NOISE REDUCTION SYSTEM

• US 3,798,559 A
• Filed: 04/17/1972
• Issued: 03/19/1974
• Est. Priority Date: 04/20/1971
• Status: Expired due to Term

• Current Assignee: Kiyoji Fujisawa, Masao Tomita
• Original Assignee: Kiyoji Fujisawa, Masao Tomita
• Inventors: Tomita, Masao, Fujisawa, Kiyoji
• Primary Examiner(s): Heyman, John S.
• Assistant Examiner(s): Anagnos, L. N.
• Application Number:
  US05/244,558
• Time in Patent Office: 701 Days
• Field of Search: 307/235R,235A,233,237,246,264 328/150,151,165,167,168,169,171,172 330/29,144,145,149
• US Class Current: 327/311
• CPC Class Codes: H03G 3/301 the gain being continuously variable
• CPC Class Codes: H03G 3/3015 using diodes or transistors
• CPC Class Codes: H03G 3/341 Muting when no signals or only weak signals are present H03G3/344, H03G3/345 take precedence
• CPC Class Codes: H03G 5/22 having semiconductor devices

Abstract

A noise reduction system which has a switching circuit, a charge-and-discharge circuit and a variable gain amplifier. When the input signal is lower than a predetermined threshold level, the charge-and-discharge circuit is changed to charging operation by the switching circuit and a control voltage is applied to the variable resistance means, whereby the resistance thereof is decreased and the input signal is attenuated. When the input signal exceeds the predetermined threshold level, the charge-anddischarge circuit is changed to discharging operation and the control voltage becomes zero, so that the input signal is not attenuated since the variable resistance means has a small resistance. Accordingly, the input noises can be removed during a pause in the input signal.

Claims

1. A noise reduction sYstem comprising:

an input terminal;

an output terminal;

variable gain means comprising a resistor, a capacitor and a variable resistance means operatively coupled together, said variable gain means connected between said input terminal and said output terminal; and

a control means coupled at one end thereof to said input terminal and coupled at another end thereof to said variable resistance means, said control means comprising a series connection of an amplifier means, a rectifier means, a switching means comprising a constant voltage source and a switching circuit which is switched in accordance with a signal generated by said rectifier means, and a charge-and-discharge means comprising a capacitor, at least one resistor and at least one diode operatively coupled together and having different time constants for the charging operation and discharging operation thereof respectively, and being switched from one state to the other state by said switching means, whereby the gain of said variable gain means is controlled in accordance with the output signal of said from said charge-anddischarge means.

2. A noise reduction system as claimed in claim 1 wherein said diode and resistor of said charge-and-discharge means are connected in parallel and said capacitor of said charge and discharge mean is grounded at one end and connected at the other end thereof to the parallel connection of said diode and said resistor.

3. A noise reduction system as claimed in claim 2 wherein said resistor of said variable gain means is coupled between said input and output terminals of said system, and said variable resistance means is connected at one end to said output terminal through said capacitor of said variable gain means and grounded at the other end thereof.

4. A noise reduction system as claimed in claim 3 wherein said variable resistance means is a transistor, the collector of which is connected to said output terminal of said system through said capacitor of said variable gain means, the emitter of which is grounded, and the base of which is connected to said charge-and-discharge means.

5. A noise reduction system as claimed in claim 3 wherein said variable resistance means is a field effect transistor, the drain of which is connected to said output terminal of said system through said capacitor of said variable gain means, the source of which is grounded, and the gate of which is connected to said charge-and-discharge means.

6. A noise reduction system as claimed in claim 3 wherein said variable resistance means comprises a series circuit comprised of two diodes and a resistor, the junction point of said series connected two diodes being connected to said output terminal of said system through said capacitor of said variable gain means, and said series circuit being connected at one end to said charge-and-discharge means through said resistor and being grounded at the other end.

7. A noise reduction system as claimed in claim 3, wherein said variable resistance means comprises a transistor and a series connection of three diodes, the emitter of said transistor being grounded through said series connection of three diodes, the base of said transistor being connected to said charge-and-discharge means and the collector of said transistor being connected to a D.C. source, and the junction point of a first diode and a second diode in said series connection of three diodes being connected to said output terminal of the noise reduction system through said capacitor of said variable gain means.

8. A noise reduction system as claimed in claim 1, wherein said switching means comprises a Zener diode, a resistor and a transistor, said Zener diode being connected to the collector of said transistor through said resistor at one end thereof and being grounded at the other end thereof, the operation of said transistor being changed between a saturation state and a cut-off state according to the output signal from said rectifier means.

9. A noise reduCtion system comprising an input terminal, an output terminal, a frequency filter means which comprises a capacitor and a variable resistance means coupled between said input terminal and said output terminal, a switching means which comprises a constant voltage source and a switching circuit, a charge-and-discharge means which comprises a capacitor, at least one resistor and at least one diode and which is coupled to said switching means and has the charging operation and discharging operation changed by said switching means in accordance with an input signal level, said charge-and-discharge means being coupled to said frequency filter, and a series circuit of an amplifier means and a rectifier means coupled between said input terminal and said switching means, whereby the cut-off frequency of said frequency filter means is controlled according to the output signal from said charge-and-discharge means.

10. A noise reduction system as claimed in claim 9 wherein said frequency filter means is a low pass filter having said variable resistance means coupled between said input terminal and said output terminal, and having said capacitor coupled between said output terminal and ground.

11. A noise reduction system as claimed in claim 10, wherein said low pass filter further comprises a resistor which is connected in series with said capacitor.