Abstract:
Systems and methods for providing bass compensation to correct for uneven bass response are disclosed. An example bass compensation system includes a low pass filter configured to receive an audio signal from an audio source and provide a filtered audio signal, the low pass filter having a roll off of at least 18 dB per octave. The bass compensation system further includes a summing amplifier coupled to the low pass filter and configured to sum the audio signal from said audio source and the filtered audio signal to provide a summed audio signal, wherein the summed audio signal provided by the summing amplifier provides a bass boost at a first frequency and mid bass cut at a second frequency greater than the first frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of Provisional Application Nos. 61/403,871 filed on Sep. 23, 2010, and 61/572,025 filed on Jul. 11, 2011, which applications are incorporated herein by reference, in their entirety, for any purpose. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to improving bass response in an automobile, and more particularly, to improving bass response by providing a system to compensate for lack of bass and problem of undesirable mid bass resonance in an automobile. 
       BACKGROUND OF THE INVENTION 
       [0003]    The acoustic environment of automobiles is difficult and may present challenges to reproduce a satisfying audio experience. For example, automobile audio system design and the acoustics of an automobile may contribute to the loss of bass. Loss of bass can be attributed to the inability of loudspeakers to reproduce lower frequencies along with a low frequency roll off attributed to factory speaker protection systems that limit bass output at higher listening levels. The physical location of the loudspeakers also becomes a factor in sound quality having an impact on the low frequency and high frequency response. Automobiles also work as sealed enclosures that produce mid bass resonance making the bass response unnatural sounding. Last the sensitivity of the ear at low frequencies is not the same at low levels as it is at higher levels as explained by Harvey Fletcher et al.,  Loudness, Its Definition, Measurement and Calculation , in Journal of the Acoustic Society of America, Vol. 5, October 1933. It is widely known that the low frequency response in an automobile given the many factors of speaker placement, listening position and automobile resonance, make it quite difficult to make the automobile audio systems sound equivalent to that of a high end stereo system. 
         [0004]    It is generally accepted that the interior of an automobile is far from being an ideal listening space. One of its most obvious flaws is a dominant resonance at 250-300 Hz that makes the bass lose tonality and unnatural sounding. This resonance reinforces the mid bass produced by a typical sound system in an automobile and while this resonance can be used to an advantage in competitive high SPL autosound competitions is not helpful to the objective of reproducing good sound in an automobile 
         [0005]    Several approaches have been taken to compensate for low frequency roll off in an automobile environment. Graphic equalizers are typically used to make the necessary compensation that is usually done after measurements have been made with a ⅓-octave real time analyzer. This requires the end user to have some skill and audio knowledge to use the analyzer and then apply that to the equalizer. 
         [0006]    The concept of an accelerated slope tone control for bass compensation was introduced by Dennis A. Bohn in  Accelerated Slope Tone Control Equalizers , J. Audio Eng. Soc., Vol. 40, No. 12, December 1992, and U.S. Pat. No. 5,046,105 issued Sep. 3, 1991 describes a system that improved the function of typical bass and treble tone controls. This concept demonstrated the use of 2nd and 3rd order filters with an added zero offset for each additional pole to eliminate phase cancellations in the mid bass. The advantage of the accelerated slope tone control concept is to reduce mid band interaction inherent in bass and treble tone controls. 
         [0007]    Additionally, numerous methods exist that improve the bass response of smaller loudspeakers with limited bandwidth. The most significant improvement in sound reproduction in a vehicle is to typically add bass loudspeakers and power amplification. What is needed is a method to optimize the use of the aforementioned improvements in a vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of a bass compensation system according to an embodiment of the present invention. 
           [0009]      FIG. 2  is a block diagram of a bass compensation system according to an embodiment of the present invention. 
           [0010]      FIG. 3  is a block diagram of a bass compensation system according to an embodiment of the present invention. 
           [0011]      FIG. 4  is a block diagram of a bass compensation system according to an embodiment of the present invention. 
           [0012]      FIG. 5  is a schematic diagram of a bass compensation system according to an embodiment of the invention. 
           [0013]      FIG. 6  is a frequency response diagram according to an embodiment of the invention. 
           [0014]      FIG. 7  is a block diagram of a bass compensation system according to an embodiment of the present invention. 
           [0015]      FIG. 8  is a block diagram of a bass compensation system according to an embodiment of the present invention. 
