JPH0365812A - Filter circuit - Google Patents

Abstract

PURPOSE:To set a harmonic components ratio to an input signal optionally by revising a transfer function in response to the frequency of an input signal. CONSTITUTION:A comparator 1 detects passing of zero volt (zero cross) of an input terminal from an input terminal 3. Moreover, a counter 5 receives the result of detection of the comparator 1 and counts the number of pulses with a prescribed frequency supplied from an oscillator 7 from the rise of the pulse till the rise of the succeeding pulse. That is, since the rise interval of the pulses corresponds to the frequency of the input signal through the zero cross detection by the comparator 1, the frequency of the input signal is detected as the number of pulse with a sufficiently high frequency. Then a coder 9 outputs a 4-bit control signal for on/off of switches SW1-SW8 in response to the result of the detection of the frequency of the input signal from the counter 5. Thus, the transfer function is changed in response to the frequency of the input signal to adjust the parameter such as a cut-off frequency is optionally adjusted.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a filter circuit.

(Prior art) Fig. 3 shows a general low-pass filter constructed using an operational amplifier OPl, where the cut-off frequency "C" is the resistor R9,
10 and the values of capacitors C1 and C2. Therefore, the characteristics are as shown in FIG. 4(A).

<Problems to be Solved by the Invention> However, as a characteristic, E is uniquely fixed by the values of resistors R9 and R10 and capacitors C1 and C2.
For example, there are problems such as: When inputting signals of frequency fl and higher frequency f2, regarding their harmonic components, in the case of pre-cancer, the overtone (2f+) component is in the middle of the attenuation characteristic of the low-pass filter, so the overtones are output together. On the other hand, in the latter case (FIG. 4(B)), the overtone (2f2) component is completely in the 'mm region, so no overtone is output (FIG. 4(C)). This means that the harmonic component ratio (the degree to which the input signal contains harmonic components) differs depending on the frequency of the input signal, resulting in an unnatural hearing when used in musical instruments or the like.

That is, in conventional circuits, parameters such as the cutoff frequency cannot be freely changed, so it is impossible to keep the harmonic component ratio relative to the fundamental wave constant regardless of frequency or to change it arbitrarily.

The present invention has been made in view of the above, and an object thereof is to provide a filter circuit in which the harmonic component ratio with respect to an input signal can be arbitrarily set.

[Structure of the Invention] (Means for Solving the Problem) In order to achieve the above object, in a filter circuit that passes a component signal in a frequency WI range corresponding to a transfer function of an input signal and attenuates other component signals. The gist of the present invention is to have a frequency detection means for detecting the frequency of an input signal, a setting means for setting a transfer function according to the detected frequency, and a changing means for changing the transfer function based on the setting. .

(Function) In the filter circuit according to the present invention, by making it possible to change the transfer function according to the frequency of the input signal, it is possible to arbitrarily adjust the parameter of the 'a discontinuous frequency.

(Example> Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a diagram showing the configuration of an embodiment in which the present invention is applied to a low-pass filter. In the figure, 1 is a comparator that detects the passage of zero volts (zero cross) of the input signal from the input terminal 3. A counter 5 counts the number of constant frequency pulses supplied from the oscillator 7 from the rise of one pulse to the rise of the next pulse in response to the detection result of the comparator 1. In other words, by the zero cross detection of comparator 1, the pulse rising interval corresponds to the frequency of the input signal, so
The counter 5 detects the frequency of this input signal as a sufficiently high frequency pulse number. Reference numeral 9 denotes an encoder, which outputs a 4-bit control signal for turning on/off the switches SW1 to SW8, which will be described later, in accordance with the result of detecting the frequency of the input signal from the counter 5.

Note that the encoder 9 is composed of, for example, a ROM or a decoder.

