Abstract:
An integrated audio codec includes a high-pass filter to prevent damage to personal computer speakers and other components. The audio codec may be compliant with HD audio standards and can operate with generic software drivers. Tuning of the high-pass filter is provided through an external pin-out where either an external capacitor or external resistors provide an ability to tune the high-pass filter. In one implementation, a tuning voltage is digitized into a tuning code used by a digital high-pass filter. In addition, multiplexers can be used to insure only the audio path leading to the speakers is filtered.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to audio codecs and specifically with the tuning of high-pass filters within audio codecs. 
     2. Related Art 
     Excessive low frequency (“bass”) sounds are often problematic for personal computers (PC). Both internal and external speakers on PCs are typically small and can be damaged if too much power is delivered at low frequencies. Excessive low frequency sound may also cause problems with components within a personal computer. For example, the vibrations from low frequency sound can lead to data corruption and even failure of a hard drive device.  FIG. 1  shows a prior art system which attenuates harmful low frequency signals. The audio signal is supplied by HD audio driver  102 , which is filtered by high-pass filter  104  and drives speaker  106 . 
     The advent of high definition (HD) audio standards in PCs has imposed certain audio performance requirement on external outputs, such as headphones or microphone jacks. However, such requirements are not imposed on integrated speakers which allows a high-pass filter to be inserted into the audio path to integrated speakers. The HD-audio standards also allow for individual digital to audio converters (DACs) to be assigned to arbitrary audio paths. Under the standard, the HD codec must operate with a generic audio driver. Therefore, if a high-pass filter is included in the audio path, specialized driver software would not be available to enable or adjust it and the high-pass filter must be automatically tuned or set in the factory by a vendor. 
     These limitations are not an obstacle to HD audio codecs where low impedance drivers, such as speaker drivers, are separate from the audio codec. A user or vendor could place a high-pass filter between the output of the codec and the speaker driver, leaving all other audio paths unfiltered. 
       FIG. 2  illustrates a HD audio system as known in the art. Integrated circuit  210  comprises DAC  202  and line driver  206  for a speaker path and DAC  204  and headphone driver  208  for the headphone path. DAC  202  and DAC  204  receive audio input from HD audio interface  260  as digital data. DAC  202  and DAC  204  each convert their respective digital audio signals into an analog audio signal. The analog audio signal passed to line driver  206  is amplified and provided to line output  252  which can be connected to a speaker driver. The analog audio signal passed to the headphone driver is provided to headphone output  254  which may be connected to a jack, where headphone  244  could be attached. The line out analog audio signal does not have sufficient power to drive a speaker, so speaker driver  232  is employed to amplify the audio signal to drive speaker  242  in a low impedance interface. In this system, speaker driver  232  is provided on integrated circuit  230  which is separate from integrated circuit  210 . In order to prevent damage to speaker  242 , high-pass filter  220  is inserted into the audio path between line driver  206  and the speaker driver  232 . This high-pass filter can be a resistor  214  connected to ground and capacitor  212  in series with the audio path as shown in the figure. 
     Because the desired frequency response of the high-pass filter is often based on the speaker characteristics, the speaker manufacturer either supplies or specifies requirements for the high-pass filter. In the architecture shown in  FIG. 2 , neither the manufacturer of integrated circuit  210  nor the manufacturer of integrated circuit  230  needs to know the requirements of the high-pass filters. 
     In today&#39;s trend of further integration, integrated circuits, audio codecs are being integrated with the speaker drivers on the same chip. This complicates access to the audio path for the insertion of a high-pass filter. Therefore, there is a need in the industry to insert a high-pass filter into an HD audio path while meeting HD audio requirements including compatibility with a generic software driver, that is, without the ability to tune the high-pass filter through software. 
     SUMMARY OF INVENTION 
     One embodiment of an integrated audio driver in an HD audio environment comprises in the speaker audio path a digital high-pass filter, a DAC that converts the filtered audio signal to an analog audio signal, a speaker driver that drives speakers with the analog audio signal and a control signal which receives external tuning information and configures the digital high-pass filter based on the tuning information. 
     In one variation, the tuning information the tuning information is a voltage received at an integrated circuit pin-out and the control circuit converts the voltage into a control code supplied to the digital high-pass filter. This voltage can be produced by an external voltage divider comprising two resistors. The digital high-pass filter can be deactivated when the voltage is essentially the ground potential. A pull-down resistor can be coupled to the integrated circuit pin-out so that the digital high-pass filter is deactivated if nothing is connected to the integrated circuit pin-out. 
