1. Field of the Invention
This invention relates to a second-order IIR type digital filter and a reference signal canceling device and method using it.
2. Description of the Related Art
In FM stereo broadcasting, an FM detected signal includes a pilot signal at a single frequency of 19 KHz which is a reference for stereo demodulation. In order to cancel such a pilot signal, the circuit as shown in
However, the above IIR filter provides changes in a central frequency and phase characteristic according to the accuracy in the coefficient of the filter. In order to remove the change, the number of bits of the coefficient must be increased. This gave raise to an increase in the circuit size and production cost.
An object of this invention is to provide a second-order IIR type digital filter with a simple arrangement and high accuracy.
Another object of this invention is to provide a reference signal canceling apparatus using such a digital filter.
In order to attain the above object, in accordance with the first aspect of the this invention, there is provided a second-order bandpass IIR type digital filter, wherein assuming that a sampling frequency is six times as large as a central frequency of a passing frequency band, a first-order input feedback coefficient b1 is set at −1+2−n and a second-order input feedback efficient b2 is set at 1−2−(n−1)(n: an odd number of 3 or larger).
In this configuration, the phases at the central frequency at the input and output of the digital filter can be made to agree to each other.
Since the coefficient is set as power of 2, it can be easily acquired with high accuracy.
In a preferred embodiment, a coefficient a0 of an zero-order output is set at 2−n (a0=2−n) and a coefficient a2 of a second-order output is set at −2−n (a2=−2−n). Therefore, the amplitudes at the central frequency at the input and output can be made to agree to each other.
In a preferred embodiment, the second-order output is subtracted from the zero-order output and a subtraction result is multiplied by 2−n. In this configuration, the digital filter can be simplified.
In accordance with the second aspect of this invention, in a reference signal canceling apparatus comprising a filter for extracting a reference signal contained in an FM detected signal and a subtracter for subtracting an output form the filter from the FM detected signal, the above digital filter according to the first aspect of this invention is used as the above filter.
In this configuration, the reference signal (pilot signal) can be removed easily and completely.
In accordance with the third aspect of this invention, a method of canceling the reference signal using the digital filter according to the first aspect is also provided.
The above and other objects and features of the invention will be more apparent from the following description taken in conjunction with the accompanying drawings.
Prior to explaining an embodiment of this invention, referring to
In
H(Z)=(a0+a1Z−1+a2Z−2)/(1+b1Z−1+b2Z−2) (1)
The amplitude characteristic M(ω) can be expressed by
M(ω)=(O/Q)0.5 (2)
The delay characteristic τ(ω) can be expressed by
τ(ω)=(P/O−R/Q)T (3)
where
Q=A2+C2
P=C(C+a2 Sin 2ωT)−A(A−a0+a2 Cos 2ω(T)
Q=B2+D2
R=D(D+b2 Sin 2ωT)+B(B−1+b2 Cos ωT)
T=sampling time (4)
where
A=a0+a1 Cos ωT+a2 Cos 2ωT
B=1+b1 Cos ωT+b2 Cos 2ωT
C=a1 Sin ωT+a2 Sin 2ωT
D=b1 Sin ωT+b2 Sin 2ωT (5)
In the general form of the second bandpass filter,
a1=0
a2=−a2 (6)
Setting the sampling frequency fs (=1/T) at six times as large as the central frequency of the bandpass filter,
ωcT=2πfcT=π/3 (7)
2ωcT=2π/3 (8)
Further
Cos 2ωT=−Cos ωT (9)
Sin 2ωT=Sin ωT (10)
2 Cos ωT=1 (11)
Assuming that
a0=a,a2=−a (12)
in Equation (5),
A=a+a Cos ωT
B=(2+b1−b2)Cos ωT
C=−a Sin ωT
D=(b1+b2)Sin ωT (13)
Therefore, in Equation (4),
O=2a2(1+Cos ωT)
Q=(2+b1−b2) 2 Cos2 ωT+(b1+b2)2 Sin ωT
P=2a2(Sin2 ωT−Cos2 ωT−Cos ωT)
R=(b1+b2)(b1+2b2)Sin2 ωT+(2+b1−b2)(b1−2b2)Cos2 ωT (14)
Assuming that
b1=−(1−a)
b2=1−2a (15)
b1+b2=−a
b1+2b2=1−3a
2+b1−b2=3a
b1−2b2=−3+5a (16)
Substituting Equation (16) into Q and R in Equation (14),
Q=9a Cos ωT+a Sin2 ωT
R=−a(1−3a)Sin2 ωT+3a(−3+5a)Cos2 ωT (17)
Considering Equation (7),
Cos2 ωT=¼, Sin2 ωT=¾ (18)
Therefore,
O=3a2
p=0
Q=3a2
R=3a(2a−1) (19)
Thus, substituting Equation (19) into Equation (3), the amplitude characteristic M(ω) is expressed by
M(ω)=(O/Q)0.51 (20)
The delay characteristic τ(ω) is expressed by
τ(ω)=(P/O−R/Q)T=(1−2a)/6afc (21)
Now, assuming that
a=2−n(n: integer of 0 or more) (22)
the delay characteristic τ(ω) is expressed by
τ(ω)=(2n−2)/6fc (23)
Assuming that
n=3,5,7,9, . . . . . . (n: positive integer other than 1) (24)
τ(ω)fc=1,5,21,85, (25)
Namely, the group delay can be taken as integer times of 1/fc and the phases of the central frequencies at the input and output agree with each other.
