WO2012177572A2

WO2012177572A2 - Optimization method for audio playback systems

Info

Publication number: WO2012177572A2
Application number: PCT/US2012/043024
Authority: WO
Inventors: Clayton Williamson
Original assignee: Beats Electronics, Llc
Priority date: 2011-06-24
Filing date: 2012-06-18
Publication date: 2012-12-27
Also published as: WO2012177572A3; AR086737A1; TW201308198A

Abstract

A software product for optimizing sound reproduction by an audio system is disclosed. In one embodiment, the software includes an equalization engine for independently adjusting characteristics of an audio signal for each of a plurality of frequency bands. The software then passes the adjusted signal to a dynamics engine having independently adjustable attack and release thresholds and attack and release rate settings to enforce a maximum excursion limit of the speaker driver knowing the voltage-to-excursion relationship for the speaker driver. The software then passes the audio signal to a bass enhancement engine configured to boost all bass frequencies below a bass roll-off frequency of the speaker driver. The boosted signal is passed to a dedicated dynamics engine for the boosted bass frequencies, the dynamics engine again having independently adjustable attack and release thresholds and attack and release rate settings to enforce a maximum excursion limit of the speaker driver.

Description

OPTIMIZATION METHOD FOR AUDIO

PLAYBACK SYSTEMS

RELATED APPLICATION

[0001] This international application claims priority to U.S. Patent Application No. 61/501 1 16, filed on June 24, 2011 , entitled OPTIMIZATION METHOD FOR AUDIO PLAYBACK SYSTEM", the contents which are incorporated herein by reference in its entirety.

BACKGROUND

[0001] This application relates generally to the optimization of audio output from a variety of audio playback devices, and particularly to software-based solutions for optimizing audio output on audio playback devices.

[0002] A goal of any audio playback system is to reproduce input audio signals to the listener without unwanted artifacts while enabling playback at high output levels. Speaker drivers, speaker enclosures, speaker placement in a listening environment, and the listening environment itself all impact a listener's experience. Many solutions are known to exist to compensate for, or otherwise accommodate, these variables. However, none of the known solutions take into consideration the physical excursion limits of the speaker driver, as well as the speaker enclosures, speaker placement, and listening environment in which the speaker driver is situated. In addition, known solutions merely monitor, process and/or adjust a portion of the audio frequency spectrum rather than monitoring and treating the entirety of the input frequencies, and the frequencies that are processed and adjusted often cause negative consequences to other, unprocessed frequencies, resulting in undesirable and frequently audible artifacts.

[0003] Music often has large voltage peaks requiring an appropriate matching of a compressor/limiter, which acts as a dynamic volume control, and the amplifier. In the absence of a compressor/limiter, as the volume of the music is turned up, the output from the amplifier may become clipped (i.e., where output is independent of input) when the amplifier can no longer reproduce the input signal at the demanded output volume or the voltage may exceed the safe limits of the speaker driver. At that point, the signal may introduce significant, undesirable distortion in the output signal. A properly designed compressor/limiter, on the other hand, can avoid amplifier clipping and excessive speaker driver excursion by dynamically adjusting the volume downward when the input voltage reaches a predetermined threshold setting. But compressor/limiters can often create artifacts, such as gain riding, where, for example, the volume of vocal frequencies is inadvertently dynamically lowered when a sudden bass transient from, for example, the kick of the bass drum, reaches the threshold setting and initiates the compressor/limiter function.

[0004] A challenging aspect of audio playback systems is to deliver a steady state audio signal that matches a target response and a transient audio signal without ringing or other unnatural transient artifacts. In the case of a laptop, a handheld MP3 player, a mobile phone, and the like having one or more relatively small speaker drivers (i.e., transducers), the amplifier in these systems is generally relatively small as well. The speaker driver is relatively safe from receiving damaging transient signals because of the relatively low power delivered to the speaker driver. Thus, larger amplitude transient signals can be safely passed to the speaker driver in these types of devices with less compression as the damaging effects are minimal in these applications. Frequently, however, in devices such as these, a process known as "digital clipping" occurs when the signal demands exceed electronics ability to reproduce it. If the signal is clipped in this fashion, the listener will almost certainly hear an unpleasant degradation of the signal.

[0005] By contrast, on the other end of the spectrum, a floor standing stereo speaker driver being driven by a 300 watt amplifier may cause significantly larger excursions of the speaker driver, potentially causing damage to the driver. Transient signals in this situation exasperate the potential for damage. As a result, larger transient signals cannot be passed to the speaker driver in these applications as doing so may damage the speaker driver.

[0006] In many audio playback systems, boosting the volume levels of certain bass frequencies is used to compensate for low volume levels, and for limitations in speaker drivers and environmental acoustics, among others. However, conventional bass frequency boost techniques are restricted to very low values to prevent exhausting either driver or amplifier limits prematurely, and thus generating excessive distortion or damaging the components.

Alternatively, conventional bass boost systems that employ a limiter in conjunction with the boost produce audible "gain-riding" artifacts as a result of the relatively small frequency range that is typically boosted, causing wideband attenuation of the signals when the limiter engages. Finally, some types of bass enhancement schemes use the psychoacoustic "missing fundamental" technique of adding harmonics to the system in order to give the illusion that the speaker systems is reproducing bass that are beyond its capabilities. However, this often generates obvious and unwanted artifacts, such as distortion, ringing, and other non-linear effects.

[0007] Another problem that is encountered is that a traditional limiter only knows a voltage threshold, which is frequency independent. In contrast, the safe voltage limits of the speaker depend strongly on frequency. For example, the excursion of a speaker below and above speaker enclosure port tuning are significantly different, and thus the amplifier output voltage that can safely be allowed to drive the speaker correspondingly different. A traditional limiter cannot protect the speaker driver below port tuning because there is no frequency dependence without sacrificing a large amount of output potential at higher frequencies. In addition, a traditional limiter is broadband because it limits all frequencies. Transient bass signals that exceed the threshold setting of the compressor/limiter cause the volume to be lowered in other frequency ranges with adverse consequences on the listener's experience. For example, a bass drum transient signal that triggers the limiter also lowers the volume of, for example, the vocals. Another problem created during traditional limiter usage is filter shifting. High pass filtering is typically employed to protect the speaker from out-of-band signals— especially those lower than what the system can reproduce. This keeps the driver excursion low and prevents the amplifier from wasting power in regions that do not produce useful output. However, since a traditional limiter operates in a frequency independent manner, voltages below the threshold are allowed to increase in value until they hit the threshold, regardless of upstream filtering. In the case of a high pass filter, for example, this has the effect of lowering the effective high pass filter frequency, thus allowing higher than intended driver excursion. This limiter-based frequency shifting forces the designer to use a combination of lower gain, lower threshold values, and higher high pass frequency filter values in order to protect the system from distortion and damage.

[0008] In addition, bass boosting methods that utilize the concept of harmonics not only ignore the capabilities of the speaker driver but may also add distortion to a signal and are in any event frequencies that are not part of the source audio material. Listeners may therefore hear unwanted artifacts caused by the addition of harmonic frequencies added to the signal by such methods.

[0009] In the case of a laptop speaker driver or other transducer-like design having an inherent inability to reproduce low frequency signals, frequency roll-off may occur below the resonant frequency, such as, for example, below 200 Hz. If, for example, a bu MZ signal is senx to a speaker driver of this type, the speaker driver could be damaged or generate excessive distortion because it physically cannot move a significant mass of air through driver excursion. In these situations, a high pass filter is typically employed to filter out low frequencies, such as those frequencies below the resonant frequency. The result is safety over tonal sound quality.

[0010] Consequently, there exists a need to reproduce audio input signals so that the output audio signals result in a maximum, safe excursion amount for a given speaker driver, taking into consideration the speaker driver enclosure and anticipated listening environment.

SUMMARY

[0011] A computer program product stored on a computer-readable medium is disclosed, where the computer program product includes computer-executable code instructions that are executable by a computer processor to optimize sound reproduction by an audio system. The computer-executable code instructions comprises: first code instructions for adjusting characteristics corresponding to a plurality of frequency bands of an audio signal;

second code instructions for reducing a voltage of the audio signal processed by the first code instructions that exceeds a first user-adjustable threshold to enforce a desired maximum excursion amount of the speaker driver; and

third code instructions for boosting bass response of the audio signal processed by the second code instructions below about a bass resonant frequency of a speaker driver of the audio system, the third code instructions comprising fourth code instructions for reducing the voltage of the audio signal processed by the third code instructions that exceeds a second user-adjustable threshold to enforce a desired maximum excursion amount of the speaker driver. [0012] The first code instructions may be configured to process the audio signal through a plurality of biquad filters corresponding to respective frequency bands for user adjustment of the characteristics. The characteristics corresponding to the plurality of frequency bands may include at least one of a gain, a center frequency of each of the plurality of frequency bands, and a Quality Factor. The gain, the center frequency, and the Quality Factor may each be independently adjustable by a user.

