Abstract:
By taking advantage of the ability to control the phase relationship between a processor&#39;s output (or portions of a processor&#39;s output) and the phase of the pre-processed signal, a controlled accentuation or enhancement of the processor&#39;s effect can be realized. In one embodiment this is achieved by providing a gain control circuit that receives and selectively amplifies the input signal prior to it being summed with the processor&#39;s output.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains to a method and apparatus for reducing the level of at least a portion of a signal&#39;s frequency spectrum. More particularly, the present invention pertains to a method and apparatus for enhancing/intensifying the effect of at least one frequency band which has previously been chosen for a reduction in level.  
           [0002]    A category of processors for altering the level of selected portions of an input signal that may have been converted to electronic analog or digital signals is known in the art. When dealing with signals in the “audio” frequency range (approximately between 20 Hz-20 kHz ) for data such as music (or other audio sound), the category includes devices known as filters, equalizers, and tone controls.  
           [0003]    FILTER SHAPES AND BANDS (STOP, PASS, and TRANSITION)—Filters are often descibed as having certain shapes, which consist of regions called bands. The four most common filter shapes are known as hi-pass, low-pass, band-pass, and band-reject (also called ‘notch’). Each of these filter shapes have areas (or bands) of the frequency spectrum that are affected (where the filter shape reduces the signal, or tries to stop it), and areas that are not affected (where the filter shape lets the signal pass through). A reduced area is called a “STOP-BAND” (i.e., the signal is stopped). An untouched area is called a “PASS-BAND” (i.e., the signal is allowed to pass). There is always an area of transition between a passband and a neighboring stopband, herein called a “TRANSITION BAND.” 
           [0004]    SOME PHASE CHARACTERISTICS OF WAVES—Wave energy (such as sound energy, alternating electrical signals, etc.) is often modeled as having peaks and troughs. Roughly speaking, a peak is where energy is at a maximum of pushing, a trough is at a maximum of pulling back, and in between the energy is shifting back and forth between this pushing and pulling.  
           [0005]    Two identical (or very similar) waves combined in certain ways produce particular results. For the simple example of a pair of identical sine waves, some basic combinations include:  
           [0006]    “IN PHASE” combination=phase difference of 0 degrees=when their high and low points (peaks and troughs) occur at the same time. Since they are pushing and pulling in the same direction, playing the two waves together like this doubles the energy, which sounds louder than one wave alone.  
           [0007]    “OUT OF PHASE” combination=phase difference of 180 degrees=when the high point of one wave combines with the low point of the other. When out of phase, one is pushing while the other is pulling. Playing two identical waves together like this completely cancels the energy. The result is complete silence.  
           [0008]    BETWEEN 0 AND 180 DEGREES: Between 0 and 180 degrees, the two waves played together will sometimes push-pull together and sometimes push-pull in opposition. At a phase difference of 120 degrees, the resulting energy is the same amount as if only one wave was playing. As you choose phase relationships going from 0 to 180 degrees, the total energy of the two waves combined gets smaller. There is double energy at 0 degrees phase (in phase); it gets gradually less until it reaches single energy at 120 degrees phase, and continues to get gradually less until it reaches zero energy at 180 degrees phase (out of phase). From 180 to 360 degrees, the process reverses (gradually gets louder).  
           [0009]    For example, in the audio engineering field, one often mixes signals, e.g. adds a bass drum microphone&#39;s output to other microphone signals present. It is known in the art that the various distances from each microphone may cause phase differences that can, at certain frequencies, cause cancellations, and so affect the tone quality of the bass drum. Many audio mixing consoles, therefore, come with switches that allow changing each microphone&#39;s phase by 180 degrees (=“reversing phase”), which is often helpful in correcting for these microphone placement problems.  