On the other hand, in FIG. 1, 11 is a filter section, which is composed of an operational amplifier 13, resistors R1-R8, capacitors C1, C2, and switches SW1-SW8, and performs filter processing on the input signal from input terminal 3. It is something to do. In this filter section 11, four resistors R1 and R are connected between the input terminal 3 and one end of the capacitor C1.
3, R5 and R7 are connected in parallel with each switch SW1.8W
3. Connection is possible via SW5.8W7. Additionally, four resistors R2 are connected between capacitors C1 and C2.
, R4, R6, , R8 are each connected in parallel with SW2.8W4
．． Connection is possible via 8W6.8W8. Therefore, the transfer function of the filter section 11 can be arbitrarily set by appropriately selecting the resistance by turning on and off the switches SW1 to SW8. The relationship between each resistance value is R1=R2, R3-R4, R5-R6゜R
7=R8, and R1:R3:R5:R7-1:2:
It is convenient to set the ratio to 4:8. In addition, switch SW1
The on/off control of ~SW8 is based on a control signal from the encoder 9.

Next, the operation of this embodiment will be explained using FIGS. 2A to 2E. In FIG. 2, (A) is the input signal, (B) is the output signal of comparator 1, (C) is the output signal from oscillator 7, (D) is the count signal in counter 5,
(E) shows a control signal from the encoder 9.

When the input signal from the input terminal 3 starts to be supplied, the comparator 1 performs zero-cross detection and sends the result to the counter 5.
(Fig. 2 (A), (B)). Counter 5 is
At the rising edge of the pulse supplied from the comparator 1, the count value a is latched, the internal register for counting is cleared, and the counting of pulses from the oscillator 7 is started until the rising edge of the next pulse. The latched count value a is sent to the encoder 9
(Fig. 2 (C), (D)). The encoder 9 outputs a required control signal (Da) to turn on only the switch predetermined according to the count value a among the switches SW1 to SW8 (FIG. 2(E))
〉. As a result, in the filter section 11, a transfer function is set, a parameter for the ``a-dissonant frequency is determined, and the required filter processing is performed on the input signal.

On the other hand, when the frequency of the input signal changes, the pulse interval output from the comparator 1 becomes narrower, and the count value in the counter 5 decreases to b (<a) (Fig. 2 (A) to (D)).
). The encoder 9 outputs a corresponding control signal (Db) according to the change in the count value, so the filter section 11
In this case, filter processing can be performed with the parameters appropriately modified (FIG. 2(E)).

Therefore, according to this embodiment, filter processing can be performed using a desired transfer function depending on the frequency of the input signal.
It has the following effects. In other words, by freely changing parameters such as the cutoff frequency, the harmonic component ratio to the input signal can be adjusted arbitrarily regardless of the frequency, so when used in musical instruments, etc., it is possible to obtain a natural hearing sensation. Even if there is a peculiar sound that is missing several notes, it can be corrected, and furthermore, special sound effects can be obtained.

Also, when used in a telephone dial tone output circuit,
Spurious can be optimally cut regardless of the output frequency (button button).

Note that although this embodiment has been described in the case of a low-pass filter, it goes without saying that it is applicable to bypass filters, band-pass filters, and the like.

Furthermore, when processing an input signal in which multiple frequencies are added, such as a chord played on a musical instrument or a telephone dial tone, -i may be input after limiting the frequencies through an appropriate filter. .

[Efficacy of invention 51! ] As explained above, according to the present invention, by making it possible to change the transfer function according to the frequency of the input signal, parameters such as the cut-off frequency can be arbitrarily adjusted. The ratio can be set arbitrarily.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the operation of this embodiment, and FIG. 3 is a diagram showing a conventional example.
FIG. 4 is a diagram for explaining the operation of the conventional example. 1... Comparator 3... Input terminal 5... Counter 7... Oscillator 9... Encoder 11... Filter section 13... Operational amplifier representative Hidekazu Miyoshi, patent attorney Figure 1 Figure 3 Figure 14 (B) is4 Figure (C)

Claims (1)

[Claims] A filter circuit that passes a component signal in a frequency band corresponding to a transfer function for an input signal and attenuates other component signals includes a frequency detection means for detecting the frequency of the input signal and a transfer function according to the detected frequency. 1. A filter circuit comprising: a setting means for setting; and a changing means for changing a transfer function based on the setting.