[0013] The gain may be continuously or discretely adjustable from about -12 dB to about 12 dB. The center frequency may be continuously or discretely adjustable from about 20 Hz to about 20 kHz. The Quality Factor may be continuously or discretely adjustable from about 0.1 to about 4.0.

[0014] The second code instructions may be configured to process the audio signal according to an independently adjustable and user-definable setting comprising at least one of an attack rate, a release rate, an attack threshold, and a release threshold. The first user- definable threshold may be continuously or discretely adjustable from about 0 to about -12 dB.

[00 5] The third code instructions may be configured to process the audio signal according to an independently adjustable and user-definable setting comprising at least one of a bass attack rate, a bass release rate, a bass attack threshold, and a fbass release threshold. The second user-definable threshold is continuously or discretely adjustable from about 0 to about - 12 dB.

[0016] In another embodiment, a system for optimizing sound reproduction by an audio system is disclosed, comprising: one or more processors;

a memory; PATENT an equalization engine stored in the memory, comprising first code instructions to cause the one or more processors to adjust characteristics corresponding to a plurality of frequency bands of an audio signal;

a dynamics engine stored in the memory, comprising second code instructions to cause the one or more processors to dynamically reduce a voltage of the audio signal processed by the equalization engine that exceeds a first user-definable threshold to enforce a desired maximum excursion amount of the speaker driver; and

a bass enhancement engine stored in the memory, comprising

third code instructions to cause the one or more processors to boost bass response of the audio signal processed by the dynamics engine below about a bass resonant frequency of a speaker driver of the audio system, and

fourth code instructions to cause the one or more processors to reduce the voltage of the audio signal processed by the bass enhancement engine that exceeds a second user-definable threshold to enforce a desired maximum excursion amount of the speaker driver.

[0017] The first code instructions may be configured to process the audio signal through a plurality of biquad filters corresponding to respective frequency bands for adjustment of the characteristics. The characteristics corresponding to the plurality of frequency bands may include at least one of a gain, a center frequency of each of the plurality of frequency bands, and a Quality Factor. The gain, the center frequency, and the Quality Factor may each be independently adjustable by a user.

[0018] The dynamics engine may include independently adjustable settings corresponding to the group consisting of an attack rate, a release rate, a delay period, a compression ratio, a threshold, a release threshold, or a look-ahead. The bass enhancement engine may include independently adjustable settings corresponding to the group consisting of a bass boost amount, an attack rate, a release rate, a delay period, a compression ratio, a threshold, a release threshold, a look-ahead, a high pass filter setting, or a soft clip setting.

[0019] In another embodiment, a method for optimizing sound reproduction by an audio system is disclosed, comprising: providing a computer-readable medium encoded with a first computer program for independently adjusting characteristics corresponding to a plurality of frequency bands of an audio signal;

providing a computer-readable medium encoded with a second computer program for dynamically reducing a voltage of the audio signal processed by the first computer program that exceeds a first user-adjustable attack threshold to enforce a maximum excursion amount of the speaker driver;

providing a computer-readable medium encoded with a third computer program for dynamically ceasing voltage reduction of the audio signal processed by the second computer program when the voltage falls below a first user-adjustable release threshold;

providing a computer-readable medium encoded with a fourth computer program for boosting bass frequencies below about a bass resonant frequency of a speaker driver of the audio system;

providing a computer-readable medium encoded with a fifth computer program for dynamically reducing the voltage of the audio signal processed by the fourth computer program that exceeds a second user-adjustable attack threshold to enforce a maximum excursion amount of the speaker driver; and providing a computer-readable medium encoded with a sixth computer program for dynamically ceasing voltage reduction of the audio signal processed by the fifth computer program when the voltage falls below a second user-adjustable release threshold to return the voltage to a boosted state.

[0020] The characteristics corresponding to the plurality of frequency bands may include at least one of a gain, a center frequency of each of the plurality of frequency bands, and a Quality Factor. The method may further include providing a computer-readable medium encoded with a seventh computer program for instantly clipping the voltage of the audio signal processed by the sixth computer program that exceeds a user-adjustable soft clip threshold to further enforce a maximum excursion amount of the speaker driver.

[0021] A computer program product stored on a computer-readable medium is disclosed, the computer program product having computer-executable code instructions that are executable by a computer processor for enhancing bass response of an audio system, the computer-executable code instructions comprising: first code instructions for boosting a voltage corresponding to bass frequencies of an audio signal below a selected frequency; and

second code instructions comprising a compressor for reducing the voltage of the audio signal boosted by the first code instructions that exceed a user-adjustable threshold, the second code instructions configured to enforce a frequency independent excursion amount of the speaker driver.

[0022] The selected frequency may correspond approximately to a bass resonant frequency of a speaker enclosure associated with the speaker driver. The computer program may further include third code instructions comprising a high pass filter for processing the audio signal before processing the audio signal by the second code instructions. The computer program product may further include fourth code instructions for recombining the reduced audio signal with the audio signal above the selected frequency. The computer program product may further include fifth code instructions for soft clipping the recombined audio signal. The computer program product may further include sixth code instructions for delaying the audio signal above the selected frequency. The compressor may comprise separate attack and release thresholds.

[0023] In another embodiment, a computer program product stored on a computer-readable medium is disclosed, the computer program product having computer-executable code instructions that are executable by a computer processor for enhancing bass response of an audio system, the computer-executable code instructions comprising: first code instructions for determining bass frequencies of an audio signal to be

boosted below a selected frequency;

second code instructions for boosting a voltage of the audio signal corresponding to the bass frequencies; and

third code instructions comprising a compressor for reducing the voltage of the audio signal, the third code instructions configured to enforce a frequency independent excursion amount of a speaker driver.

[0024] The third code instructions may be configured to process the audio signal according to an independently adjustable and user-definable setting comprising at least one of the group consisting of an attack rate, a release rate, a release delay, a compression ratio, an adjustable frequency bypass, an attack threshold, a release threshold, or a look-ahead. The computer program product may further comprise fourth code instructions comprising a frequency dependent limiter for limiting a voltage of the audio signal according to a frequency dependent threshold for creating zero filter frequency shift. The first code instructions may include a high pass filter and a low pass filter for establishing a range of bass frequencies to be boosted below the selected frequency. [0025] In another embodiment, a computer program product stored on a computer-readable medium is disclosed, the computer program product having computer-executable code instructions that are executable by a computer processor for enhancing a response of an audio system, the computer-executable code instructions comprising: first code instructions comprising a compressor for reducing a voltage of an audio signal, the first code instructions configured to enforce a frequency independent excursion amount of a speaker driver, the first code instructions configured to process the audio signal according to a user-definable setting comprising an attack rate, a release rate, a release delay, a frequency bypass, an attack threshold, and a look-ahead.

[0026] The setting may include at least one of a compression ratio and a release threshold. The computer program product may further include second code instructions comprising a low pass filter for filtering the audio signal before processing the audio signal by the first code instructions. The low pass filter may be a first order low pass filter. The computer program product may further include third code instructions for soft clipping the voltage of the audio signal processed by the first code instructions that exceeds an adjustable soft clip threshold to further enforce a maximum excursion amount of the speaker driver. The computer program product may additionally include fourth code instructions comprising a high pass filter for filtering the audio signal after processing the audio signal by the first code instructions to enforce a frequency dependant threshold. The high pass filter may be a 2nd order or higher high pass filter.

[0027] In another embodiment, a method for optimizing sound reproduction by an audio system is disclosed, comprising: providing a computer-readable medium encoded with a first computer program for filtering bass frequencies of an audio signal, the first computer program comprising a low pass filter configured to pass bass frequencies below an adjustable low pass filter setting and a high pass filter configured to pass bass frequencies above an adjustable high pass filter setting, the low pass filter setting comprising a frequency that is higher than a frequency of the high pass filter setting;

providing a computer-readable medium encoded with a second computer program for boosting bass response of the audio signal passed by the first computer program; and

providing a computer-readable medium encoded with a third computer program for dynamically reducing a voltage of the audio signal passed by the second computer program that exceeds an adjustable threshold to enforce a maximum safe excursion amount of a speaker driver associated with the audio system.

