Method for reducing noise in a microphone circuit

Abstract

A computer sound card includes a power noise isolation circuit for reducing ground plane noise generated during application of a heavy electrical load, such as a load provided by a computer GPU, to a power supply. The power isolation circuit isolates an output ground from a ground supplied to an input of the isolation circuit from the power supply. In one embodiment, the isolation circuit includes a switching circuit and a transformer to reduce power supply ripple noise that might otherwise be introduced by the power supply into an amplifier, such as a microphone preamplifier. In some embodiments, a differential amplifier stage is added to an output of the microphone preamplifier stage to further reduce noise, such as common mode noise.

Description

Technical Field

[0001] The present invention relates to a computing device having an input / output circuit. More particularly, the present invention relates to power noise control in an audio processing circuit such as, for example, a sound card of a computing device. Background Art

[0002] Over time, the importance of graphics cards and chips to computing devices has increased. To meet these growing demands, graphics chip and card manufacturers provide at least annually a new generation of graphics hardware that exhibits improved performance. With this overall increase in performance, power requirements have also increased. To meet this increased demand for power, computer manufacturers have provided external power connectors, at least in part due to the insufficient power available from motherboard buses such as the PCI / e bus.

[0003] With these high-end graphics cards, consumers have also developed high expectations for audio performance. Unfortunately, the higher power consumption of these high-performance graphics cards is also accompanied by increased power noise. Consumers use audio in many cases in combination with high-performance graphics cards. One example includes online interactive gaming, where audio can be used to communicate with other players in a multi-player environment. In another example, a consumer may want to provide commentary on a video stream of their gaming session, for example, when using a service such as "Twitch". In these and other cases, clear voice communication is important.

[0004] Microphone amplifiers and preamplifiers for providing voice communication have very high gains, such as gains of 20 - 40 dB. Any noise introduced by the power supply will also be amplified by the microphone amplifier. Typical solutions include more power filters, using amplifiers with high power supply rejection ratio (PSRR) characteristics, and better ground planes, to name just a few examples. However, given the high gains used in microphone amplifiers for voice communication, these solutions are generally not sufficient to reduce the noise to an acceptable level for high-end products. A method is needed to reduce this power noise so that very weak microphone signals can undergo high amplification and still provide a clear voice communication signal at the amplifier output. Summary of the Invention

[0005] To achieve the above object, the present invention provides, in various embodiments, an audio processing system for use with a computing device such as a laptop computer, a smart phone, or a desktop computer. The system includes an isolation block configured to receive a power supply signal including a positive voltage and a first ground reference, the isolation block being configured to reduce power supply noise present in the power supply signal by providing a positive output voltage and a second ground reference at its output, the second ground reference being isolated from the first ground reference. The audio processing system further includes a microphone preamplifier circuit configured with one input for receiving a microphone signal, a second input at the second ground reference, and an output signal. Also included is a digital signal processor block configured to receive the microphone preamplifier output signal, convert it to a digital format, and transmit the digitized microphone preamplifier output signal to the computing device. In one embodiment, the isolation block includes a switching circuit and a transformer. In such an application, the transformer is used to isolate the power ground from other system grounds and provide an isolated low-noise power supply.

[0006] In yet another embodiment, the audio processing system further includes a differential amplifier connected between the microphone preamplifier and the digital signal processor block. The differential amplifier is configured to remove common-mode noise from the microphone preamplifier output signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a diagram showing a conventional microphone amplifier for use with a computer.

[0008] Figure 2 is a block diagram showing a microphone amplifier circuit with a noise isolation circuit for use with a computing device according to an embodiment of the present invention.

[0009] Figure 3 is a diagram showing sampled power supply noise reduction in the presence of a lightly loaded graphics card according to some embodiments of the present invention.

[0010] Figure 4 is a diagram showing another view of sampled power supply noise reduction in the presence of a lightly loaded graphics card according to some embodiments of the present invention.

[0011] Figure 5 is a diagram showing sampled power supply noise reduction in the presence of a heavily loaded graphics card according to some embodiments of the present invention.