     The integrated audio driver can also comprise another DAC and headphone driver placed in the headphone audio path. The selection of either DAC for either audio path can be accomplished through three digital multiplexers and two analog multiplexers. The first selects which audio path is supplied to the digital high-pass filter, the second selects which audio path is supplied to the first DAC, and the third selects which audio path is supplied to the second DAC a second digital multiplexer operable to select the first digital audio signal or the filtered digital audio signal as an input signal to the DAC. The first analog multiplexer selects which analog signal is sent to the speaker driver and the second analog multiplexer selects which analog signal is sent to the headphone driver. In addition a high-pass filter control circuit can be used to control the three digital multiplexers on the basis of a DAC select signal and a speaker driver enable signal. 
     An alternate embodiment of the integrated audio driver in an HD audio environment comprises a DAC to convert a digital audio signal to produce an analog audio signal, an portion of an analog filter coupled to an external capacitor, where the portion of the analog filter and the external capacitor comprise a high-pass filter which filters the analog audio signal, and a speaker driver that drives speakers based on the filtered analog audio signal and to drive a speaker. 
     One embodiment of the portion of the analog filter can comprises an analog subtractor that receives the analog signal, an amplifier coupled to the analog subtractor that produces a filtered analog audio signal, a transconductance element that produces a current proportional to the filtered analog audio signal through the external capacitor. The voltage of the external capacitor is supplied to the analog subtractor where the voltage is subtracted from the analog audio signal. 
     Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows generically a system which attenuates harmful low frequency signals; 
         FIG. 2  illustrates a typical HD audio system; 
         FIG. 3  illustrates an HD audio codec with the speaker driver integrated on the same integrated circuit; 
         FIG. 4  illustrates a version of an integrated HD audio codec where the high-pass filtering is sent off chip; 
         FIG. 5  illustrates an integrated audio codec provides a tunable analog high-pass filter between a DAC and a speaker driver; 
         FIG. 6  illustrates an embodiment of an integrated audio codec employing a digital high-pass filter; 
         FIG. 7  is an example of an integrated audio codec with a simple method of external tuning of a digital high-pass filter; and 
         FIG. 8  shows an integrated audio codec which has selectable DACs. 
     
    
    
     Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
     DETAILED DESCRIPTION 
     A detailed description of embodiments of the present invention is presented below. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure. 
     For clarity, the connections between components in the figures in this disclosure are represented by a single line. However, these connections can be single-ended or differential. In addition the integrated circuit outputs and headphone outputs are shown as a single node, but may be either a single-ended or differential output. Additionally, each output node is intended to represent an output interface for an integrated circuit and can be implemented in any number of packaging techniques including but not limited to flip-chip bump, a pin, a pad, or a solder ball. While the technology may vary, each of these outputs is referred to as a pin-out. In the case of a differential output, each node represents two pin-outs. In addition since the HD audio codec may be integrated with the HD audio interface no explicit input node is shown leading to the individual DACs. In the event the HD audio codec resides on a separate IC from the HD audio interface input pin-out(s) would be needed. 
     One naïve approach to introducing a high-pass filter to the HD audio path shown in  FIG. 3  is to add the high-pass filter to the output of integrated circuit  310 . However, for a high-pass filter to operate in this configuration, the filter would have to simultaneously operate at much high power, exhibit the desired frequency response, and provide a low impedance interface for speaker  242 . Meeting three criteria above would prove to be cost prohibitive especially in a PC speaker environment and such a solution would just simply not be considered. 
     Another approach is to mimic the configuration in  FIG. 2 . The output of line driver  206  would be sent off chip where it could be filtered where the filtered signal could be brought on chip and supplied top speaker driver  232 . In this configuration, a line driver would likely be necessary. 
       FIG. 4  illustrates a version of an integrated HD audio codec where the high-pass filtering is sent off chip. Integrated circuit  410  is similar to integrated circuit  310  except that it provides access to the interface between line driver  206  and speaker driver  232  through pins  402  and  404 . High-pass filter  220  which was also described in its use in system  200  can then be connected to pins  402  and  404  to yield a circuit which is essentially the same as shown in system  200 . In this arrangement, the manufacturer of the speaker can supply the high-pass filter which best suits the characteristics of the speaker, and the manufacturer of the HD audio codec can provide an HD audio codec which is not dependent on the characteristics of the speaker. 