[0028] The low pass filter may be a fourth order low pass filter. The low pass filter setting may be settable to approximately a resonant frequency of the speaker driver. The high pass filter may be a second order high pass filter. The method may further include providing a computer-readable medium encoded with a fourth computer program for dynamically ceasing voltage reduction of the audio signal processed by the third computer program when the voltage falls below an adjustable release threshold to return the voltage to a boosted state. The method may further include providing a computer-readable medium encoded with a fifth computer program for soft clipping the voltage of the audio signal processed by the fourth computer program that exceeds an adjustable soft clip threshold to further enforce a maximum excursion amount of the speaker driver. [0029] In another aspect, a non-transitory computer-readable medium comprising computer- readable instructions for optimizing audio response from an audio playback system is provided. The non-transitory computer-readable instructions, when executed on a computer, cause the computer to perform the method steps described herein.

[0030] A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth illustrative embodiments that are indicative of the various ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Fig. 1 illustrates an audio signal filtered by a high pass filter in combination with a limiter.

[0032] Fig. 2 illustrates an audio signal filtered by a high pass filter without a limiter.

[0033] Fig. 3 illustrates a frequency independent, constant excursion of a speaker driver resulting from enforcement of an exemplary frequency dependent threshold by a

compressor/limiter of an exemplary bass enhancement engine.

[0034] Fig. 4 illustrates an exemplary user interface for an equalization engine.

[0035] Fig. 5 illustrates an exemplary user interface for a dynamics engine.

[0036] Fig. 6 illustrates an exemplary user interface for a bass enhancement engine.

[0037] Fig. 7 is a block diagram illustrating an exemplary method for optimizing an audio signal for audio playback systems. [0038] Fig. 8 is a block diagram illustrating an exemplary bass enhancement engine of the method described in Fig. 7.

[0039] Fig. 9 is a block diagram illustrating an exemplary implementation of an embodiment of the invention.

[0040] Fig. 10 is a detailed block diagram illustrating an exemplary implementation of an equalization engine of the embodiment shown in Fig. 9.

[0041] Fig. 1 1 is a detailed block diagram illustrating an exemplary implementation of an dynamics engine of the embodiment shown in Fig. 9.

[0042] Fig. 12 is a detailed block diagram illustrating an alternative implementation of a dynamics engine.

[0043] Fig. 13 shows an exemplary steady state signal response for a signal processed by an exemplary dynamics engine.

[0044] Fig. 14 shows the signal response after processing a signal through an exemplary bass soft clip processing engine of an exemplary bass enhancement engine.

[0045] Fig. 15 shows a signal before and after processing the signal through an alternative bass soft clip processing engine of an exemplary bass enhancement engine.

[0046] Fig. 16 shows a signal before and after processing the signal through another alternative bass soft clip processing engine of an exemplary bass enhancement engine.

[0047] Fig. 17 shows a signal before and after processing the signal through yet another alternative bass soft clip processing engine of an exemplary bass enhancement engine. [0048] Although the figures and the following disclosure describes one or more embodiments of an optimization method for audio playback systems, one of ordinary skill in the art would know that the teachings of the disclosure would not be limited to use solely in connection with audio playback systems, and instead would appreciate that the teachings of the following disclosure may also apply to other aspects of sound recordings.

[0049] An embodiment of the invention is directed to a software tool that enables designers and manufacturers of audio playback systems to tailor the output audio signal knowing the various characteristics of the speaker system, speaker placement, and the listening

environment. An aspect of the software tool is that it incorporates knowledge of the relationship between the excursion of the speaker driver and the voltage output for a given playback system, speaker driver, speaker enclosure, speaker placement, and listening environment.

[0050] An embodiment of the invention comprises three parts that work together toward maximizing the audio performance of audio playback systems, including playback systems incorporating loudspeakers or transducers. The three parts are an equalization engine, a dynamics engine, and a bass enhancement engine. In an embodiment, an audio input signal is first processed by the equalization engine, then processed by the dynamics engine, and then processed by the bass enhancement engine to optimize the output signal for delivery to one or more speaker drivers. In other embodiments, the signal may be processed in any order.

[0051] Once a speaker driver and enclosure are known, an output voltage-to-excursion relationship may be determined to obtain a safe voltage limit that the speaker system can handle as a function of frequency. The voltage limits will have a strong frequency dependence such that the safe limits below resonance will generally be significantly lower than those above resonance. Once the safe output voltages are known, a compressor/limiter may be programmed to enforce the limits for both regions independently.

Equalization Engine

[0052] The function of the equalization engine is to overcome and correct for the

deficiencies in the sound reproduction chain to yield the desired response at the listener's ears. For example, an equalization engine may adjust the output signal to compensate for speaker driver limitations or to correct for different listening environments. In one embodiment, an equalization engine comprises adjustable high and low pass filters, and fully parametric filters to provide maximum flexibility, accuracy, and fidelity to enable a playback device manufacturer, for example, to tailor the output signal for a given speaker driver and listening environment. A high pass filter blocks relatively low frequencies and allows all frequencies above the high pass filter set point frequency to pass. A low pass filter blocks relatively high frequencies and allows all frequencies below the low pass filter set point frequency to pass.

[0053] A parametric filter is one in which the frequency, the gain, and the Quality Factor (Q), which describes the shape of the band-pass filter, are each adjustable independent of one another. Q is defined as being equal to the center frequency divided by the bandwidth, where center frequency is defined as the frequency at the center of the frequency band and which typically represents the peak of the frequency response curve. Bandwidth is defined as the difference between the upper and lower frequencies of the frequency response curve where the gain has dropped to 0.707 of the mid-band gain (i.e., the voltage gain at the center frequency). Consequently, a relatively low Q implies a relatively wide bandwidth whereas a relatively high Q implies a relatively narrow bandwidth. Thus, parametric equalization of an audio signal provides frequency response curves ranging from relatively wide bandwidth to relatively narrow bandwidth. [0054] Parametric equalization is a powerful tool to adjust the input audio signal so that the output signal results matches the target frequency response for all audible frequencies. This is achieved, in principle, knowing characteristics of the speaker driver, speaker enclosure, speaker placement, and listening environment as well as the impact of, for example, an increase or decrease in the adjustment of gain at the listener's position relative to the speaker driver.

Dynamics Engine

[0055] Big sound, especially from small devices such as iPod's, mobile telephones, laptop computers and the like, places heavy loads on the speaker drivers and electronics of the device's playback system. In one embodiment, a dynamics engine comprising a

compressor/limiter, or dynamic volume control, ensures that these demands stay within the safe limits of both, ensuring clean, distortion-free output, even at very high output levels. It does this by enforcing a limit placed on the output voltage of the audio signal. In particular, a

compressor/limiter dynamically reduces the voltage of the audio signal when it would exceed the capabilities of the system, allowing it to play significantly louder without distortion than would otherwise be possible. A compressor/limiter in this context therefore prevents clipping of the signal by setting the compressor threshold, which sets a target output level desired to be achieved, generally slightly below the point at which the output signal may be clipped. In one embodiment, the threshold is set approximately 1-2 dB below the point at which the output signal is expected to be clipped.

[0056] A compressor/limiter sets a compression ratio, which is defined as the ratio of the input signal (dB) to the output signal (dB) when the signal is above the defined threshold. As the compression ratio tends higher, for example, having a slope of 20 to 1 , the

compressor/limiter acts as a limiter because the slope of the curve is nearly horizontal. That is, larger changes in input signal yield very little change in output. As the compression ratio tends lower, for example, having a slope of 2 to 1 , the compressor/limiter behaves more subtly and acts as a compressor with larger changes in output as a function of input variations.

[0057] Using a compressor/limiter, however, can create artifacts caused by the compression of the offending frequency range while leaving unaffected frequencies uncompressed. For example, if a steady state signal is close to the threshold setting, a sine wave audio signal may exceed the threshold and trigger the compressor/limiter, but a sample taken 2-3 ms later may already register below the threshold setting. This results in oscillation of the output volume for the triggered frequencies as the system attempts to react to the triggering of the threshold followed by the removal (release) of the compressor when compression is no longer needed. Additionally, since a traditional limiter has no frequency dependence, its operation can result in unwanted high and low pass filter frequency shifts. For example, as shown in Fig. 1 , a high pass filter together with a traditional limiter causes a frequency shift from, for example, 100 Hz to 50 Hz due to the limiter. By contrast, as shown in Fig. 2, processing an audio signal under the same conditions through a high pass filter without a limiter does not cause any frequency shift in the output signal.

