Abstract:
A digital filter having a small amount of hardware is realized by constructing a cosine roll-off filter. In the filter the first and fifth multipliers and the second to fourth multipliers are respectively shared. Three selectors are provided corresponding to the multipliers, and coefficient groups ka1 to ka3, and kb1 and kb3 are inputted by the selectors to the corresponding multipliers by time sharing. The results of multiplications by the multipliers when the coefficient groups kb1 to kb3 are inputted, are added between corresponding multipliers by adders. A final output is derived from an output selector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a digital filter and an operation method in the digital filter, and more particularly to a cosine roll-off filter and an operation method in the filter. 
     2. Description of the Prior Art 
     Generally, a cosine roll-off filter is used to adjust a rectangular wave so as to bring its waveform into a position corresponding to a multiple of the number of taps to prevent interference due to reflection when transmitting it. 
     FIG. 3 shows a configuration of a conventional digital filter. In the figure, a general 5-tap FIR-type filter is shown. Specifically, multipliers M 1  to M 5  are provided corresponding to coefficients C 1  to C 5  inputted from five taps, and outputs of the multipliers M 1  to M 5  are added by adders A 1  to A 4  to obtain a final output. Input data to the filter is successively sent to subsequent stages by four flip-flops (hereinafter simply referred to as F/F)  31  to  34 . 
     Consider the use of the filter of FIG. 3 in a double sampling mode. For input data of D 1 , D 2 , D 3 , . . . , D 5 , a filter output is as shown below. That is, output Y 1  by the double sampling and output Y 2  obtained one half cycle later are as follows. 
      Output  Y   1 = D   1 ×( C   5 + C   4 )+ D   2 ×( C   3 + C   2 )+ D   3 × C   1   
     
       
         Output  Y   2 = D   1 × C   5 + D   2 ×( C   4 + C   3 )+ D   3 ×( C   2 + C   1 )  (1) 
       
     
     Furthermore, when the filter is used as a cosine roll-off filter, since C 5 =C 1  and C 4 =C 2  are satisfied because of symmetry, the above expression (1) is replaced by: 
     
       
         Output  Y   1 = D   1 ×( C   1 + C   2 )+ D   2 ×( C   3 + C   2 )+ D   3 × C   1   
       
     
     
       
         Output  Y   2 = D   1 × C   1 + D   2 ×( C   3 + C   2 )+ D   3 ×( C   1 + C   2 )  (2) 
       
     
     It is to be noted that the coefficients C 1 , (C 1 +C 2 ), and (C 2 +C 3 ) are common in both the expressions (1) and (2). 
     Accordingly, by sharing overlapping hardware portions in FIG. 3 so that tap coefficients da 1 , da 2 , and da 3  are equal to C 1 +C 2 , C 2 +C 3 , and C 1 , respectively, a filter can be realized which is identical in function and about half in the number of components with respect to the FIR-type filter of the figure. The realized filter is shown in FIG.  4 . 
     In the figure, with multipliers M 1  to M 3  provided corresponding to coefficients da 1  to da 3  inputted from three taps, outputs of the multipliers M 1  to M 3  are temporarily stored in F/Fs  41  to  44  and are added by adders A 1  to A 4 . A final output is obtained by switching an output selector  40 . 
     In FIG. 4, an operating speed and the number of components are significantly reduced by integrating tap coefficients between oversamplings, taking advantage of the fact that input data is unchanged for the duration of oversampling in the double sampling mode. 
     Furthermore, with the filter, the speed of the multipliers commonly considered to have the lowest operating speed may be half that of those in FIG. 3, contributing to speedup. 
     Next, consider quadruple sampling in the filter of FIG.  5 . In FIG. 5, with multipliers M 1  to M 9  provided corresponding to coefficients C 1  to C 9  inputted from nine taps, outputs of the multipliers M 1  to M 9  are added by adders A 1  to A 8  to obtain a final output. Input data to the filter is successively sent to subsequent stages by four flip-flops (F/F)  51   5 o  58 . 
     In this case, as in the expressions (1) and (2), for input data of Dl, D 2 , D 3 , and so forth, filter output is as shown below. Specifically, output Y 1  by the quadruple sampling, output Y 2  obtained a quarter of the cycle later, output Y 3  obtained a quarter of that cycle later, and output Y 4  obtained a quarter of that cycle later satisfy the following expression. 
     