     While shown in  FIG. 4  as requiring an additional 2 interface nodes, in actuality there are at least a left and right channel so two interface nodes are required for each channel bringing the total to four interface nodes. In high fidelity applications, the interfaces between the DAC and the speaker driver are differential rather than single-ended so each node could actually represent 2 pin-outs. This would lead to 4 pin-outs per channel giving a total of 8 pin-outs. The additional pin-outs adds expense to the manufacture and packaging of an integrated circuit. 
       FIG. 5  illustrates an embodiment of an integrated audio codec  510  providing a tunable analog high-pass filter between DAC  202  and speaker driver  232 . An analog high-pass filter is inserted between DAC  202  and speaker driver  232  and comprises amplifier  506 , feedback subtractor  502  and transconductance element  504 . In this embodiment, a capacitor  520  is provided off-chip, to reduce the difficulty and cost associate with fabricating a capacitor with high capacitance on the chip  510 . The capacitor  520  is connected to the high-pass filter through output node  508 . 
     In addition to using an off-chip capacitor  520  to provide capacitance for the high-pass filter by selecting from difference capacitance for capacitor  520 , the high-pass filter can be tuned. For example, the cut off frequency of the high-pass filter can be adjusted by adjusting the capacitance coupled to pin  508 . In operation, transconductance element  504  produces a current proportional to the output voltage of amplifier  506 . This current is driven into external capacitor  520 . The voltage across the capacitor is proportional to the integral of the current driven through it. This voltage is sampled by feedback subtractor  504  and subtracted from the output of DAC  202  and supplied to amplifier  506  thus completing the feedback loop. 
     The embodiment of  FIG. 5  illustrates an audio high-pass filter for a single audio channel. This solution may be extended to additional audio channels, such as each channel in a stereo PC, by adding two high-pass filters along with corresponding pins and capacitors. 
       FIG. 6  illustrates an embodiment of an integrated audio codec employing a digital high-pass filter. When integrated circuit  610  receives a digital audio signal destined for speaker  242 , it passes through digital high-pass filter  602  which removes potentially damaging low frequency signals. The filtered digital audio signal is then passed to DAC  202  which converts the audio signal to analog. Speaker driver  232  drives speaker  242  with the analog audio signal. Digital high-pass filter  602  is controlled by interface circuit  604  which receives tuning information through pin  606 . In one embodiment, interface circuit  604  receives tuning information through pin  606  to control a plurality of digital high-pass filters for each of a plurality of audio channels used such as two channels in a stereo PC application. 
     The digital signal received through pin  606  includes configuration information for DHPF  602 , which may include digital filter coefficients. The digital signal may be provided by an additional external digital circuit that would be powered on and coupled to the HD audio codec or, in alternative embodiment, by discrete circuit components. 
       FIG. 7  is an example of an integrated audio codec where the digital high-pass filter is tuned using discrete components. In this embodiment, the characteristic of the high-pass filter to be tuned is the cut off frequency, which could be represented by a single numeric value. The interface circuit is ADC  702  which receives a tuning voltage at pin  704 . The tuning voltage is a fraction of a reference voltage such as the supply voltage. ADC  702  converts the voltage at pin  704  into a digital code. The digital code is used as a tuning parameter for digital high-pass filter  602 . In one embodiment, the digital code is mapped to the cut off frequency of digital high-pass filter  602 . 
     As illustrated, a pair of external resistors function as a voltage divider. The voltage at pin  704  is proportional to the ratio of the resistances of resistor  706  and resistor  708 . For example, if the resistance of resistor  706  is R 1  and the resistance of resistor  708  is R 2 . The voltage seen at pin  704  is 
                 R   2         R   1     +     R   2         ⁢       V   REF     .           
With the appropriate choice of resistances, this resistor divider can produce any voltage between ground and V REF .
 
     The tuning parameter can be read upon startup of the HD audio codec or when the speaker path is activated, and that parameter can be used until the speaker path is deactivated or the HD audio codec is shutdown. Alternatively the tuning parameter can be periodically read by the ADC and the digital high-pass filter updated. Because the operation of the ADC does not need to be instantaneous, slower methods of reading the voltage at pin  704  can be used such as the use of a successive approximation ADC which comprises a successive approximation register (SAR), a sample hold, a comparator and a DAC. A successive approximate ADC can require several clock cycles to measure the voltage seen at pin  704 . 
     In addition to selecting a tuning parameter, this interface can also enable or disable the digital high-pass filter. For example, a code of “0” received through pin  704  can be used by the ADC  702  to disable digital high-pass filter  602 . This would correspond to a ground voltage at pin  704 . Optionally, a high impedance such as pull-down resistor  712  on integrated circuit  710  could be used to tie the voltage seen at pin  704  to ground when pin  704  is not connected to external circuitry. In this fashion, if no external circuitry is connected to pin  704 , then the digital high-pass filter is disabled. If resistor  712  is selected to be high enough, it would have negligible impact on a voltage at pin  704  when a resistor divider is attached. 