[0012] Figure 6 is a diagram showing another view of an example power supply noise reduction in the presence of a heavily loaded graphics card according to some embodiments of the present invention. DETAILED DESCRIPTION

[0013] Reference will now be made in detail to the preferred embodiments of the present invention. Examples of the preferred embodiments are shown in the accompanying drawings. While the present invention will be described in conjunction with these preferred embodiments, it will be understood that it is not intended to limit the present invention to such preferred embodiments. On the contrary, the present invention is intended to cover alternatives, modifications, and equivalents that may be included within the spirit and scope of the present invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well-known mechanisms have not been described in detail so as not to unnecessarily obscure the present invention.

[0014] It should be noted herein that throughout the various drawings, like reference numerals denote like components. The various drawings shown and described herein are used to illustrate the various features of the present invention. With respect to a particular feature shown in one drawing and not in another, unless otherwise indicated or in a case where the structure inherently prohibits the combination of that feature, it should be understood that those features may be adapted to be included in the embodiments shown in the other drawings as if they were fully shown in those drawings. Unless otherwise specified, the drawings are not necessarily drawn to scale. Any dimensions provided in the drawings are not intended to limit the scope of the present invention, but are merely illustrative.

[0015] The increased use of an external power connector supplied by the computer power supply rather than power obtained from a computer bus source such as a PCIe bus has led to increased concerns about noise. The PCIe bus is a high-speed serial computer expansion bus standard. It is commonly used as a motherboard interface for computer graphics cards, hard disk drives, and Wi-Fi hardware modules, etc. The noise introduced by the power supply can generally be described as power supply voltage ripple. It is defined as an unwanted AC voltage superimposed on the output rail of the power supply. The ripple is mainly caused by the loading on the power supply. In a common example, this "loading" comes from a graphics card that draws current in a non-constant manner, such as when presenting complex scenes frame by frame. This non-constant loading causes the drawn current to change. Even when using a ground plane, the changing current will manifest itself as a ripple voltage due to the resistance (which is not zero ohms) on the power supply line. This can also be due to power surges and dips when devices connected to the same power rail are powered on or off.

[0016] To minimize the voltage ripple noise and prevent it from being amplified by the microphone preamplifier and / or differential amplifier stage, an isolation circuit is introduced between the power supply and the microphone preamplifier / amplifier. For comparison, Figure 1 a conventional microphone amplifier with a typical power rail is shown in Figure 2 while a modified circuit illustrating an embodiment of the present invention is shown inFigure 1 FIG. Figure 1 is a diagram showing a conventional microphone amplifier 100 for use with a computer. Figure 2 FIG. Figure 2 is a block diagram showing a microphone amplifier circuit having a noise isolation circuit for use with a computing device according to an embodiment of the present invention.

[0017] Figure 1 The exemplary power rails shown obtain their power from a power connector such as a PCIe power socket 102. The positive supply signal 106 shown exhibits significant ripple noise. In Figure 1 the conventional case, the negative power rail 104 is the common ground for the power socket 102, the microphone pre - amplifier 108, the digital signal processor 112, and a computer bus (e.g., a PCIe bus) 114. While it is desirable to introduce a clean voice communication signal into the DSP 112 to generate a digitized microphone signal for use by a computing device (e.g., a computer), here the power supply ripple noise becomes a problem. Due to the high amplification provided to the signal from the microphone 110, distortion will occur. This is considered to be a symptom of the connected grounds (i.e., the "common" ground). In these circuits, non - constant loading on the power rails will cause voltage ripples to appear on the "grounds" at different points on the circuit board: i.e., the ground of the microphone pre - amplifier and the ground of the ADC on the sound card will have some voltage difference. These voltage differences themselves appear as noise and will couple with the microphone signal, be amplified by the microphone pre - amplifier / amplifier, and be sent to the ADC.

[0018] When an isolated power supply as disclosed in the embodiments of the present invention is used, the ground of the microphone pre - amplifier is isolated from the main power supply and thus does not have a voltage difference caused by non - constant loading. This allows low - noise isolated power to be provided to the microphone circuit for voice communication.

[0019] Figure 2 A redesigned power rail for a sound card or audio amplification system 200 is shown to generate low - noise isolated power to prevent amplification distortion of noise from the microphone path (i.e., the microphone amplifier circuit). It should be noted that for illustrative purposes, the electrical connections shown correspond to a sound card inserted into a desktop computer, but the features of the present invention are equally applicable to other environments where power supply ripple voltage creates a problematic noise issue. That is, the features of the present invention are equally applicable to notebook / laptop computers, smart phones, and substantially any circuit that requires reducing power supply ripple before generating a clean output signal. Embodiments of the present invention are well - suited to providing clean amplified voice signals, such as those necessary for digitization by an analog - to - digital converter (ADC) for further audio processing.