       
           Y   1 = D   1 ×( C   9 + C   8 + C   7 + C   6 )+ D   2 ×( C   5 + C   4 + C   3 + C   2 )+ D   3 × C   1   
       
     
     
       
           Y   2 = D   1 ×( C   9 + C   8 + C   7 )+ D   2 ×( C   6 + C   5 + C   4 + C   3 )+ D   3 ×( C   2 + C   1 ) 
       
     
     
       
           Y   3 = D   1 ×( C   9 + C   8 )+ D   2 ×( C   7 + C   6 + C   5 + C   4 )+ D   3 ×( C   3 + C   2 + C   1 ) 
       
     
       Y   4 = D   1 × C   9 + D   2 ×( C   8 + C   7 + C   6 + C   5 )+ D   3 ×( C   4 + C   3 + C   2 + C   1 )  (3) 
     where if C 1 =C 9 , C 2 =C 8 , C 3 =C 7 , and C 4 =C 6 , 
     
       
           Y   1 = D   1 ×( C   1 + C   2 + C   3 + C   4 )+ D   2 ×( C   2 + C   3 + C   4 + C   5 )+ D   3 × C   1   
       
     
     
       
           Y   2 = D   1 ×( C   1 + C   2 + C   3 )+ D   2 ×( C   3 + C   4 + C   4 + C   5 )+ D   3 ×( C   1 + C   2 ) 
       
     
     
       
           Y   3 = D   1 ×( C   1 + C   2 )+ D   2 ×( C   3 + C   4 + C   4 + C   5 )+ D   3 ×( C   1 + C   2 + C   3 ) 
       
     
     
       
           Y   4 = D   1 × C   1 + D   2 ×( C   2 + C   3 + C   4 + C   5 )+ D   3 ×( C   1 + C   2 + C   3 + C   4 )  (4) 
       
