1. Field of the Invention
The invention relates in general to audio playback systems and specifically to reducing distortion caused by speakers.
2. Background Information
Small speakers have found prolific uses in the state of the art. They appear in computers, particularly laptops, but also exist in telephones, in particular cordless and cellular phones. Additional applications include MP3 players and video cameras. Generally for these applications, the small speakers do not have the dynamic ranges and linear frequency responses of their larger or more expensive counterparts. As a result, many small speakers are susceptible to intense distortion if power delivered in certain frequency bands become too strong.
In some applications, it is desirable to reduce distortion in a speaker. One approach is to maintain the power levels of these vulnerable frequency bands below the distortion power threshold. For the purposes of this disclosure, the distortion power threshold for a given frequency band is the amount of power in the given frequency band the speaker can tolerate before the distortion becomes unacceptable for a particular audio application. A speaker's vulnerable frequency bands are frequency bands that have particularly low distortion power thresholds that may be exceeded during speaker use.
In the present day, a trend has been moving more towards purchasing music via download rather than through a compact disc. Additionally, users are increasingly using devices such as computers, phones, MP3 players, and even combination video cameras as a means for listening to music. All of these use the small speakers mentioned. When a user listens to music, he may be forced to turn down the volume when the music causes a vulnerable frequency band to reach its distortion power threshold.
Some playback applications may allow the user to apply an equalizer during the playback. Depicted in
It should be noted that gain and attenuation are mentioned together. Gain or attenuation is applied to a signal to relatively suppress a portion of an audio signal. If the power level is actually reduced the signal is attenuated. If the power level is increased but not compared to other frequency bands, gain is applied, but relatively speaking the portion of the audio signal is actually suppressed. For the purposes of this disclosure, the terms applying gain or attenuation may be used interchangeable, but should be understood to mean a scaling of a portion of the audio signal relative to the rest of the audio signal.
While providing the end user an equalizer allows the user to tune the output to avoid distortion without necessarily turning down the overall volume, this approach has significant shortcomings. First, the bandwidth of the filter controlled by each slider is often too broad so attenuating the vulnerable frequency band has a significant impact on other frequencies negatively impacting the quality of the playback. Second, due to the nature of music, every song or composition has a different frequency profile. A given song may require attenuation to prevent a vulnerable frequency band from exceeding the distortion power threshold while a second song may not, so either the user has to readjust the equalizer or the second song is unnecessarily altered.
Another approach is to provide a preconfigured equalizer. The preconfigured equalizer can attenuate frequencies using band pass filters covering known vulnerable frequency bands. This approach has the advantage of exploiting the knowledge of the particular frequency bands that are vulnerable. Additionally, the bandwidth of the constituent band pass filters can be narrower than that of the user adjusted equalizer, thus minimizing the impact on other frequencies. However, this approach still has the shortcoming that depending on the song played a band pass filter may be unnecessarily applied to the song thus introducing undesired playback effects when not necessary. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies.
A system for reducing distortion in a speaker comprises an equalizer or equalizer module which can attenuate or suppress the vulnerable frequency bands of the speaker in an audio signal. A monitoring module measures the signal intensity for each of the vulnerable frequency bands and supplies the intensity levels to the control module which is configured to adjust the suppression or attenuation if any the equalizer applies to each vulnerable frequency band. This adjustment is made so that the resultant power remaining in the vulnerable frequency band is not sufficient to cause significant distortion in the speaker. For a given speaker the intensity level in each vulnerable frequency band sufficient to cause distortion is determined and this intensity level is referred to as the distortion power threshold. The control module receives measured signal intensities from the monitoring module and controls the equalizer to suppress the audio signal so that the intensity level in the vulnerable frequency band is lower than the intensity level sufficient to cause distortion. Additionally, the monitoring module can measure the signal before equalization or after equalization in a feedback configuration.
The system can also be pre-emptive by delaying the audio signal prior to equalization, so that a large spike in the intensity in a vulnerable frequency band can be predicted and a transition to the attenuation can be applied to avoid an abrupt transition that may produce undesirable playback artifacts.
The monitoring module can comprise a band pass filter and a module that measures the root mean square (RMS) power for each vulnerable frequency band. This band pass filter can be centered at the center frequency of the vulnerable frequency band and have a bandwidth greater than or equal to the band width of the vulnerable frequency band. The equalizer can comprise one or more band equalizers. Each band equalizer can also be centered at the center frequency of the vulnerable frequency band and have a bandwidth greater than or equal to the band width of the vulnerable frequency band.
The system can also account for an attack and release time when transitioning to and from attenuation a vulnerable frequency band. The attenuation can also account for the master volume set by the user.
The system can also be applied to a multi channel playback system such as stereo or surround sound. At the option of the user or based on a preset configuration, the attenuation applied to each channel can be locked so that the effect on each channel by the equalizer is uniform.
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.
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.
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 as defined by the appended claims.
