Patent Application
20150382104
An embodiment of the invention is related to techniques for reducing ground loop-induced interference or noise in audio devices. Other embodiments are also described.
An audio source device (e.g., a laptop computer, a tablet computer, or a smartphone) can be connected through an audio cable to an audio receiver (e.g., a powered loudspeaker, a television unit, or an audio amplifier connected to a speaker), to convert an audio signal into sound. Quite often, an appreciable difference in the ground potential of the two devices can arise during operation, typically due to the presence of a ground loop that connects the “ground planes” of the two devices to each other, e.g. through an ac wall plug, or through a grounded communications cable. This ground potential difference voltage may be modeled by a voltage source referred to as Vn. The voltage Vn causes a current through the small-but-finite-resistance of a ground wire of the audio cable, which in turn interferes with the audio signal at the receiver. This interference is often manifested as a buzz or hum that can be heard, along with the desired content in the audio signal, from a speaker that is connected to the receiver.
Audio interference due to a ground loop may be ameliorated, by using an audio cable that has a very low resistance ground wire. Such cables however are often deemed to be too bulky. Another way to reduce the effect of the ground loop is to insert a sufficiently large “ground loop break” resistance Rgb in series between a ground pin of the audio connector in the source device and the system ground of the source device. This reduces the resulting voltage drop across the cable ground wire thereby reducing the resulting interference. But making Rgb too large may cause undesirable crosstalk between left and right channels that are being carried by the audio cable, when driving certain low impedance loads such as headsets.
In some cases, when Rgb is made too large, a further problem is created, namely that the amplifier can run out of headroom, i.e. its output voltage amplitude becomes so large as to reach close to the voltage of one of the power supply rails that are feeding the amplifier. This can occur when the amplifier has a feedback input that is coupled to receive the voltage of the connector's return pin, and coupling Rgb to this feedback input results in a voltage divider being formed by series-coupled resistances of the external load and Rgb, between the amplifier's output and the amplifier's ground reference node. In this feedback scenario, increasing Rgb will lead to more of the available amplifier output voltage swing being dropped across Rgb, for a given amplifier input voltage, which in turn will feed back to the amplifier thereby causing the amplifier to respond by increasing its output voltage (so as to compensate for the larger drop across Rgb). Continued increase of the amplifier output voltage in this manner will lead to distortion of the voltage waveform produced across the external load (which includes the speaker driver), and eventually clipping of the amplifier output voltage. This undesirable effect is more likely to occur when the input impedance of the coupled external audio device is not large enough in comparison to Rgb.
An embodiment of the invention is an audio source apparatus that automatically adjusts the value of Rgb upward, during in-the-field use by its end user, in order to reduce ground loop interference when coupled to higher impedance external audio loads such as a home audio receiver or an ac wall powered external speaker. This increase however is limited, to also reduce the likelihood of distortion occurring in the amplifier output signal (due to too much signal swing).
In one embodiment, the audio amplifier has a first input coupled to an audio source to receive an input audio signal containing user audio content, a second input coupled to the return pin of a connector to obtain a feedback signal, and an output that is coupled to a signal out pin of the connector. The audio amplifier may have a ground reference node that is coupled to a ground plane of the apparatus. An amplifier headroom detector detects headroom of the audio amplifier. A dynamic ground break resistance controller has an input coupled to an output of the headroom detector. The controller is coupled to dynamically control the variable resistor circuit in response to the headroom detection, i.e. automatically during in-the-field use of the apparatus by its end-user. In this manner, when a higher impedance external load is connected, a larger value of Rgb is selected that will reduce ground loop interference but without being so large as to cause the amplifier output to run out of signal swing. When a lower impedance external load, such as a passive speaker, is connected, Rgb is reduced, as needed to maintain sufficient signal swing across the external load. Also, this approach may automatically make adjustments to Rgb (in order to help reduce ground loop interference) during the lifetime of the audio apparatus, as parasitic resistance such as connector contact resistance changes during the lifetime of the audio apparatus, due to age and usage, or when different audio cables having different cable resistance are used. In one embodiment, the headroom detection is sensitive enough to detect changes in the headroom that are caused by the relatively smaller changes in the parasitic resistance, allowing easy updates to select the appropriate value of Rgb.
The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary.
The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. Also, in the interest of conciseness, a given figure may be used to illustrate the features of more than one embodiment of the invention, or more than one species of the invention, and not all elements in the figure may be required for a given embodiment species.
Several embodiments of the invention with reference to the appended drawings are now explained. Whenever the relative positions and other aspects of the parts described in the embodiments are not clearly defined, the scope of the invention is not limited only to the parts shown, which are meant merely for the purpose of illustration. Also, while numerous details are set forth, it is understood that some embodiments of the invention may be practiced without these details. In other instances, well-known circuits, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description.
