Patent Publication Number: US-2017374456-A1

Title: Minimizing startup transients in an audio playback path

Description:
FIELD OF DISCLOSURE 
     The present disclosure relates in general to circuits for audio devices, including without limitation personal audio devices such as wireless telephones and media players, and more specifically, to systems and methods for minimizing audible effects of electrical transients during powering on or startup of an audio playback pack. 
     BACKGROUND 
     Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers. 
     Using existing approaches, during startup of an audio playback path of a personal audio device, startup electrical transients of the audio playback path may lead to audible artifacts (e.g., pops or clicks) perceptible to a listener of the personal audio device. 
     SUMMARY 
     In accordance with the teachings of the present disclosure, one or more disadvantages and problems associated with existing audio signal paths may be reduced or eliminated. 
     In accordance with embodiments of the present disclosure, a method may be provided for powering up or down a playback path comprising a digital-to-analog converter (DAC) for generating a non-ground-centered analog intermediate voltage centered at a common-mode voltage and coupled to a driver for generating a ground-centered playback path output voltage at an output of the driver wherein the output of the driver is clamped via a finite impedance to a ground voltage. The method may include transitioning continuously or in a plurality of discrete steps the analog intermediate voltage from an initial voltage to the common-mode voltage such that the transitioning is substantially inaudible at the output of the driver. 
     In accordance with these and other embodiments of the present disclosure, a method for operating an output clamp of an output driver stage of a playback path may include transitioning continuously or in a plurality of discrete steps an impedance of the output clamp in order to match an output offset of the output driver stage in order to minimize audio artifacts appearing at an output of the output driver stage. 
     In accordance with these and other embodiments of the present disclosure, a playback path may include a digital-to-analog converter (DAC) for generating a non-ground-centered analog intermediate voltage centered at a common-mode voltage, a driver coupled to the DAC for generating a ground-centered playback path output voltage at an output of the driver, an output clamp for clamping the output of the driver to a ground voltage via a finite impedance, and circuitry configured to transition continuously or in a plurality of discrete steps the analog intermediate voltage from an initial voltage to the common-mode voltage such that the transition is substantially inaudible at the output of the driver. 
     In accordance with these and other embodiments of the present disclosure, a playback path may include a driver for generating a ground-centered playback path output voltage at an output of the driver, an output clamp for clamping the output of the driver to a ground voltage via a finite impedance, and a controller for operating the output clamp by, each time the output clamp is activated or deactivated, transitioning continuously or in a plurality of discrete steps an impedance of the output clamp in order to match an output offset of the driver in order to minimize audio artifacts appearing at an output of the driver. 
     Technical advantages of the present disclosure may be readily apparent to one skilled in the art from the figures, description and claims included herein. The objects and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are not restrictive of the claims set forth in this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
         FIG. 1  is an illustration of an example personal audio device, in accordance with embodiments of the present disclosure; and 
         FIG. 2  is a block diagram of selected components of an example audio integrated circuit of a personal audio device, in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration of an example personal audio device  1 , in accordance with embodiments of the present disclosure.  FIG. 1  depicts personal audio device  1  coupled to a headset  3  in the form of a pair of earbud speakers  8 A and  8 B. Headset  3  depicted in  FIG. 1  is merely an example, and it is understood that personal audio device  1  may be used in connection with a variety of audio transducers, including without limitation, headphones, earbuds, in-ear earphones, and external speakers. A plug  4  may provide for connection of headset  3  to an electrical terminal of personal audio device  1 . Personal audio device  1  may provide a display to a user and receive user input using a touch screen  2 , or alternatively, a standard liquid crystal display (LCD) may be combined with various buttons, sliders, and/or dials disposed on the face and/or sides of personal audio device  1 . As also shown in  FIG. 1 , personal audio device  1  may include an audio integrated circuit (IC)  9  for generating an analog audio signal for transmission to headset  3  and/or another audio transducer. 
       FIG. 2  is a block diagram of selected components of an example audio IC  9  of a personal audio device, in accordance with embodiments of the present disclosure. As shown in  FIG. 2 , a microcontroller core  18  may supply a digital audio input signal DIG_IN to a digital-to-analog converter (DAC)  14 , which may convert the digital audio input signal to an intermediate analog signal Y IN . 
     DAC  14  may supply intermediate analog signal Y IN  to an amplifier  16  which may amplify or attenuate audio input signal Y IN  in conformity with a gain to provide an audio output signal V OUT , which may operate a speaker, headphone transducer, a line level signal output, and/or other suitable output. Amplifier  16  may comprise multiple amplifier stages  22 ,  24 , and  26  coupled in succession with one another and a plurality of resistors  28  such that the gain of amplifier  16  is set by the resistances of resistors  28 . As shown in  FIG. 2 , amplifier  16  may include, due to non-idealities of amplifier  16  (e.g., temperature variations, process tolerances, device mismatch, passive mismatch, etc.), a slight inherent offset  29  from a desired ground or common mode voltage associated with amplifier stage  16 , which may affect signal output V OUT . As also shown in  FIG. 2 , amplifier  16  may include a switch  34  coupled between amplifier stage  24  and amplifier stage  26 . Such switch  34  may be deactivated (e.g., open, off, disabled, etc.) when the playback path implemented by audio IC  9  is powered down, and activated (e.g., closed, on, enabled, etc.) when the playback path is powered on. In some instances, amplifier  16  may also be referred to as a “driver.” 
