as a function of frequency of an output device. 
         FIG. 3  shows the frequence response of corrected and uncorrected systems. 
         FIG. 4  is a method according to the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing one or more preferred embodiments of the invention. The scope of the invention should be determined with reference to the claims. 
     An audio system including a source device  14 , (for example, a smartphone or other portable electric device), and an output device  18  (for example, a speaker, headphones, or other transducers), are shown in  FIG. 1 . The source device  14  receives signals  12  or plays a locally stored signal (i.e., a song stored on an iPod is not received by the device  14 , but is played on or from the device  14 ), processes and amplifies the signals  12 , and provides the amplified signals to the output device  18  through second cables  16 . The output device  18  then produces sound waves  20 . Such source device  14  inherently includes a complex output impedance. The output impedance resulting from the output amplifier stage varies widely among different devices and designs ranging from about 0 ohms to over 120 ohms. 
     The output device  18  imparts a frequency dependent load (impedance). The pairing of the output device  18  load impedance with a source device  14  output impedance creates a frequency-dependent voltage divider, which voltage divider divides the voltage at every frequency, and therefore the power at every frequency, between the source device  14  and output device  18 . The formula for this frequency-dependent voltage divider is: 
                 V   out     ⁡     (   f   )       =           Z     output   ⁢           ⁢   device       ⁡     (   f   )             Z     output   ⁢           ⁢   device       ⁡     (   f   )       +       Z     source   ⁢           ⁢   device       ⁡     (   f   )           ⁢       V     i   ⁢           ⁢   n       ⁡     (   f   )               
where V and Z represent voltage and impedance, respectively, and f is the frequency.
 
     As different voltages are delivered to the output device  18  at different frequencies, the Sound Pressure Level (SPL) of the sound waves  20  produced by the output device  18  at those frequencies varies. The SPL response of the output device  18  varies by frequency as the load impedance varies by frequency when the output impedance is nonzero. Since the impedance of most audio output devices  18  can vary significantly with frequency, the resulting SPL variations for a given output and source device pairing can be vary significant, as much as 6 dB. The equalization according to the present invention applied through software adjusts this change in frequency response to maintain a consistent acoustic frequency response across device pairings. 
     Through measuring the complex impedance for the source device  14  and output device  18  across the frequency range of human hearing (typically 20 Hz to 20,000 Hz), the change in frequency response can be determined through analysis of the applied frequency-dependent voltage divider for a given device pairing. For example, a pair of MEElectronics A 151  headphones with a frequency-dependent impedance  30  shown in  FIG. 2  varies from ˜25 ohms to ˜425 ohms played out of a source device with a ˜1 ohm output impedance will have a frequency response  32  differing by upwards of 6 dB SPL at certain frequencies compared to the frequency response  34  of the same headphones played out of a source device with a ˜103 ohm output impedance. Through modeling the effect of the voltage divider, the frequency response of the interaction is determined, and the inverse EQ of this response is generated. This inverse response is applied through software running on the source device, or on the audio stream being played through the source device, to ensure consistent acoustic responses  36  across differing devices which is very close to the frequency response  32 . 
     A method according to the present invention is shown in  FIG. 4 . The method includes the steps of measuring the complex impedance of the source device at step  100 , measuring the complex impedance of the output device at step  102 , determining the frequency response of the interaction between the source device and the output device based on the measured impedance at step  104 , generating an inverse equalization of the frequency response at step  106 , and applying the inverse equalization to the signal in the source device at step  108 . Those skilled in the art will recognize that processing of the inverse filter does not necessarily have to be applied by the source device and could be done in the cloud or another stage of processing, and methods performing the inverse filter processing at other stages of processing are intended to come within the scope of the present invention. Further, the inverse equalization may be pre-loaded into the source device or downloaded from a database, for example, a cloud-based database. Such database may include a library of inverse equalizations for a multiplicity of source and output device pairings provided to uses as needed. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.