The audio source apparatus 1 may be a desktop computer, a laptop or notebook computer, a tablet computer, or other consumer electronics audio device that contains an audio source.
Still referring to
An output node of the amplifier 2 is coupled to the external audio device 6 through a signal out pin of an audio connector 7. The connector 7 also has a signal return pin, in addition to the signal out pin as shown. The signal return pin is directly coupled to a ground plane of the source apparatus 1. The amplifier 2 may also be directly coupled to the ground plane of the apparatus, through any one of several possible techniques (to be described below). It should be noted that while a single audio channel is shown in
The external audio device 6 serves to convert the audio signal received over the audio cable 4 into sound, through an acoustic transducer or speaker (not shown). As depicted in the example of
Still referring to
As seen in
A similar ground loop interference problem is created in the embodiment of
Turning now to
The controller may be activated when an audio application is running in the audio source apparatus, and/or when there is detection of an external load being attached to the connector. The controller may be implemented as a programmed digital processor in combination with hardwired circuitry, and may be implemented in a distributed fashion, e.g. a portion of it may be implemented as part of an audio codec chip while another portion may be implemented by a suitably programmed applications processor, central processing unit, CPU, or system on a chip, SoC. The controller is coupled to control the variable resistor circuit, in response to the headroom detection.
Also shown in
The advantageous result obtained by the use of the above-described voltage feedback arrangement, however, may be spoiled when making Rgb too large. To explain, recall that making Rgb larger will reduce the interference current through Rcab_G. However, an increase in Rgb will also, due to the voltage feedback described above, lead to the amplifier automatically increasing its Vout signal swing, in order to maintain the same voltage Vload_ext across the external load. In other words, as Rgb increases, more of the Vout signal swing is dropped across Rgb, such that to maintain the same Vload_ext (drop across Rext), the signal swing of Vout will have to increase. Referring now to
An embodiment of the invention uses a headroom detector that determines the present output headroom of the audio amplifier. The headroom may be viewed as a delta voltage, being a difference between a) maximum excursion or peak of the output voltage of the amplifier, while user audio content or test content is input to the amplifier, and b) a power supply voltage of the amplifier. This is shown in
However a headroom value is defined, the headroom detector may use any one of the following techniques to sense or compute a measure of the headroom. In one embodiment, a feed forward (or look ahead) type of computation is performed, by digitally analyzing the peaks of the digital audio input signal produced by the audio source, which is being converted into analog form or other wise driven by the amplifier out through the audio cable 4 (and into the external load). This analysis of the digital audio input signal may be performed in conjunction with knowledge of 1) a stored characteristic or behavioral model of the amplifier, 2) the expected total load impedance (including Rcable_L or Rcable_R, Rext, and the present value of the variable ground break resistance Rgb) which may be a stored value, and 3) a stored value representing the amplifier's power supply rail voltage.
In another embodiment, the headroom detector and the controller may behave reactively, where the headroom detector includes a circuit that senses the present output voltage of the amplifier, Vout (Vout_left or Vout_right), and optionally also sensed the present power supply rail voltage, Vcc or Vdd. The power supply rail voltage here may, alternatively, be taken as a known, regulated value, and be found as a stored expected value. A comparison is then made between a) the sensed Vout and b) the expected or sensed power supply rail voltage, Vcc or Vdd, and the results of such a comparison are signaled to the controller, e.g. as a difference or delta value.
Once a measure of headroom has been determined, the controller then compares the determined headroom to a set threshold, which may represent the minimum headroom that can be tolerated. When the headroom has shrunk down to the threshold, the controller responds by appropriately controlling the variable resistor circuit. If the determined headroom has shrunk down to a predetermined threshold or guard band (see
In yet another embodiment, the headroom detector may be a circuit that is built into the audio amplifier, e.g. as part of a constituent operational amplifier, that automatically asserts a headroom alert signal when an internal state of the op amp (e.g., certain node voltages) indicates that for example the op amp is no longer operating in a stable feedback configuration (which may mean that the output voltage of the op amp has likely risen so much as to be too close to its power supply rail voltage). In that case, the controller may not need to perform any comparison and can simply rely on the low headroom alert signal from the headroom detector, to immediately decide how to adjust downward or place an upper limit to the value of Rgb (by appropriately signaling the variable resistor circuit).
The controller may also signal the variable resistor circuit to increase Rgb, as follows. In one embodiment, the controller performs digital signal processing upon the audio input signal from the audio source (prior to amplification), or if available the amplifier output Vout, to determine when Rgb can be allowed to increase. Such processing may include any combination of signal level based dynamics such as attack and release times and hysteresis, and time based dynamics such as hold times. In another embodiment, once Rgb has reached an “optimum” level, Rgb may not be increased until the next time attachment of the connector has been detected. In that case, the controller can be configured to perform a conventional connector attachment/detachment detection process.