           [0016]      FIG. 9  is a functional block diagram of a digital signal processor for a bass compensation system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]    Embodiments of the invention provide bass compensation to compensate for difficult acoustic environments, for example, an automobile and the loss of bass from various circumstances. Additionally, it may be desirable to reduce some or all of the mid bass resonance typically experienced in automobiles. Certain details are set forth below to provide a sufficient understanding of embodiments of the invention. However, it will be clear to one skilled in the art that embodiments of the invention may be practiced without these particular details. Moreover, the particular embodiments of the present invention described herein are provided by way of example and should not be used to limit the scope of the invention to these particular embodiments. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the invention. 
         [0018]      FIG. 1  illustrates a bass compensation system according to an embodiment of the invention. The bass compensation system includes an audio input  10  to which an audio source may be coupled to provide an audio signal to a summing amplifier  11  and 18 dB per octave low pass filter  12  with a corner frequency in the region of 300 Hz. The bass compensation further includes an 18 dB per octave high pass filter  13  with a corner frequency approximately in the region of 33 Hz. The high pass filter  13  is configured to limit subsonic energy delivered to the amplifier and loudspeakers. The high pass filter  13  may have slopes other than 18 dB per octave as well. For example, the high pass filter  13  may be a 24 dB per octave high pass filter in other embodiments. The high pass filter  13  may provide subsonic loudspeaker protection whether the bass compensation is engaged or not. Other frequencies other than 33 Hz may be used, or in some embodiments the frequency is selectable to limit the maximum excursions of woofers and subwoofers in an automobile sound system. In other embodiments, the high pass filter  13  is not included, for example, limiting subsonic energy is provided by circuitry other than the bass compensation system. 
         [0019]      FIG. 2  illustrates a bass compensation system according to an embodiment of the invention. Similar to the bass compensation system of  FIG. 1 , the bass compensation system of  FIG. 2  includes a summing amplifier  11 , a low pass filter  12 , and a high pass filter  13 . The bass compensation system of  FIG. 2  further includes a level control circuit  14  configured to control the amount of bass compensation applied to the summing amplifier  11  is shown in  FIG. 2 . The level control  14  after the 18 dB per octave low pass  12  provides a means to control the amount of bass compensation applied to the summing amplifier  11 . The level control circuit  14  could precede the 18 dB per octave low pass filter as well and yield similar results. 
         [0020]      FIG. 3  illustrates a bass compensation system according to an embodiment of the invention. The bass compensation system of  FIG. 3  includes similar blocks as those illustrated in  FIG. 2 , and the same reference numbers are used for blocks previously described. The bass compensation system of  FIG. 3 , however, further includes a switch  18  coupled to the level control circuit  14  which may be used to manually switch in and out the bass compensation is shown in  FIG. 3 . The switch  18  is illustrated in  FIG. 3  as coupled after the bass compensation level control  14  and the low pass filter  12 . A mechanical switch  18  could also be located at the input of the 18 dB per octave low pass filter  12 . 
         [0021]      FIG. 4  illustrates a bass compensation system according to an embodiment of the invention. In addition to the blocks previously described with reference to  FIGS. 1-3 , the bass compensation system of  FIG. 4  includes circuits configured to automatically engage the bass compensation at a predetermined audio level. The bass compensation system includes an adjustable DC threshold potentiometer  16  that may be used to set the reference input of a comparator  17 . The output of the comparator  17  coupled to a peak detector  19  provides a signal that is used to turn on the switch  18 . Other types of semiconductor or mechanical switches may be used for the switch  18  to engage the bass compensation circuit. 
         [0022]      FIG. 5  illustrates a bass compensation system according to an embodiment of the invention. The bass compensation system of  FIG. 5  represents an analog implementation of a bass compensation system according to an embodiment of the invention, in that various blocks included in the previously described embodiments of the invention are implemented using analog circuitry. An input stage  20  includes a differential amplifier  20  driven from various signal levels, low level at line level or directly connected to loudspeaker outputs from an automobile factory radio. A single ended inverting or noninverting amplifier would also work equally as well at this stage. 
         [0023]    The input stage  20  drives a summing amplifier  11  and 3rd order Chebyshev 18 dB per octave low pass filter  12  having an output level determined by level control  14  and switch  18 . Typically a 3rd order shelving circuit with zero compensation for the two additional poles would be used with a standard tone control type circuit. 