[0020] As Figure 2As shown, the PCIe power socket 102 supplies power to the microphone audio amplification circuit. Typically, a computer power supply powers the computer motherboard, which in turn powers various slots or buses. In the case of the fast PCI (PCI Express) standard, the main power connector provides power up to the limit of the PCIe x16 slot (up to 75 watts maximum for one version) for devices such as video cards. However, the standard also defines an auxiliary power connector (e.g., a PCI Express power connector configured to connect to the power socket 102) that can deliver additional power directly from the power supply to the graphics card. A main purpose of this feature is to accommodate high-performance graphics cards. The auxiliary power 203 available from the computing device 250 is typically at 12V and is "noisy" for reasons detailed elsewhere in the specification. The power available through the auxiliary power connector can be at other voltages, including but not limited to 5V.

[0021] The isolation circuit 210 is inserted instead of applying this noisy power directly to the microphone preamplifier circuit 208. This isolates the ground 206 of the microphone preamplifier from the ground 204 of the power supply, resulting in an isolated low-noise power supply and thus providing a "clean" (in this case 12V) voltage 205 applied to the preamplifier 208.

[0022] In a preferred embodiment, the isolation circuit 210 includes a switch circuit 212, a transformer 214, and a rectifier block 215. In one embodiment, the switch circuit and the transformer 214 are connected in a push-pull configuration. In this embodiment, the switch circuit includes a control IC to regulate the switching frequency and the duty cycle, the latter value being used to control the voltage. In one embodiment, the switch circuit is in a push-pull configuration, for example, a pair of switches operating out of phase generates an AC voltage on the primary side of the transformer. This AC voltage is coupled to the secondary side of the transformer and rectified into a DC voltage.

[0023] In a preferred embodiment, the switch circuit and the push-pull configuration are not used to change the nominal value of the power supply voltage applied to the input of the isolation circuit 210, but rather to isolate the microphone preamplifier power supply 205 and ground 206 from the input power supply 203 and ground 204. This results in a low-noise isolated power supply and reduces the amount of distortion in the microphone signal output from the microphone preamplifier 208. Figure 5 and Figure 6 An improvement in noise attenuation from the isolation block is shown, which shows the improvement when the graphics card is operating with a load, such as the differential microphone signal 223 at the output of the preamplifier 208. In the switch circuit, a high switching frequency design generally results in a smaller transformer and lower output ripple, and for these reasons is preferred in one embodiment of the present invention.

[0024] In a preferred design, the switching circuit controls a transformer with separate primary and secondary windings to achieve isolation from input to output. In this embodiment, the switching IC converts 12V DC to AC, which is then coupled to the secondary winding of the transformer and then rectified back to 12V DC. The switching circuit can be fine-tuned, for example by selecting the values of L and C, to reduce any potential switching noise in the audio band or its harmonics. For a non-limiting example, in one embodiment, the switching frequency is selected to be higher than 500 kHz.

[0025] The microphone preamplifier 208 is preferably a high-gain amplifier capable of amplifying weak voice signals. The amplifier gain is preferably in the range of 20 - 43 dB or higher. It is shown as including an operational amplifier 224 with a single input, but can also include other amplifier configurations. In some embodiments, a differential amplifier 227 is added to the output of the microphone preamplifier for additional noise control. In the case of the microphone preamplifier 206, an operational amplifier (but not necessarily) can be used to amplify the microphone signal. Figure 2 Only one of the operational amplifier inputs connected to the amplifier is shown, using the "inverting" or "non-inverting" input terminal to amplify a single input signal while the other input is grounded. Since a standard operational amplifier has two inputs, an inverting and a non-inverting one, we can also connect the microphone signal to both inputs simultaneously, resulting in another common type of operational amplifier circuit called a differential amplifier. The resulting output voltage will be proportional to the "difference" between the two input voltage signals and is very suitable for reducing common-mode noise.