     
     Accordingly, as in FIG. 4, an FIR-type filter of FIG. 6 can be obtained by sharing overlapping hardware portions in FIG.  5 . 
     Tap coefficients ka 1 , ka 2 , ka 3 , kb 1 , kb 2 , and kb 3  are equal to C 1 +C 2 +C 3 +C 4 , C 2 +C 3 +C 4 +C 5 , C 1 , C 1 +C 2 +C 3 , C 3 +C 4 +C 4 +C 5 , and C 1 +C 2 , respectively. 
     In the figure, with multipliers M 11  to M 13  provided corresponding to coefficients ka 1  to ka 3  inputted from three taps, outputs of the multipliers M 11  to M 13  are temporarily stored in F/Fs  611  to  614  and are added by adders A 11  to A 14 . 
     A counterpart of the same configuration described above is provided (portion indicated by dashed lines in the figure). Specifically, with multipliers M 21  to M 23  provided corresponding to coefficients kb 1  to kb 3  inputted from three taps, outputs of the multipliers M 21  to M 23  are temporarily stored in F/Fs  621  to  624  and are added by adders A 21  to A 24 . A final output is obtained by switching an output selector  60 . 
     In the example of FIG. 6, in 2N-times (N is a positive integer) sampling operations such as quadruple or octuple sampling, a filter can be realized which is half in the number of components and ½N in multiplier operating speed with respect to normal FIR-type filters. 
     As described above, the prior art is described in, e.g., Japanese Published Unexamined Patent Application No. Hei 5-55875. Such a digital filter is very efficiently configured. However, to realize, e.g., a 113-tap octuple-sampling filter would require 60 multipliers and 112 adders. An example of configuration in this case is shown in FIG.  7 . In the figure, a number of multipliers, adders, and F/Fs (a block indicated by a rectangle in FIG. 7) are provided corresponding to incoming coefficients fa 1  to faF and fb 1  to fbF, and further, a counterpart of the same configuration is provided. Therefore, there is a drawback in that a huge amount of hardware is required. Concrete expressions of the coefficients fa 1  to faF and fb 1  to fbF are omitted. 
     There is also a problem that the operating speed of a filter depends on the switching speed of a selector at the last output stage. Specifically, the operating speed of an entire filter is determined by the operating speed of the selectors  40 ,  60 , and the like of the last output stage, making it difficult to increase an operating speed. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above-described drawbacks of the prior art, and its object is to offer a digital filter that has a small amount of hardware and a high operating speed, and an operation method in the digital filter. 
     A digital filter according to the present invention is a digital filter including multiplying means for multiplying input data and predetermined coefficients and adding means for adding the multiplication results, comprising: N (N is a natural number) multipliers in which, of first to (2N+1)-th multiplying means, M-th (M=1 to N) multiplying means and M-th (M=N+2 to 2N+1) multiplying means are shared, and to which the input data is inputted as one of inputs; N selectors, provided corresponding to the N multipliers, for inputting first and second coefficient groups to the corresponding multipliers by time-sharing; and adding means for adding, between corresponding multipliers, the results of multiplications by the N multipliers when the first coefficient group is inputted, and the results of multiplications by the N multipliers when the second coefficient group is inputted. The adding means comprises: a first adder group for cumulatively adding the results of multiplications by the N multipliers when the first coefficient group is inputted; a second adder group for cumulatively adding the results of multiplications by the N multipliers when the second coefficient group is inputted; and output selectors for outputting the results of additions of the first and second adder groups by time-sharing. The input data changes in its contents at a first cycle and inputs of the N selectors are switched at a second cycle, which is double the first cycle. As another option, first and second holding circuit groups may be further provided corresponding to the first and second coefficient groups, respectively, such that when the second coefficient group is inputted to the N multipliers, the results of additions by the first adder group are held in the first holding circuit group, and when the first coefficient group is inputted to the N multipliers, the results of additions by the second adder group are held in the second holding circuit group. 
     An operation method in a digital filter according to the present invention is an operation method in a digital filter including multiplying means for multiplying input data and predetermined coefficients and adding means for adding the multiplication results, comprising: a multiplying step for performing multiplications by N (N is a natural number) multipliers in which, of first to (2N+1)-th multiplying means, M-th (M=1 to N) multiplying means and M-th (M=N+2 to 2N+1) multiplying means are shared, and to which the input data is inputted as one of inputs; a step for inputting, by N selectors provided corresponding to the N multipliers, first and second coefficient groups to the corresponding multipliers by time-sharing; and an adding step for adding, between corresponding multipliers, the results of multiplications by the N multipliers when the first coefficient group is inputted, and the results of multiplications by the N multipliers when the second coefficient group is inputted. The adding step outputs, by time-sharing, a first addition result of cumulatively adding the results of multiplications by the N multipliers when the first coefficient group is inputted, and a second addition result of cumulatively adding the results of multiplications by the N multipliers when the second coefficient group is inputted. The input data changes in its contents at a first cycle and inputs of the N selectors are switched at a second cycle, which is double the first cycle. As another option, a holding step may be further included which uses first and second holding circuit groups provided corresponding to the first and second coefficient groups, respectively such that when the second coefficient group is inputted to the N multipliers, the results of additions by the first adder group are held in the first holding circuit group, and when the first coefficient group is inputted to the N multipliers, the results of additions by the second adder group are held in the second holding circuit group. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a block diagram showing the configuration of a digital filter according to one embodiment of the present invention; 
     FIG. 2A is a time chart showing the operation of a conventional digital filter, and 
     FIG. 2B is a time chart showing the operation of a digital filter of the present invention; 
     FIG. 3 is a block diagram showing the configuration of a conventional digital filter; 
     FIG. 4 is a block diagram showing an implementation of a filter that is identical in function and about half in the number of components with respect to the filter of FIG. 3; 
     FIG. 5 is a block diagram showing the configuration of another conventional digital filter; 
     FIG. 6 is a block diagram showing an implementation of a filter obtained by sharing overlapping hardware portions in the filter of FIG. 5; and 
     FIG. 7 is a block diagram showing a configuration of a 113-tap filter. 
    