In particular, control module 304 receives information about the vulnerable frequency bands from monitoring module 302. In one embodiment, the received information includes the root mean square (RMS) power levels of each of the vulnerable frequency bands. If the power level of a vulnerable frequency band exceeds the corresponding distortion power threshold for that band, then control module 304 adjusts equalizer 306 so that the vulnerable frequency band is suppressed. Rather than completely suppressing the vulnerable frequency band, the vulnerable frequency band need only be attenuated until the vulnerable frequency band in the resultant filtered signal is below the distortion power threshold. In an alternate embodiment, the distortion power threshold is an absolute distortion power threshold which does not change based on user volume information (such as system 250) or it can be relative based on the user volume information. For example, if the user turns up the volume, the relative distortion power threshold should be lowered so when the audio signal is amplified, the resultant signal will stay below the distortion power threshold of the speaker.
The information provided by monitoring module 302 can be reactive or predictive. If equalizer 306 adjusted in response to the immediate power levels of the vulnerable frequency bands, the information is reactive. However, if equalizer 306 is adjusted in response to the predicted or future power levels of the vulnerable frequency bands, the information is predictive.
The audio signal described above can be an analog signal and analog components can be used for monitoring module 302, control module 304, and equalizer 306. The audio signal can also be a digital signal and likewise digital components can be used. A mixture of digital and analog components can also be used. Although depicted as hardware modules, individual modules can also be implemented either in hardware, software, firmware or combination thereof. In addition the audio signal can be processed in the time domain or frequency domain.
Frequency parameter 910 is provided to both band pass filter 902 and control module 906 which passes frequency parameter 910 to band equalizer 908. The frequency parameter specifies the center frequency of the various bands used in both band pass filter 902 and band equalizer 908. Frequency parameter 910 should be set to the center frequency of the vulnerable frequency band assigned to this particular frequency band limiter stage.
Bandwidth parameter 912 is provided to control module 906 which passes it on to band equalizer 908. The bandwidth parameter specifics the width of the band of frequencies about the center frequency that the equalizer should apply the supplied gain or attenuation.
Limit parameter 914 is provided to control module 906 which specifies the absolute distortion power threshold of the speaker at the vulnerable frequency band. Control module 906 can use the limit parameter along with the master volume parameter to determine the relative distortion power threshold and adjust band equalizer 908 accordingly.
Attack time parameter 916 is an optional parameter which sets the attack time of band equalizer 908. For example, suppose a song is playing with very little power at the vulnerable frequency band and suddenly there is an abrupt crescendo in the music causing the power at the vulnerable frequency band to rise rapidly and suddenly. A sudden high attenuation by band equalizer 908 can cause undesired artifacts to the listener. As a result it may be more desirable to gradually increase the attenuation over the attack period. Furthermore, if a delay line such as shown in
Release time parameter 918 is an optional parameter which sets the release time of band equalizer 908. Just as an abrupt increase in attenuation can have undesired listening effect, so can an abrupt removal of attenuation. The release time is the interval of time for the attenuation to be removed. Typical release times range from 1 to 5 seconds.
Bandwidth bandpass parameter 920 specifies the bandwidth of bandpass filter 902. Essentially, it should be set to cover the vulnerable frequency band monitored by the monitoring module. Generally, the same value is used as bandwidth parameter 914, but there are circumstances that a different value is desired.
Finally, master volume parameter 922 specifies the volume set by the user. The parameter is used to determine the relative distortion power threshold. For example, the relative distortion power threshold is essentially the absolute distortion power threshold minus the master volume. If the power levels detected in the vulnerable frequency band then the attenuation (or gain), expressed in dBs, applied can be set to the distortion power threshold minus measured power levels minus the master volume.
Because any filtering can have an undesired effect on the music played back, the filtering of vulnerable frequency bands can be minimized to occasions when filtering is needed to avoid distortion by the speaker. Other approaches to limiting speaker distortion resort to suppression or filtering of vulnerable frequency bands constantly. The result is that music can sound thin or filtered. Furthermore, filtering performed by the systems and methods disclosed herein may be configured such that suppression of the vulnerable frequency bands during soft passages in music, if any, is small whereas the suppression may be greater during loud passages. Overall, the impact on the playback of music is reduced while maintaining the ability to reduce speaker distortion.
To further minimize the filtering performed, control module 906 can also exploit known psychoacoustic properties. Generally, speaker distortion results in artifacts in certain frequencies. A given vulnerable frequency band when the distortion power threshold is exceeded can produce artifacts in one or more artifact frequencies. In psychoacoustics, certain frequencies are known to mask other frequencies, that is when a masking frequency is present then the frequency that is masked is not perceived by the human ear. If the masking frequencies are present with enough energy to mask the artifact frequencies, then the distortion will not be heard by the human ear and attenuation of the vulnerable frequency band can be avoided in this circumstance.
In many playback systems such as stereophonic, multiple speakers are used as well as multiple channels. While the speaker distortion reduction techniques and systems described above could be used to reduce speaker distortion, not all channels necessarily would reach the distortion power threshold at the same time. If speaker distortion reduction is performed independently on each channel. The playback effect could be altered. For example, the stereo effect could be changed or lost.
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. For example, rather than monitor power, any measure of signal intensity can be used, for example the magnitude of the signal itself can be measured and an intensity based threshold can be used instead of the distortion power threshold. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.
This application claims priority under 35 U.S.C. §119 to U.S. Patent Application No. 61/155,397 filed Feb. 25, 2009, entitled “Speaker Distortion Reduction Systems and Methods.”
Number | Date | Country | |
---|---|---|---|
61155397 | Feb 2009 | US |