[0026] Specifically, the differential amplifier 227 receives the differential microphone signal 223 from the microphone preamplifier 208, and due to the nature of differential amplification, reduces the common-mode noise, resulting in a further reduction of the microphone noise reflected at the differential amplifier output signal 225. It should be noted that although Figure 2An amplifier 227 with two inputs is shown, but other embodiments of the present invention are configured to have an amplifier 227 that serves as a second-stage amplification but has only a single input. The microphone itself can be single-ended, but a differential microphone signal can be achieved by changing the way the microphone or the operational amplifier is connected. Then, this clean signal is applied to the analog-to-digital converter block 116 of the DSP 112, thereby generating a clean digitized microphone signal applied to the computer bus 114 for use by the computer 250. The ground 204 of the power supply is shown isolated from the second ground reference 206 on the secondary side of the transformer 214. This second ground reference 206 also represents the ground 206 for the microphone preamplifier, but is also isolated from the computer bus ground 211 (i.e., the third ground reference). The computer bus ground reference also represents the DSP 112 and the differential amplifier 227. The ground of the differential amplifier is the same as the ground of the DSP 112 so that the ADC can correctly convert the microphone signal into the digital domain.

[0027] Figure 3 is a diagram showing the sampled power supply noise reduction in the presence of a lightly loaded graphics card according to some embodiments of the present invention. Figure 4 is a diagram showing another view of the sampled power supply noise reduction in the presence of a lightly loaded graphics card according to some embodiments of the present invention. Figure 3 and 4 shows the sampled results of the noise improvement presented when the graphics card is running without a load. In this case, the noise present is mainly a function of the common-mode noise. In several embodiments, the sound card inputs and outputs are accessed by an external audio control module (ACM). For example, the ACM can be accessed via wires, sometimes about 6 feet in length, and can include a display, microphone preamplifier inputs, and headphone outputs. While providing convenience, this module may cause increased noise, especially common-mode noise. The differential amplifier 227 is appropriately placed to handle this noise.

[0028] Although not shown here, a computer audio card such as card 200 will also have an audio amplification circuit for conventional audio signals, i.e., non-microphone signals. Since the amplification factor of these audio circuits is much smaller than the 100× (40 dB) described for the microphone preamplifier, the power supply noise generated as power supply ripple is not a problem and does not need to be isolated.

[0029] Figure 5 is a diagram showing the sampled power supply noise reduction in the presence of a heavily loaded graphics card according to some embodiments of the present invention. Figure 6 is a diagram showing another view of the sampled power supply noise reduction in the presence of a loaded graphics card according to some embodiments of the present invention. Figure 5The comparison of oscilloscope traces 502 and 504 shows the improvement in microphone noise of the isolation block using various embodiments of the present invention. Specifically, by using the isolation block 210 to isolate the 12V power supply voltage provided by the auxiliary power connector (102) from the microphone preamplifier 208, the power supply ripple voltage, shown as up to 1 volt peak-to-peak voltage, is significantly reduced. A good estimate of the frequency of the shown power supply ripple voltage is in the audio range, here estimated at about 1 kHz, but this can vary depending on the application.

[0030] Various embodiments of the present invention have been described above, typically having an isolation circuit connected to a power rail, the circuit including at least a transformer providing isolation between the power supply and the amplification circuit. It should be noted that the present invention is not limited to an isolation circuit having a transformer. The scope of the present invention is intended to further cover any environment where the circuit in the isolation block isolates the power ground from the audio amplifier circuit ground and more specifically from the microphone amplifier circuit ground. In Figure 2 the shown embodiment, a switching network and a transformer are used for ground isolation, and there is no change in the voltage from input to output. That is, in one embodiment, the present invention uses only the isolation block for isolation purposes. In other embodiments, the switching circuit, transformer, and rectifier block of the isolation block 210 are additionally used for voltage conversion. For non-limiting examples, the voltage conversion can be 12V to 12V, 12V to 5V, 5V to 12V, or simply any voltage to any voltage within the component specifications.

[0031] By using the isolation circuit, audio range ripple is substantially eliminated to provide a clean output from the microphone preamplifier and a clean signal to the DSP. The isolation circuit or block uses an isolated switched-mode power supply to decouple the power supply from the microphone preamplifier. By applying the microphone signal in a differential drive mode, we also reduce the common-mode noise introduced by other sources. As a result, various embodiments of the present invention provide a better user experience. The advantages provided by the described noise isolation and reduction features include clear voice communication without using an external mixer.