[0058] Although many compressor/limiter solutions are known to exist, the

compressor/limiter employed in an embodiment of the invention is specially designed for use with audio playback systems, with the ability to attack the output signal slowly enough when the compressor threshold is met to allow non-damaging transients to pass through, while releasing fast enough to be acoustically transparent to the listener, processing only those parts of the signal that need it and yielding about zero distortion while doing so. To do this, the

compressor/limiter in an embodiment of the invention includes separate attack and release settings, and release delay, frequency bypass, and look-ahead functionality. In addition, filtering can be employed both before and after compression, enabling the realization of a frequency dependent threshold and zero filter frequency shift. [0059] In particular, once an audio input signal reaches the compressor/limiter threshold setting, an attack setting (which may be expressed in dB/ms or dB/sample) of the

compressor/limiter of the dynamics engine may be programmed to delay any immediate action by the compressor/limiter to reduce the output voltage of the input signal according to the attack setting. In this way, nondamaging transient signals, which may briefly exceed the compressor threshold setting, may go uncompressed thus preventing any such volume oscillations caused by triggering and subsequent release of the compressor/limiter.

[0060] A separate attack setting, therefore, determines how fast the compressor/limiter will act on an input signal that exceeds the threshold setting. A signal that exceeds the threshold and continues to exceed that threshold longer than the attack setting will immediately be acted upon by the compressor/limiter to reduce the output voltage and hence the output volume sent to the speaker driver.

[0061] By contrast, a separate release setting determines how fast the compressor/limiter will cease acting on an input signal after the input signal falls below the triggering threshold setting. In particular, once a compressed audio input signal falls below the compressor/limiter threshold setting, a release setting (which may be expressed in dB/ms or dB/sample) of the compressor/limiter of the dynamics engine may be programmed to delay any immediate action by the compressor/limiter to cease acting on signal according to the release setting. In this way, input signals may continue to be compressed (i.e., output voltage reduction) below the compressor/limiter threshold until the risk for transient signal spikes, and hence risk of for speaker driver damage, fades to a point of limited concern. This also aids in having smooth and transparent compressor response by preventing excessive toggling between attack and release.

[0062] Yet another option to the dynamics engine is a release delay feature programmed to occur at either or both of the triggering of the compressor/limiter and/or upon releasing the compressor/limiter. However, a delay feature comprising a delay setting is based on a chosen time increment and not on the actual signal, which may create artifacts in the output signal when in use. The release delay also acts to prevent excessive transitions between attack and release, and thus provides more smooth and invisible compressor response.

[0063] An additional option to the dynamics engine is a frequency bypass feature such that only frequency content below a specified frequency is processed by the compressor/limiter, while the content above the specified frequency is unprocessed except for sufficient delay to match the look-ahead and to ensure desirable signal reconstruction. Clipping of very high frequencies is neither damaging, nor audible, nor likely to occur with real-world signals, yet they can trigger the compressor/limiter to reduce gain nonetheless. The bypass feature prevents this spurious triggering of the compressor/limiter by restricting the signals to be processed to only those relevant to sound quality or speaker protection. The bypass feature may be implemented using a low pass filter, such as a first order low pass filter, and a high pass filter, such as a first order high pass filter, where the low pass filter setting is adjustably set to the same or approximately the same frequency as the adjustable high pass filter setting. In this way, frequencies below the common set point (or at least the low pass filter setting) may be operated on by the compressor/limiter while the frequencies above the common set point (or at least the high pass filter setting) may be unprocessed except for the introduction of sufficient delay as described above.

[0064] A further option to the dynamics engine is a look-ahead feature programmed to sample the digital audio signal data at some period of time ahead of the processing of the signal by, for example, the triggering of the compressor/limiter. By doing this, the dynamics engine can react ahead of time and, for example, smoothly implement the attack feature of a soon-to- be offending transient signal over a longer period of time rather than instantaneously so as to avoid audible changes in volume that may be heard by a listener. user to shape the peak output voltage using a range of filtering to match the maximum output of the amplifier with the maximum safe voltage for the speaker driver across the spectrum of audible frequencies, resulting in loud, substantially distortion free audio output where processing of the audio input signal is unapparent to the listener's ears.

Bass Enhancement Engine

[0066] The function of the bass enhancement engine is to boost bass response or volume across all bass frequencies below an adjustable low pass filter setting, which may be set at the resonant frequency of the speaker driver where the frequency begins to roll off, without adding harmonics or artifacts. Knowledge of the actual speaker driver and speaker enclosure informs a system designer of the roll-off frequency of the speaker driver/enclosure combination, and therefore, the low pass filter setting to apply to the system based on where that combination causes a roll off of low frequency response.

[0067] In an embodiment, bass frequencies may be boosted by taking the frequencies passed through a low pass filter, such as a 4th order low pass filter, which may be adjustably set to pass frequencies below, for example, the resonant frequency, and adding a

predetermined yet fully adjustable amount of gain, such as about 20 dB, to all of these bass frequencies. As mentioned above, these frequencies may all be below the point where frequency roll-off begins to occur for the speaker driver/speaker enclosure combination. A high pass filter, such as a 2nd order high pass filter, may additionally be included to pass those frequencies above an adjustable high pass filter setting and below the adjustable low pass filter setting of the low pass filter to establish a frequency range that may be boosted by the bass enhancement engine. Inclusion of the high pass filter may be useful to exclude relatively low frequencies from being boosted in instances where, for example, a particular speaker driver, such as a cell phone driver or a laptop driver, may not be physically capable of reproducing such relatively low frequencies.

[0068] In situations where a user elects not to use the bass enhancement engine, the 2nd order high pass filter may nevertheless be used in conjunction with a compressor/limiter to provide a level-independent high pass filter, even during passages of large gain reduction. To obtain this functionality, the boost or gain of the bass enhancement engine may be set to zero with the 2nd order high pass filter set at about the resonant frequency.

[0069] The bass enhancement engine may further include a bypass circuit for frequencies above the low pass filter setting. For example, the frequencies above the low pass filter setting that are blocked by the low pass filter may be passed through a high pass filter, such as a 4th order high pass filter, which may be adjustably set to the same or approximately the same frequency as the adjustable low pass filter setting. In this way, frequencies below the common set point (or at least the low pass filter setting) may be boosted by the bass enhancement engine while frequencies above the common set point (or at least the high pass filter setting) may be passed to a bypass circuit for eventual recombination with the low pass frequencies after being boosted by the bass enhancement engine. An adjustable delay may be introduced to the bypassed frequencies (i.e., frequencies above the common set point or high pass filter setting) to ensure desirable reconstruction of the signal with the frequencies below the low pass filter setting.

[0070] As a gain of about 20 dB is relatively high, the resulting signal may cause the amplifier to clip if the transient output signals exceed the capability of the amplifier or may exceed the excursion capabilities of the driver at high levels. To avoid such transient clipping or excessive excursion, an embodiment of the bass enhancement engine comprises a

compressor/limiter of the type discussed above to separately process and/or compress bass frequencies passed below the low pass filter setting to the extent the signal voltage exceeds the threshold value or safe voltage limit of the speaker driver. Below the resonant frequency of the speaker enclosure, the excursion is constant, or frequency independent. Thus, a

compressor/limiter that is set to enforce a voltage threshold setting over only this frequency range has the effect of producing a constant excursion from the driver. As before, the compressor/limiter enforces a threshold setting that is based upon the known, safe excursion limit of the speaker driver when installed in the speaker enclosure.

[0071] In principle, signal output voltage may be correlated with the measured excursion of the speaker driver to provide a voltage-to-excursion relationship. By knowing this relationship, the threshold setting of the compressor/limiter of the bass enhancement engine may be programmed or otherwise set at a voltage at or just below the maximum, safe excursion limit of the speaker driver, to protect the speaker driver from excursion-related damage. The threshold setting may alternatively be expressed in decibels. In one embodiment, the threshold setting may be set by the user to any value at or below the predetermined safe excursion limit of the speaker driver, such as about 1 to about 2 dB below that which would cause the safe excursion limit to be met or exceeded.

[0072] Turning again to the figures, wherein like reference numerals refer to like elements, Fig. 3 shows an exemplary embodiment illustrating a frequency independent excursion of a speaker drive resulting from enforcing a frequency dependent threshold by a compressor/limiter. For example, threshold 46 of a compressor/limiter may be set by a user at about 1 to about 2 dB below resonant frequency 42, which in this example, is shown as being approximately 100 Hz. A compressor/limiter that enforces threshold 46 results in constant excursion 48, which in this example is shown as being approximately 1.84 mm, and which may be at or below the maximum safe excursion limit of an exemplary speaker driver. attack and release thresholds to act upon offending input signals as discussed above. If the input signal does not exceed the threshold setting, then the compressor/limiter functionality of the bass enhancement engine need not act on the signal. However, if the input signal exceeds the threshold and triggers the compressor/limiter, the compressor/limiter may attack the signal to lower the voltage and thus the volume of the boosted bass frequencies according to the attack setting. If the input signal thereafter falls below the threshold setting, the

compressor/limiter may cease acting upon the signal thereby allowing the voltage to return to an uncompressed voltage and thus raise the volume of the boosted bass frequencies according to the release setting.

