1. Field of the Invention
The present invention relates to an improvement in a comb filter for use, for instance, to eliminate cross color (cross talks of color signals) of a video tape recorder or dot interference of NTSC color television receiver, etc.
2. Description of Prior Arts
An ultrasonic delay line delays electric signal for a predetermined time period. By utilizing the fact that the propagation velocity of the sonic wave in the delay line is about 10.sup.-5 of that of the electromagnetic wave, an electric signal is delayed by a process that the electric signal is transduced into the ultrasonic wave, and is given of a delay of several tens .mu.sec to several hundreds .mu.sec by the ultrasonic delay line, and thereafter is transduced back to the electric signal, thereby to produce a delayed electric signal.
FIG. 1 shows an example of conventional delay line, FIG. 2 shows an example of conventional comb filter utilizing the delay line of FIG. 1, and FIG. 3 is a diagram showing a characteristic curve of the conventional comb filter of FIG. 2.
An ultrasonic delay line 6 shown in FIG. 1 comprises a delay element 1 which is usually made with a glass block and the oblique corner face thereof has an input terminal comprising electrodes 2a and 2b with a piezoelectric element 3 inbetween and an output terminal comprising electrodes 2a' and 2b' with a piezoelectric element 3' inbetween. When an electric signal E.sub.i is applied across the input electrodes 2a and 2b, the piezoelectric element 3 makes mechanical motion responding to the input signal E.sub.i, thereby radiating an ultrasonic wave into the delay element 1. The ultrasonic wave propagates in the delay element 1 making a lot of reflections at the surface thereof as shown in FIG. 1, and reaches the electrode 2b'. By receiving the ultrasonic wave at the electrode 2b', the piezoelectric element 3' is moved and produces electric output signal E.sub.o across the electrodes 2a' and 2b'. Such delay lines have been used in comb filters for elimination of cross color of a video recorder signal or dot interference of an NTSC color television receiver.
FIG. 2 shows an example of conventional circuit construction of the comb filter using the delay line 6 shown in FIG. 1. A sinusoidal input signal E.sub.A applied to an input terminal 4 is given to the delay line 6 and an output signal of the delay line 6 is given to an input terminal 5b of a mixing circuit 5, and the sinusoidal input signal E.sub.A as such is applied to the other terminal 5a of the mixing circuit 5, which operates summing or subtraction of the input signals at the input terminals 5a and 5b.
Provided that:
.omega. is an angular frequency of the input signal E.sub.A, PA1 .alpha. is a constant, PA1 .beta. is a constant, and PA1 .tau..sub.0 is a delay time of the delay line 6, PA1 A.sub.max is a maximum value of the amplitude A, PA1 A.sub.min is a minimum value of the amplitude A, PA1 .omega.(n).sub.max =2.pi.f(n).sub.max is the angular frequency when the signal E.sub.C takes the value of A.sub.max, and PA1 .omega.(n).sub.min =2.pi.f(n).sub.min is the angular frequency when the signal E.sub.C takes the value of A.sub.min ;
an output signal E.sub.C issued from the output terminal 7 of the circuit of FIG. 2 is given as ##EQU1##
Hereupon, it is provided that:
then the following equations hold: ##EQU2##
Then, the frequency pitch f'.sub.H between two neighboring A.sub.max and the frequency pitch f".sub.H between two neighboring A.sub.min are given as follows: ##EQU3##
That is to say, by summing the original signal E.sub.A and a signal E.sub.B delayed by the time .tau..sub.0 from the original signal E.sub.A, the resultant signal E.sub.C has the repetitions of peaks and valleys of its spectrum distribution with the frequency pitches of 1/.tau..sub.0, and therefore band-path characteristic becomes comb-shaped as shown in FIG. 3, wherein the frequency pitches f'.sub.H and f".sub.H are called comb pitches.
The abovementioned relation can be also deduced by Fourier transformation as follows:
Provided that a unit step function .delta.(t) is used as the input signal E.sub.A, the delayed signal E.sub.B is given by .delta.(t-.tau..sub.0), and accordingly the output signal E.sub.C (t) is given by: ##EQU4## By Fourier transformation of the equation (4), the following equation holds: EQU E.sub.C (j.omega.)=1+e.sup.-j.omega..tau..sbsp.0 ( 5).
Accordingly, the frequency characteristic of the output signal E.sub.C is given as follows: ##EQU5##
The equation (6) shows that the comb filter of FIG. 2 has the comb shape characteristic having the frequency pitch of 1/.tau..sub.0.
The abovementioned conventional comb filter requires a certain size of the delay element block 1, and the size has been regarded particularly determined by the delay time .tau..sub.0.