[0032] Although the foregoing invention has been described in considerable detail for purposes of clear understanding, it is apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the embodiments of the present invention should be considered illustrative rather than restrictive, and the present invention is not limited to the details given herein but may be modified within the scope of the appended claims and their equivalents.

Claims

1. An audio processing system for use with a computing device, the system comprising: An isolation block located between a microphone preamplifier circuit and a power supply of the computing device, the isolation block being configured to receive a power supply signal including a positive voltage and a first ground reference from a power supply connector of the computing device, the isolation block being configured to reduce power supply noise present in the power supply signal by providing a positive output voltage and a second ground reference at its output, the second ground reference being isolated from the first ground reference; The microphone preamplifier circuit, configured to have one input receiving a microphone signal and to provide a microphone preamplifier output signal, and wherein the microphone preamplifier circuit is powered by the positive output voltage and the second ground reference from the isolation block, and A digital signal processor block, configured to receive the microphone preamplifier output signal, convert the microphone preamplifier output signal to a digital format, and transmit the digitized microphone preamplifier output signal to a computer bus on the computing device, wherein the computer bus includes a third ground reference, the third ground reference being isolated and different from the first ground reference and the second ground reference.

2. The system according to claim 1, wherein, The computing device is one of a laptop computer or a desktop computer.

3. The system according to claim 1, wherein The power supply connector is a PCIe power socket, and the computer bus is a PCIe bus.

4. The system according to claim 1 further includes a differential amplifier connected between the microphone preamplifier and the digital signal processor block, and wherein, The differential amplifier is configured to reduce common-mode noise from the microphone preamplifier output signal.

5. The system according to claim 1, wherein, The isolation block includes a switching circuit and a transformer.

6. The system according to claim 5, wherein, The switching circuit includes a push-pull configuration.

7. A method for reducing noise in a microphone circuit, comprising the steps of: Receiving, by an isolation block, a power supply signal including a positive voltage and a first ground reference from a power supply connector of a computing device, wherein the isolation block is located between the microphone circuit and the power supply of the computing device; Isolating a low-noise power supply from a noisy power supply by providing a positive output voltage and a second ground reference at an output of the isolation block to reduce power supply noise present in the power supply signal, the second ground reference being isolated from the first ground reference; Configuring the microphone circuit to receive power from the low-noise power supply; And Configuring a digital signal processor block to receive a microphone circuit output signal from the microphone circuit, convert the microphone circuit output signal to a digital format, and transmit the digitized microphone circuit output signal to a computer bus on the computing device, wherein the computer bus includes a third ground reference, the third ground reference being isolated and different from the first ground reference and the second ground reference.

8. The method according to claim 7, wherein The step of isolating the low-noise power supply includes isolating with a switching circuit and a transformer.

9. The method according to claim 8, further comprising applying the microphone circuit output signal to a differential amplifier to reduce common-mode noise.

10. A computer audio card for coupling to a computing device, comprising: An auxiliary power socket, configured to receive an auxiliary power connector providing supplementary power from a computing device power supply; A microphone preamplifier, configured to receive a microphone signal and amplify it before transmitting it to an analog-to-digital converter (ADC); An isolation block, located between the microphone preamplifier and the power supply of the computing device, the isolation block being configured to condition the supplementary power by isolating the supplementary power from the power delivered to the microphone preamplifier, the isolation block being configured to receive a power supply signal including a positive voltage and a first ground reference from the auxiliary power connector, the isolation block being configured to reduce power supply noise present in the power supply signal by providing a positive output voltage and a second ground reference at its output, the second ground reference being isolated from the first ground reference; And A digital signal processing block, configured to receive a digital signal from the ADC and process the digital signal, the digital signal processing block further being configured to transmit the processed digital signal to a computer bus coupled to the computing device, wherein the computer bus includes a third ground reference, the third ground reference being isolated and different from the first ground reference and the second ground reference.

11. The computer audio card according to claim 10, further comprising a microphone differential amplifier in the microphone path for reducing common-mode noise.

12. The computer audio card according to claim 10, wherein, The computing device is a desktop computer, the computer bus is a fast PCI bus, the auxiliary power connector is an auxiliary PCIe power connector, and the auxiliary power socket is configured to receive the auxiliary fast PCI power connector.

13. The computer audio card according to claim 11, wherein, The microphone amplifier is connectable to a microphone via an input jack.

14. The computer audio card according to claim 13, wherein, The microphone input jack is located on an external audio control module.

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