[0074] In one embodiment, all frequencies below the low pass filter setting are acted upon by the compressor/limiter if the input signal exceeds the threshold setting. In another embodiment, certain boosted bass frequencies or bass frequency ranges may be acted upon by the compressor/limiter if the input signal exceeds the threshold setting. For example, as described above, a high pass filter, such as a 2nd order high pass filter, and a low pass filter, such as a 4th order low pass filter, may be used in combination with one another to establish a bass frequency range that may be boosted and thereafter compressed if the signal exceeds the threshold setting.

[0075] In one embodiment, the bass enhancement engine may include soft-clip functionality downstream of the compressor/limiter of the bass enhancement engine to instantaneously attack and release a particularly damaging input signal. Soft-clip functionality acts as a back-up to the compressor/limiter to instantly clip an unsafe input signal that managed to pass through the compressor/limiter. Soft-clip functionality may be useful to permit relatively high bass boost, for example, on the order of approximately 20 dB or more, to be set by a user of the bass enhancement engine because of the layering of protection of the speaker driver while enabling the use of slower attack and release rates than would otherwise be permitted.

[0076] By virtue of the teachings of the instant disclosure, a maximum excursion bass boost strategy now exists that allows the speaker driver to make rich, clean bass up to the limits of speaker driver without incurring audible artifacts.

[0077] Turning now to Figs. 4-6, there is illustrated an exemplary user interface 10

illustrating an embodiment of the invention to maximize audio reproduction and playback performance across, for example, mobile, laptop, and standalone speaker applications. Fig. 4 shows, for example, an exemplary equalization engine user interface 20 for equalization engine 140 comprising a plurality of user-adjustable settings. More particularly, equalization engine user interface 20 includes global power on-off switch 22, prescale 24, auto prescale switch 26, a plurality of biquad-type filters 30, and save/load file functionality.

[0078] Like most user-adjustable settings in user interface 10, prescale 24 may be adjusted or set using a continuous slide, or alternatively, may be adjusted or set by selecting a button that enables manual entry of prescale 24 or by selecting prescale 24 from options in a drop down list. Prescale 24 may range from about 0 dB to about -20 dB. Prescale 24 acts to prevent digital clipping by reducing the gain of the signal prior to being boosted by equalization engine 140, which would otherwise exceed digital zero dB. Prescale 24 may be toggled on or off by a user using auto prescale switch 26 and selecting either the "on" or the "off position as desired.

[0079] Equalization engine user interface 20 of Fig. 4 shows 5 biquad filters 30, each corresponding to a respective frequency band for user adjustment. Another embodiment may include a fewer or greater number of filters to more finely or more coarsely adjust the signal waveform. In the embodiment of Fig. 4, each biquad filter 30 includes fully parametric controls for frequency, gain, and Q to independently adjust each of these settings. A user may set frequency 31 from about 20 Hz to about 20 kHz, continuously or in about 1/48th octave increments or less. Gain 32 may vary from about -12 to about 2 dB, continuously or in increments of about 0.5 dB or less. Quality Factor (Q) 33 may vary from about 0.1 to about 4.0, continuously or in increments of about 0.05 or less. As shown in Fig. 4, frequency 31 , gain 32, and Q 33 may each be adjusted or set using a continuous slide, or alternatively, may be adjusted or set by selecting a button that enables manual entry of these settings or by selecting from among options in a drop down list. Biquad filter 30 may further include switch 35, as shown in Fig. 4, for toggling and displaying filtering types comprising peak, dip, bypass, adjustable Q high pass, adjustable Q low pass, bass shelf, notch, bandpass, allpass, and treble shelf, the effect of which is graphically displayed in graph 38.

[0080] Turning now to Fig. 5, there is shown an exemplary dynamics engine user interface 50 for dynamics engine 150 comprising independently adjustable settings for attack 52, release 54, delay 56, compression ratio 58, frequency limit (F limit) 60, threshold 62, release threshold 64, and optionally, look-ahead 66. Dynamics engine user interface 50 further includes on/off switch 72 for engaging or disengaging dynamics engine 150, postscale setting 68 for providing make-up gain so that the peaks after compression are near 0 dB, auto postscale on/off switch 70, as well as clip indicator light 74 and reset button 76 for resetting dynamics engine user interface 50 if the signal is clipped. Clip indicator light 74 provides a visual indication that at least one sample of the signal exceeds digital 0 dB and was therefore clipped. In addition, a real-time display of signal level and peak signal level hold may be displayed in dynamics engine user interface 50. When reset button 76 is selected by a user, clip indicator light 74 turns off and the peak signal level hold and detector is reset to 0.

[0081] In the embodiment of Fig. 5, attack 52 may vary from about 0 to about 10 dB/ms, continuously or in increments of about 0.1 dB/ms or less, with a typical value of about 0.4 dB/ms. Release 54 may vary from about 0 to about 10 dB/ms, continuously or in increments of about 0.01 dB/ms or less, with a typical value of about 0.04 dB/ms. Delay 56 may vary from about 0 to about 100 ms, continuously or in increments of about 1 ms, with a typical value of about 10 ms. Threshold may vary from about 0 to about -12 dB, continuously or in increments of about 1 dB or less. Compression ratio 58 represents the ratio of input to output for the portion of the signal above threshold, and may range from about 1 :1 to about 20:1 , continuously or in increments of about 1. Frequency (F) limit 60 represents the frequency limit above which dynamics engine 150 will not process / bypass signals, based on complementary high pass / low pass filters set at that frequency. Release threshold 64 provides a separate threshold setting for releasing the compressor/limiter, and may range from about 0 to about -12 dB, continuously or in increments of about 1 dB or less. Look-ahead 66 may vary from about 0 to about 10 ms, continuously or in increments of about 0.1 ms. As before, each of these settings may be independently adjusted by a continuous slide, by selecting a button for manually entering the respective settings, or by selecting an appropriate setting from a drop down list.

[0082] Turning now to Fig. 6, there is shown an exemplary bass enhancement engine user interface 80 of bass enhancement engine 160 comprising frequency 82, boost 84, threshold 86, release threshold 88, look-ahead 90, bass high pass filter 92, attack 94, release 96, delay 98, compression ratio 100, soft clip 102, and soft clip master on/off switch 104.

[0083] A user may independently adjust frequency 82 from about 20 Hz to about 1000 Hz, continuously or in increments of about 1 /48th octave or less. For maximum performance, frequency 82 may be set at or near the resonant frequency of the speaker driver/speaker enclosure system where the bass begins to roll off. As described above, the bass enhancement engine processes all frequencies below this frequency set point according to the value of boost setting 84. Boost setting 84 may vary from about 0 dB to about 40 dB, continuously or in increments of about 1 dB or less. [0084] Threshold 86, release threshold 88, look-ahead 90, attack 94, release 96, delay 98 and compression ratio 100 are each adjustable and function in the same manner as described above for dynamics engine user interface 50. Bass high pass filter 92 is an optional setting to cause bass enhancement engine 160 to filter out any frequencies below the indicated set point. Bass high pass filter 92 may also be used to prevent frequency shifting if bass enhancement is not engaged. Soft clip 102 instantaneously attacks and/or releases a particularly damaging input signal and, therefore, lowers the volume of the output signal according to the soft clip setting, thus preventing any digital clipping. In one embodiment, soft clip 102 is set to a point just below 0 dB to help ensure that the soft clip functionality is triggered only when absolutely necessary.

[0085] Turning now to Fig. 7, there is shown system 125 showing an exemplary order of operations for processing input audio signal 130. Input audio signal 130 enters equalization engine 140, which operates to balance or correct for any deficiencies in the sound reproduction chain to provide a target frequency response at the listener's ears. Once the signal is adjusted according to the algorithms and settings programmed in equalization engine 140, signal 130 is then passed to dynamics engine 150, which compresses any transients that may exceed, for example, threshold 62 up to, for example, F limit 60. From there, signal 130 is passed to bass enhancement engine 160 to boost the bass response across all bass frequencies below, for example, the setting of frequency 82. As shown in more detail in Fig. 8, signals entering bass enhancement engine 160 are first passed through low pass filter 162, which may be a fourth order low pass filter, according to the setting of frequency 82. Any transient signals that exceed, for example, threshold 86, may then be compressed using compressor/limiter 164 according to threshold 86 as well as any of the other attack, release, delay, etc. settings shown in Fig. 6. Upon exiting bass enhancement engine 160, output signal 170 may then be amplified and delivered to the speaker driver by the playback system. [0086] In one embodiment, system 125 provides capability to allow the user to load at least two different saved groups of settings encompassing all signal processing settings of each to allow the user to toggle between them in real-time. In this way, a user may quickly compare the output signal processed by each. In addition, system 125 may apply all signal processing settings to the left and right channels, to one or the other, simultaneously, or staggered in time or phase.