         [0024]    In further detail, still referring to the invention of  FIG. 5  a 3rd order 18 dB per octave high pass filter  13  is included to limit energy below 33 Hz. A potentiometer  16  coupled to a negative input of a comparator  17  may be adjusted to select a DC voltage between ground and +B volts. A positive input of the comparator  17  is coupled to the output of the input stage  20 . The output of the comparator  17  is provided to a peak detector  19  that is used to control the switch  18  and provide bass compensation engaged at a predetermined audio input level. The use of peak detector  19  may be useful if the compensation is intended to be used for bass roll off that only occurs at high input levels or be additionally engaged at low levels compensating for the audible bass roll off the human ear at low levels. The peak detector  19  may be configured to have a long time constant such that the switch  18  is closed (i.e., conductive) during musical intervals. The circuits illustrated in  FIG. 5  may be used for various ones of the blocks of the bass compensation systems of  FIGS. 1-4 , previously discussed. 
         [0025]    In a broad embodiment, the invention may be used in one or many channels depending on the audio system. Typically for use with subwoofers this either is one or two channels. If two channels were required the level control  14  of  FIG. 5 , for example, would typically use a dual type potentiometer to control the bass compensation simultaneously to both channels. 
         [0026]      FIG. 6  illustrates frequency response curves for a bass compensation system according to embodiment of the invention. The frequency response  29  represents the response with the bass compensation level control  14  in a minimum level position. The frequency response  30  represents the response with the bass compensation level control  14  in a mid level position. The frequency response  31  represents the response with the bass compensation level control  14  in a maximum level position. Referring to  FIG. 6  the frequency responses  29 ,  30 , and  31  for a minimum, mid, and maximum levels illustrate frequency rolling off at 50 Hz due to the 18 dB per octave high pass filter  18 . 
         [0027]    In more detail referring to  FIG. 6  the bass compensation system behaves as a bass boost circuit up to the point phase shift produces a controlled amount of attenuation  32  in the mid bass region, for example, at approximately 300 Hz. This reduction may be useful in reducing the effect of resonance within an automobile. For example, with reference to the bass compensation system of  FIG. 5  and assuming a maximum bass boost provided by the level control circuit  14  of  FIG. 5  a boost of approximately 18 dB at 50 Hz as shown in the curve  31  and a −7 dB dip at approximately 300 Hz  32  of  FIG. 6 . is provided. The maximum amount of bass compensation may be changed by changing the resistances and/or range of resistances of the level control circuit. 
         [0028]      FIG. 7  illustrates a bass compensation system according to an embodiment of the invention. An analog audio source  33  typically of two channels is provided to analog to digital converters  34  and the output of the analog to digital converters  34  provided to the DSP digital signal processor  35 . The analog to digital converters  34  are configured to convert the analog audio signals into digital audio signals that are provided to the DSP  35 . The DSP  35  is configured to transform the digital audio signals to provide a transfer function as previously described with reference to illustrated in  FIG. 6 . The transformed digital output of the DSP  35  is provided to digital to analog converters  36  which is configured to convert the transformed digital audio signals to analog audio signals. The analog audio signal provided by the digital to analog converters  36  are typically used to drive an audio power amplifier or crossover  37 . 
         [0029]      FIG. 8  illustrates a functional block diagram for the DSP  35  for transforming the digital audio signals according to an embodiment of the invention. Digital audio signals  38  provided to the DSP  35  are provided to summing amplifier  42  and to a low pass filter  39 . The low pass filter  39  is illustrated in the embodiment of  FIG. 8  as having a corner frequency in the region of 300 Hz, but other frequencies may be used as well in other embodiments. The low pass filter  39  is then provided to a gain stage  40 . The output of the gain stage is then provided to an attenuator  41  that will determine the magnitude of the applied bass compensation. The attenuator  41  is illustrated for the embodiment of  FIG. 8  as having a range of 0 to −40 dB. Other ranges of attenuation may be used as well, however. The digital output of the attenuator  41  is provided to the summing amplifier  42  where the unprocessed digital audio signals are summed with the processed digital audio signals provided by the attenuator  41 . The digital output of the summing amplifier  42  is provided to a high pass filter  43 . The high pass filter  43  may be used to limit subsonic energy delivered to the amplifier and loudspeakers. The high pass filter  43  may provide subsonic loudspeaker protection whether the bass compensation is engaged or not. The corner frequency of the high pass filter  43  may be approximately 33 Hz, although other frequencies may be used as well. In some embodiments, the high pass filter  43  is not included. For example, circuitry other than the DSP  35  are used to limit the subsonic energy. The digital output of the high pass filter  43  is then provided to be converted to analog audio signals, for example, by D/A converter  36  shown in  FIG. 7 . 