    
     DETAILED DESCRIPTION 
     Next, an embodiment of the present invention is described with reference to the accompanying drawings. In the description below, the same reference numbers are used throughout the different figures to designate the same or similar components. 
     FIG. 1 is a block diagram showing one embodiment of a digital filter according to the present invention, and shows a configuration of a quadruple-sampling 9-tap filter. In the figure, a signal inputted from an input terminal is inputted to multipliers  1 ,  2 , and  3 . The multipliers  1 ,  2 , and  3  multiply tap coefficients ka 1 , ka 2 , and ka 3  selected by the selectors  11 ,  12 , and  13 , respectively, and the above-described input signal. 
     An output signal of the multiplier  1  is inputted to F/Fs  24  and  34 , and an adder  6 . An output signal of the multiplier  2  is inputted to adders  4  and  7 . An output signal of the multiplier  3  is inputted to an adder  5  and F/Fs  27  and  37 . 
     At this time, the selectors  11  to  17  are set to select the position a (the selectors  11  to  13  use ka  1  to ka 3 , and the selectors  14  to  17  use F/Fs  24  to  27 ). 
     Inputs of the adder  4  are the output of the multiplier  2  and the output of F/F  24 ; inputs of the adder  5  are the output of the multiplier  3  and the output of F/F  25 ; inputs of the adder  6  are the output of the multiplier  1  and the output of F/F  26 ; and inputs of the adder  7  are the output of the multiplier  2  and the output of F/F  27 ; 
     Next, if the selectors are set to select the position b, inputs of the multiplier  1  are the input signal and a tap coefficient kb 1 ; inputs of the multiplier  2  are the input signal and a tap coefficient kb 2 ; and inputs of the multiplier  3  are the input signal and a tap coefficient kb 3 ; 
     Since the selectors select the position b, inputs of the adder  4  are the output of the multiplier  2  and the output of F/F  34 ; inputs of the adder  5  are the output of the multiplier  3  and the output of F/F  35 ; inputs of the adder  6  are the output of the multiplier  1  and the output of F/F  36 ; and inputs of the adder  7  are the output of the multiplier  2  and the output of F/F  37 . This is represented by the following expression. 
     
       
           Y   1 = D   3 × ka   3 + D   2 × ka   2 + D   1 × ka   1   
       
     
     
       
           Y   2 = D   3 × kb   3 + D   2 × kb   2 + D   1 × kb   1   
       
     
     
       
           Y   3 = D   1 × kb   3 + D   2 × kb   2 + D   3 × kb   1   
       
     
     
       
           Y   4 = D   1 × ka   3 + D   2 × ka   2 + D   3 × ka   1   (5) 
       
     
     Y 1  is the output of time slot  1 , Y 2  is the output of time slot  2 , Y 3  is the output of time slot  3 , and Y 4  is the output of time slot  4 . 
     Tap coefficients ka 1 , ka 2 , ka 3 , kb 1 , kb 2 , and kb 3 , are, like FIG. 6, equal to C 1 +C 2 +C 3 +C 4 , C 2 +C 3 +C 4 +C 5 , C 1 , C 1 +C 2 +C 3 , C 3 +C 4 +C 4 +C 5 , and C 1 +C 2 , respectively. 
     In other words, the same output as that of the filter of FIG. 6 is obtained. Thereby, a filter that is half in size with respect to the digital filter of FIG. 6 can be realized. Furthermore, an operating speed can also be increased because the operating speed of the last selector part operating at the highest speed may be no more than half that of the digital filter of FIG.  6 . 
     FIG. 2A is a time chart showing the operation of a conventional digital filter, and FIG. 2B is a time chart showing the operation of a digital filter of the present invention. Referring to FIG. 2A, in the conventional digital filter, the coefficients ka 1  and kb 1  are fixed since the coefficients are not changed by time-sharing. Accordingly, for each of input data D 1 , D 2 , and D 3 , the output of the output selector  10  changes sequentially like “1”, “2”, “3”, and “4”. 
     Referring to FIG. 2B, in the digital filter of this embodiment, the coefficients are changed by time-sharing so that the coefficients ka 1  and kb 1  are alternately inputted. For each of input data “D1”, “D2”, and “D3”, the output of the output selector  10  alternately repeats “1” and “2”. Accordingly, the operating speed of the output selector  10  of the last output stage may be half that of a conventional digital filter, so that the highest operating speed can be doubled in comparison with the conventional one. 
     Also for samplings other than quadruple and octuple samplings, with the same configuration, the filter can be reduced in size and increased in operating speed. 
     As has been described, the conventional filter shown in FIGS. 6 and 7 has the same configuration repeated therein, while the present invention processes tab coefficients by time-sharing and stores tab outputs, thereby reducing components. In other words, the present invention shares and controls overlapping hardware portions by time-sharing, thereby halving the number of components in comparison with the conventional filter. The operating speed of a selector of the last output stage may be half that of the conventional filter, so that the highest operating speed can be doubled with respect to the conventional one. 
     With such a configuration, the operating speed of multipliers can be reduced to less than half (depending on a time-sharing count) that of multipliers in normal FIR-type filters. Furthermore, the operating speed of the last selector required to operate at the highest speed can be halved with respect to filters employing conventional technologies, so that the highest operating speed can be increased. 
     An operation method as described below is performed in the above-described digital filter including multiplying means to multiply input data and predetermined coefficients, and adding means to add the multiplication results. Specifically, it comprises: a multiplying step for performing multiplications by N (N is a natural number) multipliers in which, of first to (2N+1)-th multiplying means, M-th (M=1 to N) multiplying means and M-th (M=N+2 to 2N+1) multiplying means are shared, and to which the input data is inputted as one of inputs; a step for inputting, by N selectors provided corresponding to the N multipliers, first and second coefficient groups to the corresponding multipliers by time-sharing; and an adding step for adding, between corresponding multipliers, the results of multiplications by the N multipliers when the first coefficient group is inputted, and the results of multiplications by the N multipliers when the second coefficient group is inputted. 
     The adding step outputs, by time-sharing, a first addition result of cumulatively adding the results of multiplications by the N multipliers when the first coefficient group is inputted, and a second addition result of cumulatively adding the results of multiplications by the N multipliers when the second coefficient group is inputted. 
     The input data changes in its contents at a first cycle and inputs of the N selectors are switched at a second cycle, which is double the first cycle. 
     A holding step may be further included which uses first and second holding circuit groups provided corresponding to the first and second coefficient groups, respectively such that when the second coefficient group is inputted to the N multipliers, the results of additions by the first adder group are held in the first holding circuit group, and when the first coefficient group is inputted to the N multipliers, the results of additions by the second adder group are held in the second holding circuit group. 
     By adopting the above-described operation method, tap coefficients are processed by time-sharing and tap outputs are stored, so that more components can be cut in comparison with conventional digital filters. The operating speed of a selector of the last output stage may be half that of conventional digital filters, so that the highest operating speed can be doubled in comparison with the conventional ones. 
     As has been described above, the present invention has an effect in that tap coefficients are processed by time-sharing and tap outputs are stored, whereby overlapping hardware portions can be shared and more components can be cut in comparison with conventional digital filters. Another effect is that the operating speed of a selector of the last output stage may be half that of conventional digital filters, so that the highest operating speed can be doubled in comparison with the conventional ones. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is, therefore, contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.