         SUMMARY OF THE INVENTION  
         [0010]    A method and apparatus of the present invention makes constructive use of the phase canceling effect. In an embodiment of the present invention, a device can be constructed so that one or more of the post-processor stopbands are 180 degrees out of phase with the original (pre-processor) signal, while one or more of the post-processor passbands remain in phase (0 degrees) with the original preprocessor level. By mixing the above post-processor signal with the original pre-processor signal in approximately equal amounts, the in-phase passbands end up being somewhat louder (the greatest available increase with no added pre-processor gain is 2×). The out-of-phase stopbands become significantly reduced because the phase cancellation allows a (theoretically) infinite decrease. The present invention makes constructive use of this effect, and provides many benefits, which include:  
           [0011]    an enhancement/extension/magnification of the processor&#39;s effect, often beyond the ability of the processor itself,  
           [0012]    an ability to provide a variable “Q”/depth for circuits, some of which are normally difficult to do so, by allowing a variable slope/depth adjustment to a processor&#39;s stop bands,  
           [0013]    the possibility of providing a continuum from plain post-processor output to enhanced processing to no processing, all via a single control (it can also be automated for a particular setting, notably maximum enhancement).  
           [0014]    An example of use with the single control embodiment is described below. Assume the processing involves two equalizers, one set to reduce frequencies below 200 Hz, the other set to reduce the frequencies between 2 kHz and 3 kHz. With the control set to include none of the pre-processor signal (fully ‘off’), one sets each processor element to a desired sound. As the phase cancellor control is turned to gradually add pre-processor signal, the listener notices an enhancement of the processor&#39;s effect (due to the increasing phase cancellation in the stop-bands, as explained below). There will gradually be even less and less signal below 200 Hz, and less and less signal between 2 kHz and 3 kHz, until a maximum cut is reached where the control is set so that the pre- and post-processor signal mix is about even. As the control is used to add more pre-processor signal beyond that point, the regions of reduction become LESS reduced. Eventually, the pre-processor signal overwhelms the post-processor signal, until the signal is, in practice, the same as the original pre-processor output (an arrangement can be made to make it a purely pre-processor signal).  
           [0015]    The single phase-canceling control as described can serve several purposes. Although one can make processors that have as much cut as that produced by the phase cancellation method, they are often difficult or expensive; for many filter types, having a variable cut like this would be very difficult to produce. Also, tuning with a processor with a very steep cut is often not intuitive, so this two step method (process, then enhance) is often easier to do (though some filter types commonly include the two-step method achieved another way). And since the cancellation occurs because of RELATIVE signal phase, a single phase-cancelling control can work to produce this cancellation with an entire group of processor elements: note that in the example above, the two equalizing processors are first tuned individually, whereas the enhancement to (and control of) both takes place simultaneously. In particular, this ‘global’ nature of the present invention, the ability to control the intensity of an ENTIRE GROUP of processors with a single knob, is a previously unavailable effect that has an intuitive result and is easy to use. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a general block diagram of an apparatus constructed according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 2 is a detailed circuit diagram for an embodiment of the apparatus of FIG. 1.  
         [0018]    [0018]FIG. 3 is a pair of graphs which show the frequency response for the filters of FIG. 2, and the phase difference between the signal at the input and output of these filters of FIG. 2.  
         [0019]    [0019]FIG. 4A is a graph which shows the output of the circuit of FIG. 2 while R 3  is varied from 0% (no cancellation) to 2% (maximum cancellation).  
         [0020]    [0020]FIG. 4B is a graph which shows the output of the circuit of FIG. 2 while R 3  is varied from 2% (maximum cancellation) to 100% (approximates original input signal).  
         [0021]    [0021]FIG. 5 is a simplified block diagram of the apparatus of FIG. 1, and modified to include the placement of elements required for an implementation of a feature which can automatically seek a maximum of reduction in a stopband.  
         [0022]    [0022]FIG. 6 is a more detailed block diagram of the apparatus of FIG. 5, showing the elements of one implementation of an automatic seek feature in more detail.  
         [0023]    [0023]FIG. 7 is a graph which shows the result of a portion of the circuit of FIG. 2 (with the circuit marked  120  removed) while R 3  is varied from 0% (no effect) to 2%. 
     
    
     DETAILED DESCRIPTION  
       [0024]    Referring to FIG. 1, a block diagram of an apparatus for performing phase cancellation according to an embodiment of the present invention is shown. The phase cancellation can be referred to as performing the following steps:  
         [0025]    1. For those portions of the frequency spectrum output by the processors ( 120 ,  130 ,  131 ,  132 ,  133 ,  134 ,  135 ) which are desired (i.e., the passband(s)) and are therefore kept in phase with the input signal, those portions will add to their like portions of the frequency spectrum in the signal output by gain control circuit  140 , producing an increase in the level of those portions. As used herein, gain control includes amplification and/or attenuation of a signal.  