The controller may also signal the variable resistor circuit to decrease Rgb, as follows. The total amplifier load impedance may be detected using any conventional scheme; for smaller total load impedances, a smaller Rgb should be used (with the understanding that Rgb will in most cases be smaller than the external load impedance). The slope of the amplifier output voltage, or the slope of the amplifier input voltage, dV/dt, may be detected, and on that basis Rgb may be decreased. This process may be performed in real-time, by allowing a margin, favoring an “early” reduction in Rgb and then increasing Rgb based on the detected slope. Alternatively, if a digital form of the amplifier input signal (audio user content, or test content) or amplifier output signal is available, the controller can perform a digital signal processing-based look ahead scheme to compute the slope.
In one embodiment, the controller adapts the ground break resistance to different types of amplifier loads (based on the headroom detection described above), by adjusting the variable resistor circuit accordingly so that the ground break resistance is kept as large as possible to alleviate ground loop interference but without becoming so large as to cause output signal swing of the amplifier to become distorted (due to coming too close to the power supply voltage). The variation in amplifier load may be due to different line-in or receiver input impedance, different passive speaker input impedance, different types or lengths of audio cables, and different connector contact resistance (e.g., as the source audio apparatus ages).
In one embodiment, the variable resistor circuit may have two or more discrete resistance states, obtained by for example a configurable discrete passive resistor network. In another embodiment, the variable resistor circuit may exhibit continuously variable resistance, e.g. using a transistor such as an insulated gate field effect transistor whose Vgs is variable, or other types of transistors and active devices that can present a variable resistance in the desired range. In yet another embodiment, a switched capacitor network or a rapidly switched (well beyond the audio range) resistor may be used to emulate the desired variable resistance. A combination of the above may be used to obtain the variable resistance. In all such cases, the variable resistance may be digitally controllable, by the controller, e.g. from essentially zero ohms (which may be used when a passive headphone has been connected), to one or more non-zero resistance values (which may be used when the amplifier is connected to an audio receiver or line input). The controller may have a stored lookup table of headroom values and associated variable resistor circuit settings, which may be accessed by the controller whenever a new headroom value has been determined (in order to update the variable resistor circuit setting).
Returning to
The concern with the occurrence of user-audible artifacts when changing the ground break resistance may also be addressed by designing the variable resistor circuit to have sufficient granularity or resolution in its variable resistance, such that the controller can change the ground break resistance in smaller increments. This may give the amplifier enough time to adjust its output voltage gradually (in response to the feedback voltage from the Rgb node changing more gradually). This technique could be used as an alternative to relying on the ground current zero crossing detection scheme, or it could be used in conjunction therewith when updating the decision to change the ground break resistance.
In the case where the controller computes an estimate of the load impedance of the amplifier, there may be a need for sensing the load current. In that case, the sensed ground current (by the ground current detector) may be used, as representing the current through a circuit path that dc couples the ground plane of the source apparatus to the return pin of the connector, through the variable resistor circuit. The controller thus obtains or computes a measure of load current of the amplifier, based on the sensed ground current, and then uses it to compute the estimate of the load impedance (e.g., together with a measure of the amplifier output voltage). This may be done for frequency components that are below the audible frequency range.
An embodiment of the invention is a method for audio signal processing, in which an audio amplifier is driving an external load through a connector. While doing so, the amplifier is configured to respond to an input audio signal (e.g., a test signal or user audio content), and a signal from a return pin of the connector (which return pin is dc coupled to a ground plane). The method includes detecting output headroom of the amplifier (while the amplifier is driving the load). A variable resistor circuit that is coupled to provide variable resistance between the return pin of the connector and the ground plane is then automatically adjusted, i.e. without user input, in response to the detected output headroom of the amplifier. The amplifier is configured with feedback, such that when the input audio signal has fixed amplitude, the amplifier produces a voltage across the external load that also has fixed amplitude, despite changes in the variable resistance. In other words, the effective voltage gain to the external load remains fixed despite the dynamically changing ground break resistance.
In one embodiment, detecting output headroom of the audio amplifier comprises analyzing 1) a peak of the input audio signal, 2) a characteristic or behavioral model of the amplifier, 3) the amplifier's load impedance, and 4) the amplifier's power supply rail. In another embodiment, detecting output headroom of the audio amplifier comprises sensing output voltage of the amplifier, and comparing the sensed output voltage to a sensed or known value representing voltage of the amplifier's power supply rail. Based on the detected output headroom being smaller than a threshold, an indication is given that the variable resistance cannot be made larger or the variable resistance is simply lowered.
Turning now to
The system in
While certain embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that the invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. For example, although the figures depict an audio system having two channels, namely a left channel and a right channel, the concepts described above are applicable more generally to audio systems having a single output channel or more than two output channels, and also those having both output and input (e.g., microphone or sound pick up) channels. The description is thus to be regarded as illustrative instead of limiting.
