Abstract:
Challenges to the implementation of equalization in the 2.1 environment arise from the constraints imposed by HD audio requirements and Windows® Vista™. A hybrid software hardware solution overcomes many of the challenges by exploiting the software capability for equalization and using a hardware codec to perform the separation into high frequency and low frequency audio streams needed to drive stereo speakers and a subwoofer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to audio applications and specifically with equalization in audio applications. 
     2. Related Art 
     A common audio configuration uses two stereo channels and a subwoofer channel, known as 2.1. The two stereo channels drive a left and a right speaker, respectively, which respond well to high frequency audio content. The subwoofer channel drives a subwoofer which is generally larger and will respond to low frequency audio content. 
       FIG. 1  illustrates a conventional codec that drives a 2.1 system. A digital stereo signal is separated into high and low frequency components by crossover filter  110  which comprise high pass filter  102  and low pass filter  104 . High pass filter  102  extracts the high frequency portion of digital audio signal and low pass filter  104  extracts the low frequency portion of the digital audio signal. The frequency dividing the high and low frequency portions is often referred to as the “crossover” frequency. The high frequency portion of the audio signal is converted to an analog signal using digital to analog converter (DAC)  108 . This analog signal can then drive speakers  122 . Additional analog processing functions  114  can comprise an analog amplifier and line driver. Similarly, the low frequency portion is converted to an analog signal using DAC  112 . Unlike the high frequency components which have a left and right channel. Subwoofers generally use only a monaural channel. Mono mixer  106  is used to convert the stereo signal into a monaural signal. The monaural analog audio signal can then drive subwoofer  124 . Additional analog processing functions  116  may also be provided. 
     Although shown as single components, the connections shown in the drawings can represent multiple channels. Furthermore, many components are shown as a single block but in practice, may be implemented with multiple components. For example, a “stereo” DAC (e.g., DAC  108 ) may be implemented as two separate DACs one for each stereo channel. 
     Equalization is used to alter the frequency response of an audio system to enhance the listening experience. For example, output transducers, speakers and headphones have varied frequency responses. The defects in the frequency response of the output transducer can be compensated for by selectively attenuating or applying gain to the signal at particular frequencies. Equalizers can be implemented algorithmically, or through the use of passive or active electrical components. 
     Traditionally, the application of equalization to a 2.1. audio system employs a high frequency equalizer to equalize the high frequency portion of the audio signal destined for the stereo speakers and a low frequency equalizer to equalize the low frequency portion of the audio signal destined for the subwoofer. 
     Under Intel&#39;s High Definition Audio standard (HD-audio) commonly used in personal computers, a 2.1 channel audio stream is not specifically recognized. As a result, contemporary HD-audio implementations use a separate DAC and audio stream for the subwoofer. Software can be used to perform the crossover filtering and the equalization. 
       FIG. 2  illustrates an exemplary HD-audio implementation. Software module  210  separates the audio stream into two audio paths that are applied to high-pass filter  214  and low pass filter  216 , respectively, which are software implementations of high pass filter  102  and low pass filter  104 , respectively. High frequency equalizer  218  applies equalization to the high frequency audio path while low frequency equalizer  220  applies equalization to the low frequency audio path. Mono mixer  224  on the low frequency audio path is a software implementation of mono mixer  106 . The two audio paths traverse HD-audio interface  230  to communicate with hardware codec  240 . Interface  232  provides an HD-audio interface between software module  210  and hardware codec  240  for the high frequency audio path. Likewise interface  234  provides an HD-audio interface between software module  210  and hardware codec  240 . Hardware codec  240  comprises DAC  242  and DAC  244  which function similarly to DAC  108  and DAC  112  as described for  FIG. 1 . Block  246  and block  248  are similar to that described for block  114  and block  116 , respectively. 
     While this architecture provides a workable system for providing equalization in a 2.1 audio system, it suffers from several draw backs. First, in order to perform the separation in the software, the incoming audio stream has to be duplicated so that high pass filter  214  can filter one copy of the audio stream and low pass filter  216  can filter another copy of the audio stream. To accomplish this, copy operation  212  is performed, which causes a certain amount of latency. Because both audio streams must remain synchronous, the audio stream sent to the high frequency audio path needs to be delayed to maintain synchronicity with audio stream sent to the low frequency path, thus producing additional delay. 
