Abstract:
The separation of a adjacent band pass filters is improved, without changing the filters, by inverting the output signals from alternate filters and not inverting the remaining output signals. All the output signals are then summed. The result is a deeper notch in the frequency response of adjacent filters.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application includes disclosure contained in application Ser. No. 09/466,313, filed Dec. 17, 1999, entitled “Band Pass Filter from Two Notch Filters”, assigned to the assignee of this invention. The entire contents of the earlier application is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates to band pass filters and, in particular, to improving the separation of parallel band pass filters without changing the filters themselves. 
     Frequently, a plurality of band pass filters are coupled in parallel and the outputs are summed at an active or passive summation node. As more fully described below, this has the effect of broadening the response curve of each filter in a pair of adjoining filters. 
     Parallel band pass filters are used in many and diverse applications, such as equalizers, hearing aids, telephones, and other audio and radio frequency applications. To consider but one example, a single telephone may include several sets of filters, e.g. for detecting multiple tone dialing signals, for noise reduction, and for echo cancellation. Devices known as complementary comb filters have been used to eliminate echoes by having the signal to a speaker filtered through the pass bands of a first comb filter, thereby falling within the stop bands of a second, complementary comb filter coupled to a microphone. 
     Comb filters are used primarily because the “Q” of most filters is relatively low, less than twenty and typically about ten. One definition of “Q” is the ratio of the bandwidth at −3 dB to the center frequency. The center frequencies in a comb filter are widely spaced, relative to the bandwidth, and band reject or stop band filters are used in between pairs of band pass filters. The “Q” of a filter is not a very good description of the frequency response of a filter because Q does not describe the shape of the response curve of the filter, particularly the “skirts” of the curve. It is desired that the skirts be as close to vertical as possible, although a vertical skirt can only be approached as a limit. 
     In view of the foregoing, it is therefore an object of the invention to improve the separation of parallel band pass filters without changing the filters themselves. 
     Another object of the invention is to reduce the bandwidth of parallel filters without affecting the nominal “Q” of the filter. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in this invention in which the separation of a adjacent band pass filters is improved, without changing the filters, by inverting the output signals from alternate filters and not inverting the remaining output signals. All the output signals are then summed. The result is a deeper notch in the frequency response of adjacent filters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of parallel band pass filters constructed in accordance with the prior art; 
     FIG. 2 is a block diagram of parallel band pass filters arranged in accordance with the invention; 
     FIG. 3 is a chart of curves comparing the frequency responses of the circuits illustrated in FIGS. 1 and 2; 
     FIG. 4 is a schematic of a preferred embodiment of a band pass filter; 
     FIG. 5 is a partial block diagram of a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, band pass filters  11 ,  12 ,  13 ,  14 , and  15  have progressively higher center frequencies and are connected in parallel between input  10  and summation circuit  17 . The center frequencies of the filters are typically geometrically related, e.g. each center frequency is 1.7 times the next lower center frequency, and the “Q” of the filters is typically ten. Other apparatus can be included in such circuitry, depending upon application. For example, an equalizer also includes a variable gain stage coupled to the output of each filter. 
     FIG. 2 is a block diagram of an audio processing circuit constructed in accordance with the invention in which alternate filters are coupled through an inverting amplifier to the summation network. As illustrated in FIG. 2, the output of filter  12  is coupled through inverting amplifier  21  to summation circuit  17  and the output of filter  14  is coupled through inverting amplifier  22  to summation circuit  17 . Either filters  11 ,  13 , and  15  or filters  12  and  14  could have their outputs inverted. The effect of inverting alternate outputs is illustrated in FIG.  3 . 
     In FIG. 3, curve  31  represents the frequency response of filter  11 , curve  32  represents the frequency response of filter  12 , curve  33  represents the frequency response of filter  13 , curve  34  represents the frequency response of filter  14 , and curve  35  represents the frequency response of filter  15 . The frequency responses of the filters overlap, although the point at which the response curves intersect may be far down on the curve. The response curves intersect approximately half-way between the center frequencies of the filters. 
     The sum of the outputs of filters  11  and  12  follows curve  37  in the region between the center frequencies. Note that the nadir of curve  37  is distinctly higher than the point at which curves  31  and  32  intersect. The outputs of the remaining filters combine to produce similar curves. 
     When alternate outputs are inverted in accordance with the invention, the sum of the outputs of filters  11  and  12  follows curve  38  in the region between the center frequencies. The difference, represented by double ended arrow  39 , is approximately 8.5 dB in one embodiment of the invention. Not only is this a large additional attenuation between the bands but the skirts of the response curves are also steeper below approximately −10 dB. 
     FIG. 4 is a schematic of a band pass filter incorporating the invention disclosed and claimed in the application cross-referenced above. Specifically, a preferred band pass filter is made by subtracting the outputs from two notch filters. The particular notch filter chosen is not critical. The notch filter illustrated in FIG. 4 is known as a twin-T filter with positive feedback; see “ Electronic Filter Design Handbook ” by Williams and Taylor, Third Edition, McGraw-Hill, Inc., 1995, pages 6.38 and 6.39. This particular filter was chosen because of its simplicity, depth of notch, and because the gain can be adjusted easily, by changing the ratio of resistors R 7  and R 8  or R 13  and R 14 , to modify the frequency response of the resulting band pass filter. Band pass filter  40  includes two channels,  41  and  42 , each containing a notch filter and each connected to input  43 . The outputs of the channels are subtracted in amplifier  44 . 
     FIG. 5 is a partial schematic of a preferred embodiment of the invention including five filters such as filter  40  (FIG. 4) each having a different center frequency. In one embodiment of the invention, the center frequencies were 316 Hz, 534 Hz, 900 Hz, 1525 Hz, 2577 Hz, and the filters were one-third octave. 
     Input  50  is coupled to each of band pass filters  51 ,  52 ,  53 ,  54 , and  55 . The outputs of filters  51 ,  53 , and  55  are coupled to the non-inverting input of operational amplifier  59 , which preferably has unity gain. The outputs of filters  52  and  54  are coupled to the inverting input of amplifier  59 . This is a simpler circuit than that illustrated in FIG. 2 but achieves the same result; namely, subtracting alternate bands from the remaining bands. 
     In operation, band pass filters constructed in accordance with FIG. 4 can have the skirts of the response curve individually adjusted and, in particular made steeper than obtainable with filters of the prior art. Even so, the circuit of FIG. 5 increases the depth of the notch between filters by approximately 8.5 dB, a significant improvement over filters of the prior art. Despite the increased depth between pass bands, a signal passing through two such filters, as in a telephone for example, showed 3 dB less attenuation than identical filters in which alternate pass bands were not inverted. In other words, the output signal was twice as loud with the invention as without the invention. 
     The invention thus reduces the bandwidth of parallel filters without affecting the nominal “Q” of the filter and provides improved separation of parallel band pass filters without changing the filters themselves. Thus, the invention can be used with any band pass filter. 
     Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, the invention can be used at any frequency and can be implemented in analog or digital form; e.g. with impedance elements as shown or as a finite impulse response (FIR) filter or as an infinite impulse response (IIR) filter. The inverting amplifier could precede the filter in alternate channels but this is not preferred. Depending upon the application and the number of filters, one may not invert the output of every other filter. For example, if fifteen band pass filters were used, one might invert the outputs of only filters five, seven, nine, and eleven or invert the outputs of only filters one, three, five and nine.