[0087] Turning now to Figs. 9-11 , there is shown an exemplary implementation of an embodiment of the invention. As shown in Fig. 9, system 200 comprises prescale engine 202, equalization engine 210, dynamics engine 250, bass enhancement engine 300, and postscale engine 323. An input signal is first received and processed by prescale engine 202, then by equalization engine 210 to correct for deficiencies in the sound reproduction chain to obtain a desired response at the listener's ears, then by dynamics engine 250 to ensure that the audio signal remains within desired limits of the speaker driver to ensure distortion-free output even at very high output levels, then by bass enhancement engine 300 to boost the bass response across a wide spectrum of bass frequencies while maintaining desired limits of the speaker driver and while ensuring distortion-free audio output, and then by postscale engine 323. In other embodiments, processing of the signal may be in any order.

[0088] Prescale engine 202 is configured to reduce the gain of the input audio signal, if needed, prior to equalization by equalization engine 210, thereby allowing sufficient digital headroom so that any peaks will not exceed digital 0 dB. Postscale engine 323 is configured to increase the gain of the output audio signal, if needed, to make up for any gain reduction by prescale engine 202 and by, for example, compressor 258 to cause the output of the system to be as high as possible without exceeding the excursion limit of the speaker driver. [0089] A signal passed to equalization engine 210 from prescale engine 202 is filtered by parametric filtering engine 211 using, for example, user-determined parametric settings for frequency 220, gain 221 , and Quality Factor (Q) 222. Such settings may be selected or determined by a user using, for example, equalization engine user interface 20 described above.

[0090] Turning to Fig. 10, there is shown a detail block diagram of an exemplary equalization engine 210. As shown, equalization engine 210 comprises input block 208 for input audio signal 209, parametric filtering engine 211, and output block 232 for audio signal 233 after processing by parametric filtering engine 211. Parametric filtering engine 211 comprises filtering engine 213 and filter settings 219 corresponding to each of a plurality of frequency bands desired by a user to be filtered. In the embodiment of Fig. 10, for example, there is shown a total of five frequency bands operating in series, but any integer less than or greater than five is possible up to the limits of available processing power by the computer processing unit and available memory. For many applications, five frequency bands may be a reasonable compromise between resource demands on the host system and ability to adequately achieve the target response from the system. Alternatively, a space-optimized cell phone implementation, for example, may utilize only three frequency bands to achieve the target performance, while a high-powered PC implementation may utilize seven frequency bands. In another embodiment, the frequency bands may be processed by equalization engine 210 in parallel for multi-way systems, where the input would be stereo, but the output may be 4 or more channels, such as a channel for each of the left woofer, left tweeter, right woofer, right tweeter, etc.

[0091] Filtering engine 213 comprises bypass engine 214, adjustable Q high pass engine 215, adjustable Q low pass engine 216, peak processing engine 217, and bass shelf processing engine 218, any, all, or none of which may be selected by a user for any, all, or none of the PATENT bands as selected by the user. Bypass engine 214 bypasses the filter bank, so no signal processing is performed. Adjustable Q high pass engine 215 produces a variable Q second order high pass filter. Adjustable Q low pass engine 216 produces a variable Q second order low pass filter. Peak processing engine 217 produces a peak or dip filter. Bass shelf processing engine 218 produces a second order adjustable Q bass shelf filter.

[0092] For each frequency band of parametric filtering engine 211, an input audio signal is first processed by filtering engine 213 according to user-selectable settings for bypass engine 214, adjustable Q high pass engine 215, adjustable Q low pass engine 216, peak processing engine 217, or bass shelf processing engine 218, whichever is selected. The signal is then, processed using filter settings 219 according to user-selectable settings for frequency 220, gain 221 , and Q 222.

[0093] As shown in Fig. 9, after processing an audio signal by equalization engine 210, the signal is then passed to dynamics engine 250 for processing. The signal passed to dynamics engine 250 may be filtered by dynamics filtering engine 251 having, for example, first order high pass filtering engine 253 and a first order low pass filtering engine 254 for passing respective high and low frequencies to be further processed by dynamics processing engine 257.

Dynamics processing engine 257 may include, for example, delay processing engine 276 and compressor 258 (see also Fig. 11). As shown in Fig. 9, delay processing engine 276 processes frequencies passed to it by first order high pass filtering engine 253 while compressor 258 processes frequencies passed to it by first order low pass filtering engine 254.

[0094] Turning to Fig. 11 , there is shown a detail block diagram of an exemplary dynamics engine 250. As shown, dynamics engine 250 comprises input block 248 for input audio signal 249, dynamics filtering engine 251, dynamics processing engine 257, dynamics integration engine 282, and output block 288 for audio signal 289 after processing by dynamics engine 250. [0095] Dynamics filtering engine 251 comprises bypass filtering engine 252 having, for example, first order high pass filtering engine 253 and first order low pass filtering engine 254. Although first order filters may provide a seamless transition between processed and bypass components, higher order or adjustable-order filters may be utilized instead. Dynamics processing engine 257 comprises limiter settings block 260, limiter processing engine 268, compressor settings block 271 , look-ahead delay engine 275, and gain reduction engine 279. Look-ahead delay engine 275 comprises delay processing engine 276, which receives the output signal passed to it from first order high pass filtering engine 253 and processes that signal according to a user-specified delay setting, such as look-ahead 66 described above. The output signal from delay processing engine 276 is passed to dynamics integration engine 282 for combining this signal with the signal that is filtered by first order low pass filtering engine 254 according to, for example, user-specified frequency limit 60, and which is processed by compressor 258.

[0096] By contrast, compressor 258 of dynamics processing engine 257 receives the output signal passed to it from first order low pass filtering engine 254 and processes that signal according to various user-specified settings to protect the speaker driver from damage while maximizing volume output. For example, as shown in Fig. 1 1 , limiter settings block 260 of compressor 258 include attack rate 261 , release rate 262, threshold 263, release threshold 264, and release delay 265, each of which may be selected or determined by a user using, for example, dynamics engine user interface 50 described above. One of ordinary skill would appreciate that alternate embodiments of limiter settings block 260 may include one or more of these user-selectable settings.

[0097] The output of first order low pass filtering engine 254 that exceeds the user-specified threshold setting 263 will be received by limiter engine 269 and passed to lookahead delay engine 275 and gain reduction engine 279 where gain reduction is applied by gain reduction settings block 260 and compressor settings block 271 , respectively. As shown in Fig. 11 , the output of first order low pass filtering engine 254 is passed as signal 266, which is processed by look-ahead delay processing engine 277 according to a user-specified setting, such as look- ahead 273. In this way, the gain reduction calculated by limiter engine 269 and applied by gain reduction engine 279 leads signal 266 by the defined look-ahead time. The gain reduction calculated by limiter engine 269 according to compression ratio 278 is applied to the filtered and delayed signal by gain reduction processing engine 280.

[0098] The output signal from gain reduction processing engine 280 of compressor 258 is passed to recombination engine 283 of dynamics integration engine 282 for combining this signal with the signal that is filtered by first order high pass filtering engine 253 and processed by look-ahead delay engine 275, which ensures that the processed and unprocessed signals remain perfectly time-aligned. The output of dynamics integration engine 282 forms output signal 289 of output block 288, which forms the input signal for bass enhancement engine 300.

[0099] A variation of the embodiment of Fig. 11 is shown in Fig. 12. Fig. 12, for example, further includes threshold shaping engine 285 having high pass filtering engine 286. High pass filtering engine 286 may comprise a 2nd order or higher high pass filter. By providing a filter after compressor 258, a voltage threshold may be tailored to match the excursion capabilities of a speaker driver.

[0100] Turning again to Fig. 9, bass enhancement engine 300 comprises bass filtering engine 301 , bass protection engine 305, bass boost engine 309, bass compressor engine 313, bass integration engine 317, and bass soft clip engine 320. As shown, the signal is filtered by bass filtering engine 301 having, for example, fourth order high pass engine 302 and fourth order low pass engine 303 for passing respective high and low frequencies for further processing by bass protection engine 305, bass boost engine 309, and bass compressor engine 313. Frequencies that pass from fourth order high pass engine 302 are passed to bass boost engine 309 and processed by delay processing engine 310 according to a user-specified delay setting, such as delay 98 described above. The output signal from fourth order high pass engine 302 is not otherwise processed by bass protection engine 305, bass boost engine 309, or bass compressor engine 3 3, as shown by bypass blocks 306, 314 of Fig. 9.