     Another drawback to this approach is that to meet HD-audio requirements in certain operating systems, such as Microsoft Corporation&#39;s Windows® Vista™ or Windows® 7, any audio stream used by the HD-audio interface must be exposed as an audio output stream to the operating system and ultimately to the end user. In order to use a separate stream for the subwoofer and to satisfy the constraints imposed by Windows® Vista™, a subwoofer object will show up as a user-accessible output endpoint on an “audio control panel.” 
     For example,  FIG. 3  shows an exemplary “control panel” in Windows® Vista™. Object  302  is used to select the output speakers. Selection of object  302  will also select the subwoofer through a redirection in the software. In order to satisfy the HD-audio requirements of the operating system, object  304  is also displayed to direct an audio stream to the subwoofer alone. The display of object  304  is required by the HD-audio requirements, which is unnecessary because selection of object  302  will produced the desired speaker selection. With two objects available, it becomes possible for the end user to select the subwoofer as the destination of the audio stream. If this occurs the end-user would experience no high frequency sounds. In addition, if multiple 2.1 channel outputs are available to the end-user, the number of objects required to be display would increase resulting in more confusion to the end-user and increasing the probability of error by the end-user in selecting the audio playback device. 
     SUMMARY OF INVENTION 
     In a system, a 2.1 audio system has a software module including a digital full band equalizer in communications with a hardware codec. The hardware codec includes a crossover filter for separating a stereo signal into high frequency and low frequency components. The hardware codec also includes a mono mixer which combines the low frequency components of the stereo signal a monaural audio signal. The hardware codec also includes a stereo DAC for converting the high frequency components of the audio signal to an analog signal which can undergo additional analog processing such as an amplifier and/or line driver and which ultimately drives a pair of speakers. The hardware codec also includes a subwoofer DAC for converting the low frequency monaural audio signal into an analog signal which can also undergo additional analog processing and which ultimately drives a subwoofer. 
     In an alternate embodiment, a 2.1 audio system comprises a hardware codec which receives a stereo audio signal from an HD-audio interface. The hardware codec comprises a crossover filter and mono mixer, but also comprises a digital high frequency equalizer and a low frequency equalizer to process the high frequency and low frequency components of the received stereo 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  illustrates a conventional codec that drives a 2.1 system; 
         FIG. 2  illustrates an exemplary HD-audio implementation of a 2.1 system; 
         FIG. 3  shows an exemplary “control panel” in Windows® Vista™; 
         FIG. 4  illustrates an embodiment of the present invention; 
         FIG. 5  illustrates an alternative embodiment of the present invention; 
         FIG. 6  shows an exemplary system employing the 2.1 audio equalization, according to one embodiment of the present invention; and 
         FIG. 7  shows an embodiment of the control panel interface provided by the operating system. 
     
    
    
     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. 
     It should be noted that for clarity the diagrams shown each audio stream is represented by a single line, but can comprise several channels. For example, between high pass filter  442  and high frequency equalizer  446 , a single audio path is shown in  FIG. 4 . However, this audio path comprises a left channel and a right channel. 
       FIG. 4  illustrates an embodiment of a hardware solution providing equalization in a 2.1 environment. Hardware codec  440  comprises a crossover filter here shown as comprising high pass filter  442  and low pass filter  444 . Hardware codec  440  further comprises high frequency equalizer  446 , low frequency equalizer  448 , mono mixer  450 , DAC  242 , DAC  244  and processing blocks  460  and  470 . Processing blocks  460  and  470  perform additional analog processing. As an example processing blocks  460  and  470  each are shown as comprising amplifiers  462  and  472 , respectively and line drivers  464  and  474 , respectively. In this embodiment, each of the components of hardware codec  440  is implemented in hardware, without software performing any processing. 
     The embodiment of  FIG. 4  resolves many of the drawbacks found in the prior art. As illustrated, only one audio stream is need for communication between software module  410  and hardware codec  440  through interface  230 . Therefore, the operating system and the end-user are exposed to a single audio stream. In addition, because the audio stream is duplicated in hardware, the latency issue is eliminated. 