         [0026]    2. For those portions of the frequency spectrum output by the processors ( 120 ,  130 ,  131 ,  132 ,  133 ,  134 ,  135 ) which are not desired (the stopband(s)) and are therefore made to be out of phase with the input signal, those portions will cancel/subtract from their like portions of the frequency spectrum in the signal output by gain control circuit  140 , producing a decrease in the level of those portions.  
         [0027]    Input signal  110  is provided to at least one processor  120  where the signal is modified. For example, the signal can be modified by a processor which decreases the level of a portion of the frequency band of the signal (e.g., as in an audio filter). Input signal  110  is also provided to a gain control circuit  140 , which supplies an independent source of the unaltered input signal, but with a level control. The outputs of the processor(s)  120  . . . and the gain control circuit  140  are mixed by summing amplifier circuit  150 . If desired, the phase cancelled signal can be provided to an output device  190 , such as a recording device, speaker, etc.  
         [0028]    In the example of FIG. 2, the input signal  110  is passed through an optional gain amplifier. It is known in the art that a device which filters a region of frequencies can easily be constructed. It is also known in the art to construct such filters so that the stopband (filtered region) becomes out of phase in relationship to the original signal, while the passband (unfiltered region) remains in phase with the original signal. Examples of such circuits are shown in FIG. 2 as elements  120  (a fixed pole high-pass filter with an output volume control) and  130  (a variable frequency low-pass filter).  
         [0029]    [0029]FIG. 3 shows the phase relationships between the pre-processor signal of  140  (0 degrees) and the post-processor signals of  120  (line  325 ) and  130  (line  335 ). The scale for line  335 &#39;s phase degree is at the left side, and the scale for line  325 &#39;s phase degree is at the right side. The frequency response of low pass filter  130  (FIG. 2) is seen as line  330 , whereas its phase is seen as line  335  (scale at left). Similarly, the frequency response of high pass filter  120  (FIG. 2) is seen as line  320 , whereas its phase is seen as line  325  (scale at right). The signals of the two filters are combined by summing amp  150 . The topmost curve of FIG. 4A, where R 3 =0%, shows the resulting frequency response (sub-circuit  140  has no effect on the output when its potentiometer R 3  is set to 0%). In the example of FIG. 2, gain control circuit  140  is shown as a simple passive device. Some embodiments may benefit from an active device and/or other configurations. The output of circuit  140  has a phase of 0 degrees by definition, and is provided to summing amp  150  along with the outputs of circuits  120  and  130 . As R 3  of  140  is increased from its “0%” position, more of the pre-processor signal is added to the post-processor signals by the summing amp.  
         [0030]    The stopband regions of the combined result become more attenuated, while the passband regions remain largely unaffected, as shown in FIG. 4A. With R 3  set at 2%, an equal ratio of in- and out—of phase is reached, and this produces the maximum nadir. Also, with R 3  set at 2%, the transition between the high and low passbands and the stop band is at its steepest slope. FIG. 4B shows the results as R 3  is increased beyond 2%, where the pre-processor signal from circuit  140  gradually overwhelms the outputs of elements  120  and  130 , so that an essentially flat response is eventually reached for R 3 =100%. If an even ‘flatter’ response is desired, a method for increasing the ratio of the signal from element  140  to that from elements  120  and  130  can be provided: for example, a gain stage can be added to element  140  so that its signal is significantly greater than the combined outputs of elements  120  and  130 , or a bypass switch could be added to eliminate the outputs of elements  120  and  130 . Alternatively, R 3 &#39;s value can be limited to values less than 100% for situations where reaching the upper lines of FIG. 4B is preferably avoided.  