[0101] By contrast, bass protection engine 305 of bass enhancement engine 300 receives the output signal passed to it from fourth order low pass engine 303 and processes that signal via second order high pass engine 307. The output of second order high pass engine 307 is then passed to bass boost engine 309 where user-specified gain 311 is applied to boost the signal. The boosted signal is then passed to bass compressor processing engine 315 of bass compressor engine 313 to compress offending transient signals that exceed a user-specified threshold setting, such as threshold 86 discussed above, in combination with, for example, other user-specified settings. One of ordinary skill in the art would appreciate that bass compressor processing engine 315 may include many or all of the elements discussed above for

compressor 258 and shown in Fig. 11 except that such elements configured as part of bass enhancement engine 300 would process only those signals passed to it from bass boost engine 309, namely comprising primarily bass frequencies, and only to the extent such frequencies exceed a user-specified threshold setting. A user interface, such as bass enhancement engine user interface 80, may be implemented in connection with bass enhancement engine 300 to provide user-selectable inputs for use by bass enhancement engine 300. The output from bass compressor processing engine 315 is passed to recombination engine 318 of bass integration engine 317 for combining this signal with the signal that is filtered by fourth order high pass engine 302 and processed by delay processing engine 310. [0102] To further enforce a maximum excursion limit of the speaker driver, as shown in Fig. 9, the combined signal from bass integration engine 317 is passed to bass soft clip engine 320 and processed via bass soft clip processing engine 321. As discussed above, soft-clip functionality employed downstream of the bass compressor engine 313 of the bass

enhancement engine 300 may be configured to instantaneously attack and release a particularly damaging input signal as a back-up to bass compressor engine 313 to instantly clip an unsafe input signal that manages to pass through bass compressor processing engine 315 without introducing audible distortion.

[0103] By passing the signal to bass soft clip engine 320 after processing the signal by bass compressor engine 313, bass soft clip engine 320 may be configured to address potentially damaging, transient signals that pass through bass compressor processing engine 315 and compressor 258 of dynamics engine 250. Bass soft clip engine 320 is a more aggressive compressor than bass compressor engine 313 but less aggressive than a pure limiter and can be configured by a user to process the signal according to user specifications.

[0104] As shown in Fig. 9, signals processed by bass soft clip processing engine 321 are then passed to postscale processing engine 324 of postscale engine 323.

[0105] Turning to Fig. 13, there is shown an exemplary steady state signal response for a signal processed by an exemplary dynamics engine 250 or an exemplary bass enhancement engine 300. A representative output signal is shown plotted against a representative input signal for points below and above a threshold setting 330 of -6 dB and for a compression ratio of 4:1. Below threshold setting 330, curve 332 has a 1 : 1 linear slope reflecting the fact that the compressor is not triggered and that the output signal is the same as the input signal. For input signals that exceed the -6 dB value of threshold setting 330, the compressor reduces the voltage of the signal according to the slope of the compression ratio; in this example, at the rate of 4: 1 , as shown by compressor curve 333. The compression ratio allows the user to select the desired degree of compression above threshold over a wide range to best suit the application and match the capabilities of the system. In all cases, the distortion is zero under steady state conditions.

[0106] Turning to Figs. 14-16, there is shown a series of graphs reflecting a signal before and after processing of the signal by bass soft clip processing engine 321 of bass soft clip engine 320 according to various user-specified soft clip inputs. The peak digital signal can vary from peak value of 0 to 1. For example, Fig. 16 shows that output sine wave 346 closely tracks input sine wave 345 when soft clip input is set to a relatively small signal value of 0.25. Fig. 14, therefore, reflects virtually no compression of the signal when the level is low.

[0107] By contrast, Figs. 15-17 illustrate the effect of increasing the soft clip input from .25 to, for example, .50 (Fig. 15), 1.0 (Fig. 16), and 2.0 (Fig. 17). Fig. 15 shows only a slight decrease in the peak maxima and minima of output sine wave 351 as compared to input sine wave 350, reflecting very light compression for a signal with peaks that ½ of the full scale. Fig. 16 shows a slightly greater decrease in the peak maxima and minima of output sine wave 356 as compared to input sine wave 355, reflecting modest compression for a signal that would otherwise have just reached the limits of digital clipping. Fig. 17 shows yet an even greater decrease in the peak maxima and minima of output sine wave 361 as compared to input sine wave 360. In the exemplary embodiment of Fig. 17, bass soft clip processing engine 321 causes output sine wave 361 to gracefully flatten at peak maxima and minima values of slightly less than 1 and -1 , respectively, preventing what would otherwise be gross distortion from an input signal of twice the allowable limit, while minimally affecting smaller signals.

[0108] In principle, system 200 having user interface 10 may be implemented on, for example, a personal computer or laptop platform, using, for example, Flowstone digital signal Such computer or platform may include a memory element, which may include a computer- readable medium for implementing system 200 and user interface 10 for optimizing audio response from audio playback systems.

[0109] System 200 and user interface 10 may be implemented in software, firmware, hardware, or any combination thereof. In one mode, system 200 having user interface 10 is implemented in software, as an executable program, and is executed by one or more special or general purpose digital computer(s), such as a personal computer (PC; IBM-compatible, Apple- compatible, or otherwise), personal digital assistant, workstation, minicomputer, mainframe computer, computer network, "virtual network" or "internet cloud computing facility." In another mode, system 200 having user interface 10 is implemented in firmware in, for example, a laptop, a handheld MP3 player, a mobile telephone, an automotive stereo system, a home stereo system, or any other mobile or fixed platform or device. Implementation and operation of system 200 and user interface 10 is independent of the operating system of the underlying computer or device, and will work on any operating system, such as Android, iOS, Microsoft Windows, Linux, Unix, and the like.

[0110] In one embodiment, system 200 may be used by a user to "tune" a given audio playback system to maximize the volume output of all frequencies by the system without creating unwanted artifacts or distortion in the signal. Development software may be used in connection with system 200 to further "tune" the audio response of the audio playback system. System 200 may be implemented in connection with a user interface, such as user interface 10, to permit a user to select values for each of the inputs described above for user interface 10. A consumer version of user interface 10, which may have fewer consumer-selectable inputs, may be implemented in connection with system 200. [0111] In terms of hardware architecture, a computer or device on which system 200 operates includes a processor, memory, and one or more input and/or output (I/O) devices (or peripherals) that are communicatively coupled to one another via a local interface. The local interface may be, for example, but is not limited to, one or more buses or other wired or wireless connections, as is known in the art. The memory can include any one or a combination of volatile memory elements (e.g., random access memory (RAM, such as DRAM, SRAM, SDRAM, etc.)) and nonvolatile memory elements (e.g., ROM, hard drive, tape, CDROM, etc.). The memory may also incorporate electronic, magnetic, optical, and/or other types of storage media.

[0112] System 200 and user interface 10 may comprise one or more software programs, which may be stored on any computer-readable medium for use by or in connection with any computer related system or method. A computer-readable medium is an electronic, magnetic, optical, or other physical device or means that can contain or store a computer program for use by or in connection with a computer related system or method. System 200 and user interface 10 can be embodied in any type of computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions.

[0113] If implemented in hardware, system 200 and user interface 10 may also be implemented with any of the following technologies, or a combination thereof, which are each well known in the art: a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate combinational logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc. [0114] While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the disclosure herein is meant to be illustrative only and not limiting as to its scope and should be given the full breadth of the appended claims and any equivalents thereof.

Claims

What is claimed is:

1. A computer program product stored on a computer-readable medium, the computer program product having computer-executable code instructions that are executable by a computer processor to optimize sound reproduction by an audio system, the computer- executable code instructions comprising:

first code instructions for adjusting characteristics corresponding to a plurality of frequency bands of an audio signal;

second code instructions for reducing a voltage of the audio signal processed by the first code instructions that exceeds a first user-adjustable threshold to enforce a desired maximum voltage amount of a speaker driver of the audio system; and third code instructions for boosting bass response of the audio signal processed by the second code instructions below about a bass resonant frequency of a speaker driver, the third code instructions comprising fourth code instructions for reducing the voltage of the audio signal processed by the third code instructions that exceeds a second user-adjustable threshold to enforce the desired maximum excursion amount of the speaker driver.

2. The computer program product of Claim 1 , wherein the first code instructions are

configured to process the audio signal through a plurality of biquad filters corresponding to respective frequency bands for user adjustment of the characteristics.