     In certain applications, equalization can be expensive to implement in hardware. Equalization can use substantial configuration information from an end-user manipulating a graphic equalizer interface or equalization parameters determined from speaker or headphone characteristics. A hardware equalizer that is both tunable to the number of equalization parameters and that provides good frequency response can be costly. In HD-audio environments, the HD-audio requirements can impose additional constraints on the amount of configuration information made available to the hardware codec. 
       FIG. 5  illustrates an alternate embodiment of the present invention that includes an equalizer implemented in software. The configuration comprises software module  510  which communicates to hardware codec  540  through HD-audio interface  230 . Only one interface  230  is needed to support the 2.1 audio stream. Software module  510  comprises a full band equalizer  512  that functions as a high frequency equalizer when processing high frequency audio components in an audio stream and a low frequency equalizer when processing low frequency audio components in an audio stream. In one embodiment, the two processes occur simultaneously on an input audio stream provided as a time domain signal. The equalized audio stream is passed through HD-audio interface  230  to hardware codec  540 . Hardware codec  540  duplicates the stream into a high frequency audio path and a low frequency audio path. Because the duplication is done in hardware the latency problem of a software copy is eliminated. The copy of the stereo audio stream sent to the high frequency audio path is high pass filtered by high pass filter  442  and converted to a stereo analog signal with DAC  242 . The stereo analog signal is then driven by block  460  to drive stereo speakers  122 . The copy of the stereo audio stream sent to the low frequency audio path is low pass filtered by low pass filter  444  and the stereo audio stream is mixed into a single monaural audio signal by mono mixer  450 . The resultant monaural audio stream is converted to a monaural analog signal by DAC  244  and subwoofer  124  is driven by block  470 . 
     The embodiment of  FIG. 5  has many advantages over the prior art. The HD-audio interface only uses one audio stream to communicate between the software module and the hardware codec, presenting the user with a single “2.1 speaker” endpoint, rather than a stereo speaker endpoint and a subwoofer endpoint. By moving the crossover separation to the hardware the latency problem experienced by a software stream copy is eliminated. Digital filters are implemented in hardware leaving the more complex equalization to the software module, thus reducing the cost of implementation. 
       FIG. 6  shows an exemplary system employing the 2.1 audio equalization, according to one embodiment of the present invention. In the embodiment of  FIG. 3 , system  600  includes a controller or central processing unit (CPU)  602 , optionally mass storage device  604 , main memory  610 , and hardware audio codec  540  which was described previously in  FIG. 5 . System  600 , which can be for example, a personal computer (PC), a laptop computer or a personal electronics device, can also include input devices, a display, read only memory (ROM), an input/output (I/O) adapter, a user interface adapter, a communications adapter, and a display adapter, which are not shown in  FIG. 6 . System  600  can further include a compact disk (CD), a digital video disk (DVD), and a flash memory storage device, which are also not shown in  FIG. 6 , as well as other computer-readable media as known in the art. 
     As shown in  FIG. 6 , CPU  602  is coupled to mass storage device  604  and main memory  610  typically through a bus which provides a communications conduit for the above devices. CPU  602  can be a microprocessor, such as a microprocessor manufactured by Advanced Micro Devices, Inc., or Intel Corporation. Mass storage device  604  can provide storage for data and applications and can comprise a hard drive or other suitable non-volatile memory device. Main memory  610  provides temporary storage for data and applications and can comprise random access memory (RAM), such as dynamic RAM (DRAM), or other suitable type of volatile memory. Also shown in  FIG. 6 , main memory  610  includes software applications  612 , which can include client applications that employ audio, operating system  614 , which can be Windows® Vista™ or Windows® 7, and software module  510  which was described previously in  FIG. 5 . 
     It should be noted that software module  510 , software applications  612 , and operating system  614  are shown to reside in main memory  610  to represent the fact that programs are typically loaded from slower mass storage, such as mass storage device  604 , into faster main memory, such as DRAM, for execution. Software module  510  is coupled to hardware codec  540  through HD audio interface  230 . The particular audio endpoint is selectable through the use of a control panel interface provided by operating system  614 . 
       FIG. 7  shows an embodiment of the control panel interface provided by the operating system. Unlike the prior art control panel interface of  FIG. 3 , the end user is only offered 2.1 single endpoint  702  for both the two stereo speakers and the subwoofer. 
     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.