Assuming that
n=2m+1(m: integer of 0 or more) (26)
Equation (23) can be expressed by
τ(ω)=(22m−1)/3fc (27)
Using n in Equation (26), a in Equation (22) can be expressed by
a=2−(2m+1) (28)
Further, b1 and b2 in Equation (15) can be expressed by
b1=−(1−2−(2m+1))
b2=1−2−2m (29)
The procedure described above can be summarized as follows.
Assuming that
1. the sampling frequency fs is six-times of the central frequency of the bandpass filter,
2. n is an odd number of 3 or greater,
3. a0=2−n,a1=0,a2=−2−n
4. b1−(1−2)−n and
5. b2=1−2−(n−1),
6. the amplitude characteristic M(ωc)=1 and
7. the delay characteristic τ(ωc)=(2n−2)/6fc.
Otherwise, assuming that
4′. the sampling frequency fs is six-times of the central frequency of the bandpass filter,
2′. m is an integer of m>3,
3′. a0=2−(2m+1),a1=0,a2=−2−n
4′. b1=−(1−2−(2m+1)) and
5. b2=1−2−m,
6. the amplitude characteristic M(ωc)=1 and
7. the delay characteristic τ(ωc)=(22m−1)/3fc. (31)
Therefore, the phases and amplitudes at the central frequency at the input and output agree to each other.
Further, a0=2−(2m+1), the coefficient can be calculated by “bit shift”, thus facilitating the operation. In addition, because of no coefficient error, the operation can be carried with high accuracy.
The band of the filter becomes narrow as a0 approaches 0. For this reason, the objective bandwidth can be acquired by selecting a0 according to the object.
Now referring to
In
The frequency fs of the sampling pulse is set at six times of the central frequency fc of the bandpass filter as expressed by Equation (30). Assuming that n is an odd number of 3 or more, the first-order input feedback coefficient (b1) 3 is set at −(1−2−n) and the second-order input feedback coefficient (b2) 4 is set at 1−2−(n−1).
The zero-order output coefficient (a0) 7 is set at 2−n and the second-order output coefficient (a2) 8 is set at −2−n.
In the operation with the above settings, at the input and output of the filter, the phase and amplitude can be caused to agree to each other.
Since the coefficient is set as power of 2, it can be easily acquired with high accuracy.
Now referring to
In the first embodiment, the zero-order output coefficient (a0) 7 was set at 2−n (a0=2−n) and the second-order output coefficient (a2) was set at −2−n (a2=−2−n).
Specifically, a0=−a2. The zero-order output was multiplied by a0, and the second-order output was multiplied by −a2. The outputs thus acquired were added to acquire an output signal.
The second embodiment intends to reduce the number of multiplication.
Specifically, as seen from
A reference signal canceling apparatus according to this invention can be constructed using the second-order IIR digital filter hitherto explained. Specifically, in the second-order IIR digital filter, the central frequency is set at 19 KHz of the pilot signal and the sampling frequency is set at six times as large as this frequency. The digital filter with such settings is substituted for the bandpass filter in the pilot signal canceling apparatus described with reference to
Further, for example, in an RDS broadcasting, a reference signal at 57 KHz is superposed on a detected signal for data demodulation. In this case, by setting the sampling frequency at six times as large as 57 KHz, the reference signal at 57 KHz can be easily canceled.
Number | Date | Country | Kind |
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2000-099766 | Mar 2000 | JP | national |
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Number | Date | Country | |
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20010026601 A1 | Oct 2001 | US |