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'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'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.