         [0031]    Referring to FIG. 5 and FIG. 6, block diagrams are shown of an apparatus that automatically seeks a maximum of rejection in a stopband. In FIG. 5, Gain Control Circuit  140  has been modified to include a voltage controlled resistor, so that its gain can be adjusted by the automatic setting circuit  500 . A momentary contact switch (not shown) is pressed by the user, which activates the auto-set circuit. The first action of this circuit is to shift the three shown switches from their positions during normal operation to their positions as needed for the circuit to operate. Input Signal  110  is replaced by a Test Signal  504 . Control of Gain Control Circuit  140  is transferred from manual control  502  to the automatic circuit  500 . The output of summing amp  150  is removed from output device  190 , and is redirected to automatic circuit  500 . Arrow  506  shows an extra output signal from gain control circuit  140 , directed to automatic circuit  500 , which may be required for some implementations of an automatic circuit (though not used for the example of FIG. 6).  
         [0032]    FIG.,  6  shows one possible implementation of an automatic setting device, which may be used with the circuit of FIG. 2. For this example, we choose the test signal  504  to be a sine wave that can be incremented from 10 Hz to 20 kHz by {fraction (1/12)} octave steps (={fraction ( 1 / 2 )} step in musical terminology). Control signal T is initialized for a starting frequency of 10 Hz. Control signal R is initialized so that gain control circuit  140 &#39;s output X is 0 (R=0%). Output Y of summing amp  150  is stored to Register  1  ( 621 ), and then copied from there to Register  2  ( 622 ). The current value for control signal T ( 614 ) is stored in Register T ( 612 ).  
         [0033]    Control block T ( 610 ) then performs the following loop, which sets test signal  504  to the nadir of the stopband:  
         [0034]    1—Increment T to yield an increase in the test signal frequency of {fraction (1/12)} octave  
         [0035]    2—Store the value of Y to Register  1  ( 621 )  
         [0036]    3—If Register  1 &lt;Register  2 , then:  
         [0037]    a) copy Register  1  to Register  2 , and  
         [0038]    b) store current setting of T in Register T.  
         [0039]    4—If T is set for&gt;20 kHz, end loop; otherwise, repeat loop.  
         [0040]    The value in Register T is now that value which will set test signal  504  to the nadir of the stopband. The current value  614  for control signal T is now set to be the value in Register T. After this, the value of output Y is stored in Register  1 . Control Block R ( 640 ) then performs the following loop:  
         [0041]    1—Copy Register  1  ( 621 ) to Register  2   
         [0042]    2—Increment R by one unit to increase output X of circuit  140   
         [0043]    3—Store the value of Y to Register  1  ( 621 )  
         [0044]    4—If Register  1 &lt;Register  2 , restart loop  
         [0045]    5—If Register  1 &gt;Register  2 , decrement R to decrease X, then revert to Normal operation.  
         [0046]    6—If Register  1 =Register  2 , revert to Normal operation.  
         [0047]    R has now been set to the value which produces the maximum rejection at the stopband frequency set by T. The switches of circuit  500  in FIG. 5 are reset to their “Normal” positions. Manual control  502  can now be used by the operator to make adjustments as deemed necessary. This makes the job of finding the setting of maximum stopband rejection easier, particularly for an inexperienced operator.  
         [0048]    [0048]FIG. 7 shows an example of a unique filter response created by the method. The results are from the circuit of FIG. 2, but with the circuit in area  120  removed. The remaining filter circuit  130  is a common 2 nd  order low pass filter, whose unaltered response is seen on FIG. 7 as the line where R=0%. As R is increased, a region is created that is much steeper than the original  2 nd order filter, though at the expense of a newly added shelf in the high frequency region of this example. Where R=0.2%, the graph shows an extra reduction of about 24 dB for a frequency near 900 Hz. The high frequency shelf that appears remains below −53 dB from the source (which was+10 dB), still more than enough for many applications (most audio applications, for example). This approximates a ‘variable depth’ low pass filter. Under some circumstances, this would be an excellent filter shape. In general, the new shapes created by the present invention are as diverse as the processors ( 120 ,  130 - 5 ,etc.) with which it is used, but once the signal source and processors are known, the results of the present method can be predicted.  
         [0049]    Although embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. The embodiments of the present invention allow the phenomenon of phase cancellation to provide a variety of benefits when dealing with wave phenomena. For situations where a continuous control is desired/needed to have a position that affords a pure pre-processor signal at one end of travel, an arrangement is easily made to do so.