         [0030]      FIG. 9  illustrates a functional block diagram for the DSP  35  for transforming the digital audio signals according to an embodiment of the invention. The embodiment illustrated in  FIG. 9  includes elements previously described with reference to the embodiment illustrated in  FIG. 8 . The reference numbers of  FIG. 8  are also used in  FIG. 9  to references common elements. The embodiment of  FIG. 9 , however, further includes functional blocks for engaging and disengaging bass compensation. For example, the embodiment of  FIG. 9  includes a comparator  45  that is configured to determine if the digital audio signals  38  are above or below a threshold selected by a user. The user selection is represented by the adjustable threshold value provided to the comparator  45 . The output of the comparator  45  is provided to a one shot flip flop and timer logic  46 . In operation, if the magnitude of the digital audio signal  38  exceeds the selected threshold, the output of the comparator  45  outputs a logic 1 to the one shot flip flop  47  causing it to change its state at the output. The change of state at the output of the one shot flip flop is provided to a multiplexer  48  that couples the output of the attenuator  41  to the summing amplifier  42  to engage the bass compensation. When the digital audio signal level falls below the user selected threshold timer logic  46  times out and resets the one shot flip flop  47 . Resetting the one shot flip flop opens the multiplexer  48  disengage bass compensation. 
         [0031]    It will be appreciated by those ordinarily skilled in the art that with different values of components, embodiments of the invention could also be applied to other sound system with different acoustic characteristics such as boats, motor homes, etc. 
         [0032]    Embodiments of the invention may be used to provide reasonably accurate bass compensation for a typical automobile without requiring the operator to have knowledge of auto sound acoustics and electronics. It provides the listener with an effective means to compensate for the limitations of bass reproduction in an automobile that occurs naturally in the human ear especially at lower levels. In many cases, the geometry and volume of most car interiors are reasonably similar with respect to the wavelengths of bass and mid bass frequencies. The bass compensation system may be used to apply equalization similar to that of a house curve used in professional audio to compensate for the acoustics in a room. 
         [0033]    In some embodiments of the invention, the bass compensation system uses the inherent steeper slope of a 3rd order filter. Additionally, the inherent mid bass cancellation caused by phase shift greater than 90 degrees may be used as well. The cancellation is strategically placed and limited to reduce the effects of the resonance in the mid bass region. 
         [0034]    A 3rd order filter used as tone control would typically incorporate a 2-pole phase compensated circuit to minimize the induced cancellations caused by phase shift. In some embodiments, the bass compensation system described strategically places this cancellation in the 200-300 Hz region where a dominant resonance in an automobile typically exists. Therefore the bass compensation presented here is used as an advantage to construct a bass boost and mid bass cut. 
         [0035]    Incorporating a threshold for engaging the bass compensation circuit permits a user to employ the compensation for those bass limitations that occur only at higher volume levels. It permits the user to use the bass compensation circuit for limitations placed on the bass response by factory head units at higher levels. The threshold may also be set to act as a loudness function at low levels of operation where the bass response of the human ear falls off. The time constant of the circuit that engages the bass compensation circuit may be set long enough to stay engaged between typical musical intervals and thus refrain from being switched in or out during music. 
         [0036]    The bass compensation level control gradually adds compensation as needed or desired by the user. This control allows the user to vary the compensation with respect to the type of music they are playing, their personal taste and amount needed for their specific system. The car audio enthusiast generally likes more bass, natural sounding or not, thus in this design a maximum boost of +18 dB at 50 Hz may be provided. 
         [0037]    In some embodiments of the invention, a 3rd order low pass filter is positioned at 200 Hz and summed with the main channel via the bass compensation level control. The 3rd octave low pass filter has a rising slope that peaks at 50 Hz where a 3rd octave high pass loudspeaker protection system at 33 Hz rolls off at 18 dB per octave. A field effect transistor (FET) may be used as a gate that applies bass compensation as a function of input level. For example, when the audio input to the circuit is high enough, as determined by the threshold control, the peak voltage charges an electrolytic capacitor that holds the FET on for a time sustained by normal music intervals. 
         [0038]    A 3rd order high pass circuit may be used to limit the subsonic energy sent to the amplifier and loudspeaker or loudspeakers. It may be desirable to reduce the non-audible, subsonic energy sent to the loudspeaker. A lower order filter may be used if the maximum bass compensation were reduced to a much lower value. 
         [0039]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.