     In the embodiments of  FIGS. 5 ,  6  and  7 , a high-pass filter is placed in the audio path for a speaker. This high-pass filter can be tuned and enabled through the use of an external circuit, such as two external resistors as a resistor divider. One advantage of the digital solutions over the analog solution is that by using a digital high-pass filter, the frequency response for a given tuning parameter can be selected, whereas often times in an analog filter several capacitors or inductors may be needed to maintain a desired frequency response for a particular cut off frequency. 
     The HD audio specifications permit an HD audio interface to receive instructions from the operating system to select which of a plurality of DACs in an HD audio codec is connected to a particular output (e.g., headphone or speaker) and to enable or disable the particular output driver (e.g., headphone driver or speaker driver.) 
       FIG. 8  shows an embodiment of an integrated audio codec which has selectable DACs. Codec  810  supports the HD audio feature permitting the selection of the DAC and insures that when a DAC is connected to the speaker path, the digital high-pass filter is inserted into the digital portion of the signal path. In this example, two DACs are shown, but it should be understood this approach works for an arbitrary number of DACs. Codec  810  comprises DAC  812  and DAC  814 , multiplexers (mux)  822  and  824 , speaker driver  232  and headphone driver  208 . Signal  852  received from an HD audio interface is the DAC selection signal. Signal  852  can assign DAC  812  to either headphone driver  208  or speaker driver  232 . It can also simultaneously assign DAC  814  to the opposite driver (e.g., if DAC  812  is assigned to the headphone driver then DAC  814  can be assigned to speaker driver  232 .) Alternatively, signal  852  can leave DAC  812  or DAC  814  unassigned. Signal  854  is the headphone driver enable signal which enables or disables headphone driver  208 . Signal  856  is the speaker driver enable signal which enables or disables speaker driver  232 . Signals  854  and  856  are received from the HD audio interface. 
     In order to incorporate the tuned high-pass filter into the appropriate signal path, codec  810  includes high-pass filter control circuit  802 . High-pass filter control circuit  802  receives a copy of DAC selection signal  852  and speaker driver enable signal  856 . Additionally, codec  810  includes mux  832 , mux  834  and mux  836  along with digital high-pass filter  804  and ADC  806 . If selection signal  852  assigns DAC  812  to speaker driver  232  and signal  856  enables speaker driver  232 , high-pass filter control circuit  802  enables ADC  806  which can then read the tuning voltage from pin  704 . ADC  806  converts the tuning voltage into a tuning parameter for digital high-pass filter  804  as described with reference to  FIG. 7 . 
     High-pass filter control circuit  802  inserts digital high-pass filter  804  into the signal path of the speaker by controlling muxes  832 ,  834 , and  836 . Specifically, mux  836  is directed to select digital audio signal  842  to be processed by digital high-pass filter  804 . Mux  832  is directed to then select the filtered digital audio signal to be processed by DAC  812 , which is then sent to speaker driver  232  via mux  824 . Meanwhile, mux  834  selects digital audio signal  844  to be processed by DAC  814  directly, which may or may not be processed by additional downstream components (e.g., headphone driver  208 ). Alternatively, if selection signal  852  assigns DAC  814  to speaker driver  232  and signal  856  enables speaker  232 , high-pass filter control circuit  802  again enables ADC  806  so that it can be used to tune digital high-pass filter  804 . However, mux  836  is now directed to select digital audio signal  844  to be processed by digital high-pass filter  804 . Mux  834  is now directed to select the filtered digital audio signal to be processed by DAC  814 , which is then sent to speaker driver  232  via mux  824 . Mux  832  now selects digital audio signal  842  to be processed by DAC  814  directly which may or may not be processed by additional downstream components. 
     Using approach described above for HD audio codec  810 , a digital high-pass filter can be inserted into any audio path connected to a speaker leaving all other audio paths unfiltered. This approach leaves the tuning to a tuning voltage which can be generated with two external resistors allowing the HD audio codec to be fully compatible with a generic a generic software driver. In a more advanced audio system, there may be additional speaker drivers. In that case, additional digital high-pass filters could be added controlled by a high-pass filter control. In alternate embodiments, the tuning mechanism can be shared or additional tuning mechanisms could be added. 
     It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.