3. The computer program product of Claim , wherein the characteristics corresponding to the plurality of frequency bands comprise at least one of a gain, a center frequency of each of the plurality of frequency bands, and a Quality Factor.

4. The computer program product of Claim 3, wherein the gain, the center frequency, and the Quality Factor are each independently adjustable by a user.

5. The computer program product of Claim 4, wherein the gain is continuously or discretely adjustable from about -12 dB to about 12 dB.

6. The computer program product of Claim 4, wherein the center frequency is continuously or discretely adjustable from about 20 Hz to about 20 kHz.

7. The computer program product of Claim 4, wherein the Quality Factor is continuously or discretely adjustable from about 0.1 to about 4.0.

8. The computer program product of Claim , wherein the second code instructions are configured to process the audio signal according to an independently adjustable and user-definable setting comprising at least one of an attack rate, a release rate, an attack threshold, and a release threshold.

9. The computer program product of Claim 1 , wherein the first user-definable threshold is continuously or discretely adjustable from about 0 to about - 2 dB.

10. The computer program product of Claim 1 , wherein the third code instructions are

configured to process the audio signal according to an independently adjustable and user-definable setting comprising at least one of a bass attack rate, a bass release rate, a bass attack threshold, and a bass release threshold.

1 1. The computer program product of Claim , wherein the second user-definable threshold is continuously or discretely adjustable from about 0 to about -12 dB.

12. A system for optimizing sound reproduction by an audio system, comprising: one or more processors;

a memory;

an equalization engine stored in the memory, comprising first code instructions to cause the one or more processors to adjust characteristics corresponding to a plurality of frequency bands of an audio signal;

a dynamics engine stored in the memory, comprising second code instructions to cause the one or more processors to dynamically reduce a voltage of the audio signal processed by the equalization engine that exceeds a first user-definable threshold to enforce a desired maximum excursion amount of a speaker driver of an audio system; and

a bass enhancement engine stored in the memory, comprising

third code instructions to cause the one or more processors to boost bass response of the audio signal processed by the dynamics engine below about a bass resonant frequency of the speaker driver, and fourth code instructions to cause the one or more processors to reduce the voltage of the audio signal processed by the bass enhancement engine that exceeds a second user-definable threshold to enforce the desired maximum excursion amount of the speaker driver.

13. The system of Claim 12, wherein the first code instructions are configured to process the audio signal through a plurality of biquad filters corresponding to respective frequency bands for adjustment of the characteristics.

14. The system of Claim 12, wherein the characteristics corresponding to the plurality of frequency bands comprise at least one of a gain, a center frequency of each of the plurality of frequency bands, and a Quality Factor.

15. The system of Claim 14, wherein the gain, the center frequency, and the Quality Factor are each independently adjustable by a user.

16. The system of Claim 12, wherein the dynamics engine includes independently

adjustable settings corresponding to at least one of the group consisting of an attack rate, a release rate, a delay period, a compression ratio, a threshold, a release threshold, or a look-ahead.

17. The system of Claim 12, wherein the bass enhancement engine includes independently adjustable settings corresponding to at least one of the group consisting of a bass boost amount, an attack rate, a release rate, a delay period, a compression ratio, a threshold, a release threshold, a look-ahead, a high pass filter setting, or a soft clip setting.

18. A method for optimizing sound reproduction by an audio system, comprising:

providing a computer-readable medium encoded with a first computer program for independently adjusting characteristics corresponding to a plurality of frequency bands of an audio signal;

providing a computer-readable medium encoded with a fifth computer program for dynamically reducing the voltage of the audio signal processed by the fourth computer program that exceeds a second user-adjustable attack threshold to enforce a maximum excursion amount of the speaker driver; and providing a computer-readable medium encoded with a sixth computer program for dynamically ceasing voltage reduction of the audio signal processed by the fifth computer program when the voltage falls below a second user-adjustable release threshold to return the voltage to a boosted state. frequency bands comprise at least one of a gain, a center frequency of each of the plurality of frequency bands, and a Quality Factor.

The method of Claim 18, further providing a computer-readable medium encoded with a seventh computer program for instantly clipping the voltage of the audio signal processed by the sixth computer program that exceeds a user-adjustable soft clip threshold to further enforce a maximum excursion amount of the speaker driver.

A computer program product stored on a computer-readable medium, the computer program product having computer-executable code instructions that are executable by a computer processor for enhancing bass response of an audio system, the computer- executable code instructions comprising:

first code instructions for boosting a voltage corresponding to bass frequencies of an audio signal below a selected frequency; and

The computer program product of Claim 21 , wherein the selected frequency

corresponds approximately to a bass resonant frequency of a speaker enclosure associated with the speaker driver.

The computer program product of Claim 21 , further comprising third code instructions comprising a high pass filter for processing the audio signal before processing the audio signal by the second code instructions.

24. The computer program product of Claim 21 , further comprising fourth code instructions for recombining the reduced audio signal with the audio signal above the selected frequency.

25. The computer program product of Claim 24, further comprising fifth code instructions for soft clipping the recombined audio signal.

26. The computer program product of Claim 21, further comprising sixth code instructions for delaying the audio signal above the selected frequency.

27. The computer program product of Claim 21 , wherein the compressor comprises

separate attack and release thresholds.

28. A computer program product stored on a computer-readable medium, the computer program product having computer-executable code instructions that are executable by a computer processor for enhancing bass response of an audio system, the computer- executable code instructions comprising:

first code instructions for determining bass frequencies of an audio signal to be

boosted below a selected frequency;

29. The computer program product of Claim 28, wherein the third code instructions are

configured to process the audio signal according to an independently adjustable and user-definable setting comprising at least one of the group consisting of an attack rate, a release rate, a release delay, a compression ratio, an adjustable frequency bypass, an attack threshold, a release threshold, or a look-ahead.

30. The computer program product of Claim 28, further comprising fourth code instructions comprising a frequency dependent limiter for limiting a voltage of the audio signal according to a frequency dependent threshold for creating zero filter frequency shift.

31. The computer program product of Claim 28, wherein the first code instructions comprise a high pass filter and a low pass filter for establishing a range of bass frequencies to be boosted below the selected frequency.

32. A computer program product stored on a computer-readable medium, the computer program product having computer-executable code instructions that are executable by a computer processor for enhancing a response of an audio system, the computer- executable code instructions comprising:

first code instructions comprising a compressor for reducing a voltage of an audio signal, the first code instructions configured to enforce a frequency independent excursion amount of a speaker driver, the first code instructions configured to process the audio signal according to a user-definable setting comprising an attack rate, a release rate, a release delay, a frequency bypass, an attack threshold, and a look-ahead.

33. The computer program product of Claim 32, wherein the setting further comprises at least one of a compression ratio and a release threshold.

34. The computer program product of Claim 32, further comprising second code instructions comprising a low pass filter for filtering the audio signal before processing the audio signal by the first code instructions.

35. The computer program product of Claim 34, wherein the low pass filter is a first order low pass filter.

36. The computer program product of Claim 32, further comprising third code instructions for soft clipping the voltage of the audio signal processed by the first code instructions that exceeds an adjustable soft clip threshold to further enforce a maximum excursion amount of the speaker driver.

37. The computer program product of Claim 32, further comprising fourth code instructions comprising a high pass filter for filtering the audio signal after processing the audio signal by the first code instructions to enforce a frequency dependent threshold.

38. A method for optimizing sound reproduction by an audio system, comprising:

providing a computer-readable medium encoded with a first computer program for filtering bass frequencies of an audio signal, the first computer program comprising a low pass filter configured to pass bass frequencies below an adjustable low pass filter setting and a high pass filter configured to pass bass frequencies above an adjustable high pass filter setting, the low pass filter setting comprising a frequency that is higher than a frequency of the high pass filter setting;

providing a computer-readable medium encoded with a third computer program for dynamically reducing a voltage of the audio signal passed by the second computer program that exceeds an adjustable threshold to enforce a maximum safe excursion amount of a speaker driver associated with the audio system. PATENT

39. The method of Claim 38, wherein the low pass filter is a fourth order low pass filter.

40. The method of Claim 38, wherein the low pass filter setting is settable to approximately a resonant frequency of the speaker driver.

41. The method of Claim 38, wherein the high pass filter is a second order high pass filter.

42. The method of Claim 38, further comprising providing a computer-readable medium encoded with a fourth computer program for dynamically ceasing voltage reduction of the audio signal processed by the third computer program when the voltage falls below an adjustable release threshold to return the voltage to a boosted state.

43. The method of Claim 42, further providing a computer-readable medium encoded with a fifth computer program for soft clipping the voltage of the audio signal processed by the fourth computer program that exceeds an adjustable soft clip threshold to further enforce a maximum excursion amount of the speaker driver.