       FIG. 2  also depicts an output clamp  30  having a finite variable resistor  32  coupled between the output of amplifier  16  and ground. When the playback path implemented by audio IC  9  is powered down, a controller (e.g., controller  20 ) may cause variable resistor  32  to have a low impedance, such that audio output voltage V OUT  is forced to approximately ground voltage. When the playback path implemented by audio IC  9  is powered on, a controller (e.g., controller  20 ) may cause variable resistor  32  to have a high impedance (ideally infinite), such that audio output voltage V OUT  may be driven by amplifier  16 . In some embodiments, variable resistor  32  may be implemented using a plurality of parallel switches each having a finite impedance, such that an impedance of variable resistor  32  is controlled by selectively activating and deactivating individual switches. Controller  20  may comprise any system, device, or apparatus for controlling clamp  30  and the resistance of variable resistor  32 . 
     In operation, in order to reduce audio artifacts caused by electrical transients occurring at startup of the playback path implemented by audio IC  9 , one or more components of audio IC  9  may cause analog intermediate voltage V IN  to transition continuously or in a plurality of discrete steps from an initial voltage (e.g., ground voltage) to a common-mode voltage of DAC  14  such that the transition is substantially inaudible at the output of amplifier  16 . When the analog intermediate voltage V IN  is transitioned to any non-zero value, a level-shifting current may flow from the output of DAC  14  to the output node of amplifier  36  (e.g., via the network of resistors  28 ) and through clamp  30 . If clamp  30  was an ideal clamp with zero impedance, such current would cause no change in voltage V OUT  at the output node of amplifier  16 . However, clamp  30 , no matter how well constructed or engineered, may have a finite impedance associated with it and when the level-shifting current flows through such impedance, a voltage step in output voltage V OUT  may be created at the output. However, in the present disclosure, analog intermediate voltage V IN  may be transitioned in a manner in which a sequence of voltage steps created at the output node of amplifier are sufficient to minimize or eliminate audible artifacts. For example, in some embodiments, such transition may be performed by microcontroller core  18  or another component of audio IC  9  forcing digital input signal DIG_IN to DAC  14  to a maximum (e.g., maximum positive or maximum negative value) such that analog intermediate voltage Y IN  is initially set to the ground voltage when DAC  14  is powered on due to start-up of the playback path, and then microcontroller core  18  may transition digital input signal DIG_IN continuously or in a plurality of discrete steps such that analog intermediate voltage Y IN  transitions from the initial voltage (e.g., ground voltage) to the common-mode voltage such that the transitioning is substantially inaudible at the output of amplifier  16 . In other embodiments, a variable current source  36  (or a plurality of current sources in parallel with each other and DAC  14 ) may be coupled to the output of DAC  14  and may be configured to increase the current output by such variable current source  36  continuously or in a plurality of discrete steps such that analog intermediate voltage Y IN  transitions from the initial voltage (e.g., ground voltage) to the common-mode voltage such that the transitioning is substantially inaudible at the output of amplifier  16 . 
     In addition or alternatively, in operation, controller  20  may operate output clamp  30  by, each time the output clamp  30  is activated (e.g., impedance decreased to low impedance upon powering off of playback path) or deactivated (e.g., impedance decreased to low impedance upon powering on of playback path), transitioning continuously or in a plurality of discrete steps variable resistance  32  of output clamp  30  in order to match voltage offset  29  of amplifier  16  to the audio output signal V OUT  in order to minimize audio artifacts appearing at the output of the amplifier  16 . In some embodiments, one of stages  22  or  24  of amplifier  16  (shown in  FIG. 2  as stage  24 ) may in effect be configured as a comparator to compare analog output signal V OUT  to voltage offset  29 . In other words, by sensing an output of one of the stages of amplifier  16  other than the final output stage  26 , the output of such stage  22  or  24  may indicate whether transitioning of the impedance of output clamp  30  has caused audio output signal V OUT  to reach a voltage approximately equal to voltage offset  29 , and controller  20  may cease the transitioning upon occurrence of audio output signal V OUT  reaching a voltage approximately equal to voltage offset  29 . In these and other embodiments, controller  20  may be configured to transition variable resistance  32  of output clamp  30  in order to match voltage offset  29  of amplifier  16  to the audio output signal V OUT  such that when output clamp  30  is activated or deactivated, audio output signal V OUT  experiences no substantial change. 
     Although the various systems and methods described herein contemplate reduction of audio artifacts in audio paths of personal audio devices, the systems and methods herein may also apply to any other audio systems, including, without limitation, home audio systems, theaters, automotive audio systems, live performances, etc. 
     This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative. 
     All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present inventions have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure.