Abstract:
The present invention relates to a digital filter capable of computing a tap without output delay due to the filter operation in a symbol time, and a digital broadcasting receiver having the same. Particularly, filter output is obtained within a clock period, one multiplier and one adder are used to perform coefficient update for a plurality of taps, and the multiplier performs the output operation for each tap, whereby the number of multipliers and adders is reduced inversely proportional to the number of taps being operated for one clock period. Thus, the digital filter of the present invention can be very advantageously used for resolving the filter size problem in multi-tap filters.

Description:
This application claims the benefit of the Korean Patent Application No. 10-2004-0001681, filed on Jan. 9, 2004, which is hereby incorporated by reference as if fully set forth herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a digital filter capable of computing a tap without output delay due to the filter operation in a symbol time, and a digital broadcasting receiver having the same. 
   2. Background of the Related Art 
   A digital filter based on the LMS (Least Mean Square) adaptive algorithm is a filter capable of updating or adapting coefficients on an ongoing basis. The LMS adaptive digital filter is usually used for an equalizer or a noise eliminator housed in a digital broadcasting receiver, in order to compensate the distortions generated by a channel or a system itself. 
   The LMS adaptive digital filter includes a multiplier and an adder for the coefficient adaptation for each tap, and an additional multiplier for the output (filtering). 
     FIG. 1  illustrates the general structure of an LMS adaptive filter, more particularly, a 2-tap LMS adaptive filter. As shown in  FIG. 1 , the LMS adaptive filter includes four serial delays D 11 , D 12 , D 21  and D 22 , D 11  and D 12  for delaying an input signal x 0  in sequence and D 21  and D 22  for delaying a delayed input signal xd 0 , and a first and a second coefficient updating unit  10 ,  20 . 
   Each of the delays D 11 , D 12 , D 21  and D 22  operates according to a clock (clk) signal, and a first and a second tap, i.e., the first and the second coefficient updating unit  10 ,  20  have the identical structure. 
   Here, the input signal x 0  is outputted to the delay D 11  and at the same time to a multiplier  14  of the first coefficient updating unit  10 . The delay D 11  delays the input signal x 0  by one clock, and outputs the one-clock-cycle delayed signal to the delay D 12  and a multiplier  24  of the second coefficient updating unit  20  at the same time. 
   The delayed input signal xd 0  is simultaneously outputted to the delay D 21  and a multiplier  11  of the first coefficient updating unit  10 . The delay D 21  delays the delayed input signal xd 0  by one clock, and outputs the delayed signal simultaneously to the delay D 22  and a multiplier  21  of the second coefficient updating unit  20 . The delay D 12  delays the delayed signal x 1 , which was delayed by the delay D 11 , by one clock before outputting the signal. The delays D 22  delays the delayed signal xd 1 , which was delayed by the delay D 21 , by one clock before outputting the signal. 
   The multiplier  11  of the first coefficient updating unit  10  multiplies the delayed input signal xd 0  by a feedback error signal e, and outputs the result to an adder  12 . The adder  12  adds an old coefficient c 0  to the output from the multiplier  11  for the coefficient update, and outputs the updated coefficient to a delay  13 . The delay  13  delays the updated coefficient in the adder  13  by one clock, and outputs the delayed coefficient to the adder  13  and the multiplier  14 . The multiplier  14  then multiplies the output from the delay  13  by the input signal x 0  to obtain a first output y 0 . 
   The multiplier  21  of the second coefficient updating unit  20  multiplies the delayed input signal xd 1  by a feedback error signal e, and outputs the result to an adder  22 . The adder  22  adds an old coefficient c 0  to the output from the multiplier  21  for the coefficient update, and outputs the updated coefficient to a delay  23 . The delay  23  delays the updated coefficient in the adder  23  by one clock, and outputs the delayed coefficient to the adder  23  and the multiplier  24 . The multiplier  24  then multiplies the output from the delay  23  by the input signal x 1  to obtain a second output y 1 . That is, the outputs y 0  and y 1  are obtained by multiplying the input signals (x 0 , x 1 ) by the coefficients (c 0 , c 1 ) for each tap, respectively. 
   Recently long-term fading channels are often found because of temporally distant media like ground wave digital TVs. Thus the fading problem should be resolved to facilitate the broadcast receiving operation. However, to compensate the long-term fading by the temporally distant media a multi-tap equalizer or a noise eliminator. 
   Unfortunately though, the size of the multi-tap filter is so big that the implementation of the filter becomes difficult. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a digital filter and digital broadcasting receiver having the same that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide a digital filter and digital broadcasting receiver having the same, in which the number of operators for a plurality of taps is reduced and filter output is obtained within a clock period, whereby the filter size problem found in a related art LMS adaptive filter can be resolved. 
   Particularly, the present invention is characterized of obtaining filter output within a clock period, using one multiplier and one adder to perform coefficient update for a plurality of taps, wherein the multiplier performs the output operation for each tap, and thereby reducing the number of multipliers and adders inversely proportional to the number of taps being operated for one clock period. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a digital filter, including: a first data input unit for sequentially delaying input data for a clock period, and sequentially and selectively outputting one of the input data and delay values for the calculation of a filter output value; a second data input unit for sequentially delaying a delayed input data for a clock period, and sequentially and selectively outputting one of the input data and delay values for coefficient update; a multiplier for multiplying the data value that is sequentially and selectively outputted from the second data input unit by an error value; a coefficient update unit for sequentially updating a coefficient by adding an output from the multiplier to an old feedback coefficient, storing updated coefficients in each delay that operates synchronously with a clock signal with a phase difference of 1/N period (N is the number of filter taps), and feedbacking a sequentially selected updated coefficient as the old coefficient; and an output unit for multiplying updated coefficients sequentially and selectively outputted from the coefficient update unit by data sequentially and selectively outputted from the first data input unit, storing in each delay which operates synchronously with a clock signal with a phase difference of 1/N period, adding all outputs from the delays for a predetermined summation period, and outputting the summed value. 
   In the exemplary embodiment, the first data input unit includes: N-number of serial delays for synchronizing the input data with the clk and sequentially delaying the data; a selection part for sequentially selecting, according to a selection signal sel, one of the input data x 0  and delay values that are delayed respectively by the N-number of delays, and outputting the selected value to the output unit. 
   In the exemplary embodiment, the second data input unit includes: N-number of serial delays for synchronizing the delayed input data xd 0  with the clk and sequentially delaying the data; a selection part for sequentially selecting, according to a selection signal sel, one of the delayed input data x 0  and delay values that are delayed respectively by the N-number of delays, and outputting the selected value to the output unit. 
   In the exemplary embodiment, the coefficient update unit includes: an adder for adding an output from the multiplier to an old feedback coefficient and thereby, updating the old coefficient; N-number of parallel delays, each operating synchronously with a clock signal clk 1 ˜clkN−1, clk with a phase difference of 1/N period and storing the updated coefficient; and a selection part for sequentially selecting, according to a selection signal sel, one of outputs from the N-number of parallel delays, and simultaneously feedbacking the selected value to the adder and outputting the value to the output unit. 
   In the exemplary embodiment, the output unit includes: a multiplier for multiplying a data value that is sequentially and selectively outputted form the first data input unit by an updated coefficient that is sequentially selectively outputted from the coefficient update unit; N-number of parallel delays, each operating synchronously with a clock signal clk 1 ˜clkN−1 with a phase difference of 1/N period, and storing and outputting the multiplication result of the multiplier; and an adder for adding, for the predetermined summation period, outputs from the (N−1)-number of delays and a N-th output value outputted without delay and thereby, obtaining a final output. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIG. 1  is a schematic block diagram of a related art 2-tap digital filter; 
       FIG. 2  is a schematic block diagram of an N-tap digital filter according to the present invention; 
       FIG. 3  illustrates operation timing diagrams of a procedure for updating coefficients in the N-tap digital filter of  FIG. 2 ; and 
       FIG. 4  illustrates operation timing diagrams of a filter output procedure in the N-tap digital filter of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
   The present invention provides an improved filter architecture computing a plurality of taps within a symbol time by sharing adders and multipliers so that the size of a multi-tap filter can be reduced. The present invention filter is very useful for an LMS adaptive digital filter used in a time domain equalizer or a noise eliminator in a VSB digital television receiver. 
     FIG. 2  is a detailed block diagram of an LMS adaptive digital filter, more particularly an N-tap LMS adaptive digital filter (N is an arbitrary number), according to the present invention. 
   As shown in  FIG. 2 , the digital filter includes a first data input unit  100  for delaying input data x 0  sequentially, and sequentially and selectively outputting one of the input data x 0  and delay values for the calculation of a filter output value; and a second data input unit  200  for delaying a delayed input data xd 0  sequentially, and sequentially and selectively outputting one of the input data xd 0  and delay values for use with coefficient updating. In addition, the digital filter includes a multiplier  300  for multiplying the data value that is sequentially and selectively outputted from the second data input unit  200  by an error value; a coefficient update unit  400  for updating a coefficient by adding an output from the multiplier  300  to an old feedback coefficient, storing updated coefficients in each delay that operates synchronously with a clock signal with a phase difference of 1/N period, and feedbacking a selected updated coefficient as the old coefficient; and an output unit  500  for multiplying updated coefficients sequentially and selectively outputted from the coefficient update unit  400  by data sequentially and selectively outputted from the first data input unit  100 , storing in each delay which operates synchronously with a clock signal with a phase difference of 1/N period, adding all outputs from the delays, and outputting the summed value. 
   The first data input unit  100  includes N number of serial delays  111 - 11 N operating synchronously with a clock signal clk and sequentially delaying an input data x 0 ; and a selection part  120  for sequentially selecting, according to a selection signal sel, one of the input data x 0  and delay values delayed respectively by the delays  111 ˜ 11 N, and outputting the selected value to the output unit  500 . The clk is a symbol clock in each clock period. 
   The second data input unit  200  includes N number of serial delays  211 ˜ 21 N, each operating synchronously with a clock signal clk and sequentially delaying a delayed input data xd 0 ; and a selection part  220  for sequentially selecting, according to the selection signal sel, one of the delayed input data xd 0  and delay values delayed respectively by the delays  211 ˜ 21 N, and outputting the selected value to the multiplier  300 . The delayed input signal xd 0  is generated by delaying the input signal x 0  for at least one symbol clock. 
   The coefficient update unit  400  includes an adder  410  for adding an output from the multiplier  300  to an old feedback coefficient and thereby, updating the old coefficient; N-number of parallel delays  421 ˜ 42 N, each operating synchronously with a clock signal clk 1 ˜clkN−1, clk with a phase difference of 1/N period and storing the updated coefficient; and a selection part  430  for sequentially selecting, according to a selection signal sel, one of outputs from the N-number of parallel delays  421 ˜ 42 N, and simultaneously feedbacking the selected value to the adder  410  and outputting the value to the output unit  500 . It is assumed that each of the N-number of delays  421 ˜ 42 N receives only an input signal at a rising edge of a clock and outputs the signal through an output terminal, and tends to maintain its original state. Examples of the clocks inputted to the clock terminals of the N-number of delays  421 ˜ 42 N include clk 1 ˜clk (N−1), and clk. Here, a clock period represents a symbol time, and clk 1  represents a delayed clk by 1/N period. In like manner, clk 3  represents delayed clk by 3/N period, and clk(N−1) is a delayed clk by (N−1)/N period. 
   The output unit  500  includes a multiplier  510  for multiplying a data value that is sequentially and selectively outputted form the first data input unit  100  by an updated coefficient that is sequentially selectively outputted from the coefficient update unit  400 ; and N-number of parallel delays  521 ˜ 52 N−1, each operating synchronously with a clock signal clk 1 ˜clkN−1 with a phase difference of 1/N period, and storing the multiplication result of the multiplier  510 . Again it is assumed that each of the N-number of delays  521 ˜ 52 N−1 receives only an input signal at a rising edge of a clock and outputs the signal through an output terminal, and tends to maintain its original state. Examples of the clocks inputted to the clock terminals of the N-number of delays  521 ˜ 52 N−1 include clk 1 ˜clk (N−1). As aforementioned, a clock period represents a symbol time, and clk 1  represents a delayed clk by 1/N period. 
   Also, the identical selection signal sel is inputted to the first and the second data input unit  100 ,  200 , each of the selection parts  120 ,  220 , and the selection part  430  of the coefficient update unit  400 , respectively. The selection signal sel is generated sequentially by dividing a symbol time by the number of taps (N). That is to say, N-number of selection signals sel are produced within a symbol time. 
     FIG. 3  illustrate operation timing diagrams depicting the relation between clocks clk, clk 1 ˜clk(N−1), the selection signal sel, and a procedure for updating coefficients in the LMS adaptive filter. As shown in the drawings, a new coefficient (c) is calculated sequentially according to the selection signal sel, and the new coefficient is synchronized to a clock signal clk, clk, . . . , and clk(N−1) with a phase difference by 1/N period, and stored in each of the delays  421 ˜ 42 N as new c 0 , new c 1 , . . . , and newc(N−1). 
     FIG. 4  illustrate operation timing diagrams depicting the relation between clocks clk, clk 1 ˜clk(N−1), the selection signal sel, and output signals. 
   According to the digital filter of the present invention, a total of N+1 signals (from the input signal x 0  to xN) are selectively outputted, according to the selection signal sel, from the selection part  120  of the first data input unit  100 , and a total of N+1 signals (from the delayed input signal xd 0  for coefficient update to xdN) are selectively outputted, according to the selection signal sel, from the selection part  220  of the second data input unit  200 . At this time, each of the signals is transferred to the selection parts  120 ,  220 , respectively, through the delays  111 ˜ 11 N,  211 ˜ 21 N operating synchronously with the clock signal clk. In other words, the period of a clock signal clk is equal to a symbol time. 
   The signals which are selectively outputted from the first data input unit  100  are outputted to the multiplier  510  of the output unit  500 . Also, the signals which are selectively outputted from the second data input unit  200  are outputted to the multiplier  300 . 
   In the multiplier  300  the output data from the second data input unit  200  are multiplied by an error value (e), and the multiplication result is outputted to the adder  410  of the coefficient update unit  400 . As such, even though the related art filter required multipliers as many as filter taps, the present invention filter shares a single multiplier  300  regardless of the number of taps available. Here, the error value (e) remains the same for a symbol time. 
   The adder  410  of the coefficient update unit  400  adds the output of the multiplier  300  to an old feedback coefficient for coefficient update, and outputs the new coefficient to each of the parallel delays  421 ˜ 42 N. The updated coefficients are stored in the delays  421 ˜ 42 N that are activated by corresponding clock signals (clk, clk 1 ˜clkN−1). 
   In other words, the delays  421 ˜ 42 N are designed to operate synchronously with clocks (clk 1 ˜clkN−1, clk) that are delayed from clk by 1/N period in sequence with respect to N-number of coefficients (c 0 ˜c(N−1)). Updated coefficients through the adder  410  are activated by corresponding clocks and stored in the delays, respectively. Here, clk 1  represents a delayed clk by 1/N period, clk 2  represents delayed clk by 2/N period, and clk(N−1) is a delayed clk by (N−1)/N period. The rest of the clocks are delayed likewise. 
   For example, suppose the selection part  220  of the second data input unit  200  outputs a delayed input signal xd 0  that was delayed by the same selection signal (i.e., sel=0) as in  FIG. 3(   d ). This delayed input signal xd 0  is then multiplied, at the multiplier  300 , by the error value (e) and outputted to the adder  410  of the coefficient update unit  400 . The adder  410  of the coefficient update unit  400  adds an old feedback coefficient to the output value (=e*xd 0 ) of the multiplier  300 , thereby updating the coefficient. At this time, the old coefficient which is feedbacked to the adder  410  becomes co by the selection signal (i.e., sel=0) inputted to the selection part  430  of the coefficient update unit  400 . Thus, the new coefficient outputted from the adder  410  becomes co + e*xd 0  as in  FIG. 3(   e ). 
   The updated coefficient co + e*xd 0  is outputted simultaneously to those N-number of parallel delays  421 ˜ 42 N. However, the N-number of delays  421 ˜ 42 N are designed to be activated at a rising edge only. In addition, different clock signals are inputted to the delays  421 ˜ 42 N, respectively. This means that the new coefficient is stored only in the delays which are activated when the updated coefficient co + e*xd 0  is outputted. 
   Referring back to  FIG. 2 , clk 1  is inputted to the first delay  421 , where the clk 1  as shown in  FIG. 3(   b ) is a delayed clk by 1/N period. 
   As such, when the updated coefficient co + e*xd 0  is outputted only the first delay  421  is activated at a rising edge of the clk 1 , and stores the coefficient co + e*xd 0  therein and at the same time outputs the coefficient co + e*xd 0  to the multiplier  510  of the output unit  500  through the selection part  430 . Then the selection signal sel inputted to each selection part  120 ,  220 ,  430  is changed to sel=0 as shown in  FIG. 3(   d ). Hence, the selection part  430  selects a second old coefficient c 1  and feedbacks the c 1  to the adder  410 . As illustrated in  FIG. 3(   f ) the first delay  421  maintains the input coefficient co + e*xd 0  until a next rising edge of the clk 1 . In the course of this operation, the rest of the delays  422 ˜ 42 N remain inactive, none of them receiving the new input co + e*xd 0 . 
   The multiplier  510  of the output unit  500  multiplies the updated coefficient that is selectively outputted from the selection part  430  of the coefficient update unit  400  by the output data from the first data input unit  100 , and outputs the multiplication result to the N-1 parallel delays  521 ˜ 52 N−1. More specifically speaking, the multiplier  510  multiplies the updated coefficient co + e*xd 0  outputted from the coefficient update unit  400  by the input signal x 0  that is selectively outputted from the selection part  120  of the first data input unit  100  according to the selection signal (i.e., sel=0), and outputs the multiplication result to each of the delays  521 ˜ 52 N-1. As described before, the delays  521 ˜ 52 N−1 are designed to be activated at a rising edge only. In addition, different clock signals are inputted to the delays  521 ˜ 52 N−1, respectively. This means that the multiplication result is stored only in the delays which are activated when the multiplication result is outputted from the multiplier  510 . 
   Here, the clock inputted to the first delay  521  among the delays  521 ˜ 52 N−1 is clk 1  as shown in  FIG. 4(B) . Therefore, the multiplication result y=co*x 0  from the multiplier  510  is inputted and stored only in the first delay  521  that is activated at a rising edge of the clk 1  as shown in  FIG. 4(   f ), and at the same time outputted through the output terminal. As depicted in  FIG. 4(   f ) the first delay  521  maintains the input value y˜co*x 0  until a next rising edge of the clk 1 . In the course of this operation, the rest of the delays  522 ˜ 52 N−1 remain inactive, none of them receiving the multiplication result y=co*x 0  from the multiplier  510 . 
   Accordingly, as illustrated in  FIG. 4(   e ), the filter output is sequentially generated through the multiplication (performed by the multiplier  510 ) of the input signal from the first data input unit  100  and the coefficient from the coefficient update unit  400 . A first output y 0  is stored in the delay  521  operating synchronously with the clk 1 , the delayed clk by 1/N period, and outputted at the same time. In like manner, a second output y 1  is stored in the delay  522  operating synchronously with the clk 2 , the delayed clk by 2/N period, and outputted at the same time; and an (N−1)th output y(N−2) is stored in the delay  52 N−1 operating synchronously with the clk(N−1), the delayed clk by (N−1)/N period, and outputted at the same time. The rest of outputs are stored likewise, except for the N-th output y(N−1) whose delayless value is outputted from the output unit  500  as it is. 
   Each output y 0 ˜yN−1 for an N tap, therefore, can be calculated within the clk, and if the summation of all filter outputs is outputted prior to the next clk it is possible to get the total output of the filter within the clk when the input signal is received. In other words, as shown in  FIG. 4(   h ), in the summation period prior to the clk, N-tap filter outputs are produced at the same time. If the outputs of every tap are summed up for the summation period, it becomes possible to get the total output of the filter within the clk when the input signal is received. As for the summation an adder (not shown) can be utilized to add the outputs y 0 ˜y(N−2) of the N−1 delays  521 ˜ 52 N−1 and the delayless N-th output y(N−1). 
   In conclusion, according to the present invention digital filter and digital broadcasting receiver having the same, a single multiplier and a single adder are shared regardless of the number of filter taps to perform coefficient update for each tap, whereby the number of operations is much reduced and the filter output can be obtained within the clk. Thus, the digital filter of the present invention can be very advantageously used for multi-tap filters. 
   The forgoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.