Noise reduction system

• US 4,703,507 A
• Filed: 07/25/1986
• Issued: 10/27/1987
• Est. Priority Date: 04/05/1984
• Status: Expired due to Fees

• Current Assignee: Thomas W. Holden
• Original Assignee: Thomas W. Holden
• Inventors: Holden, Thomas W.
• Primary Examiner(s): Isen, Forester W.
• Application Number:
  US06/890,617
• Time in Patent Office: 459 Days
• Field of Search: 455/225, 455/221, 455/222, 455/223, 455/302, 455/303, 455/304, 455/305, 455/306, 381/94
• US Class Current: 381/94.5
• CPC Class Codes: H03G 3/341 Muting when no signals or only weak signals are present H03G3/344, H03G3/345 take precedence

Abstract

A noise reduction system and method for reducing noise occurring during pauses in information bearing audio signals has an audio input connected to one input of a differential output circuit. A variable gain circuit has an input connected to the audio input and an output connected to the other input of the differential circuit. A dc control signal is derived from the incoming signals and controls the variable gain circuit to cause cancellation of a noise only signal during periods of no information bearing audio signals.

Claims

1. A noise reduction system for connection to the output of a radio receiver or the like comprising:

a signal input for receiving an intelligence plus noise signal or a noise only signal;

a differential output circuit having a first input connected to said signal input;

an envelope detector connected to said signal input for producing a slowly varying do control signal proportional to the envelope of the intelligence and noise signals at said signal input; and

a variable gain circuit including a bridge circuit having a pair of input terminals connected to said signal input and a pair of output terminals, said bridge having a controllable resistance in one leg thereof, said controllable resistance operatively connected to said envelope detector for receiving said do control signal whereby said controllable resistance is varied to vary the balance of said bridge circuit, and a differential amplifier having a pair of differential input terminals connected to said bridge output terminals and an output terminal connected to a second input of said differential output circuit, said differential amplifier having a high output when said bridge is greatly unbalanced and a low output when said bridge approaches balance, said do control signal applied to said variable gain circuit for controlling the gain thereof, said variable gain circuit controlled to produce an output signal at said second input of said differential output circuit essentially equal in amplitude and phase to a signal at said first input thereof when a noise only signal is present at said signal input and to produce an output signal at said second input of said circuit differing in amplitude from a signal at said first input thereof when an intelligence plus noise signal is present at said signal input.

2. The system as recited in claim 1 which said envelope detector includes:

a rectifier; and

a low pass filter having a cutoff frequency low with respect to the audio frequency content of an audio signal at said signal input and high with respect to the frequency of the envelope of said audio signal.

3. The system as recited in claim 1 in which said envelope detector includes:

a rectifier; and

a low pass filter having a cutoff frequency low with respect to the audio frequency content of an audio signal at said signal input and low with respect to the frequency of the envelope of said noise signal.

4. The system as recited in claim 1 in which said controllable resistance is a field effect transistor.

5. The system as recited in claim 1 in which said controllable resistance is a plurality of field effect transistors connected in parallel.

6. The system as recited in claim 1 in which said differential output circuit includes a potentiometer having a movable arm, said movable arm connected to said noise reduction system output.

7. The system as recited in claim 1 in which said first input is connected to said signal input via an adjustable delay circuit.

8. The system as recited in claim 7 in which said adjustable delay circuit is a phase shift network.

9. In a communications receiver, a system for cancelling noise signals occurring during pauses in an information bearing signal comprising:

(a) a plurality of bandpass filters, each having an input and an output, the bandpasses of said filters overlapping such that a total bandpass of said filters encompasses the passband of said information bearing signals;

(b) a plurality of noise cancelling subsystems wherein one of said subsystems is connected to the output of each of said plurality of bandpass filters, each of said noise cancelling subsystems having

(i) a subsystem input for receiving information bearing signals plus noise signals from one of said bandpass filters.

(ii) a differential output circuit having first and second inputs, and a noise cancelling subsystem output,

(iii) an adjustable delay having an input connected to said subsystem input and an output connected to said first input of said differential output circuit,

(iv) a variable gain circuit having a signal input connected to said subsystem input, a signal output connected to said second input of said differential output circuit, and a control input for de control signal for varying the gain of said variable gain circuit,

(v) a rectifier connected to said subsystem input, and

(vi) a low pass filter having an input connected to said rectifier for filtering a varying dc voltage from said rectifier, thereby providing a dc control voltage proportional to the envelopes of said information bearing signals and said noise signals, said low pass filter having an output connected to said control input of said variable gain circuit, said variable gain circuit causing the noise signal at said second input of said differential output circuit to be essentially equal in amplitude to the noise signal at said first input thereof when said dc control signal amplitude is low during such pauses and to cause the information bearing signals at said second input to differ in amplitude relative to the information bearing signal at said first input; and

(c) a mixer circuit having a plurality of mixer inputs wherein one of said mixer inputs is connected to the output of each of said noise cancelling subsystems, said mixer having a mixer output circuit for the information bearing audio signal within said total bandpass.

10. In a communication system for reception of intelligence bearing signals plus thermal and atmospheric noise signals in which such intelligence bearing signals include periods of relatively low amplitude, the method of reducing the amplitude of said noise signals during such periods comprising the steps of:

producing a dc control signal from said intelligence bearing plus thermal and atmospheric noise signals having a slowly varying amplitude proportional to the envelope of said intelligence bearing plus thermal and atmospheric noise signals when present and proportional to the envelope of said thermal and atmospheric noise signals when the amplitude of said intelligence bearing signals is low;

passing said dc control signal through a low pass filter having a cutoff frequency low with respect to the frequency of said intelligence bearing signal and high with respect to the lowest frequency components of the envelopes of said intelligence bearing signals, said cutoff frequency low with respect to the lowest frequency components of the envelopes of said thermal and atmospheric noise signal;

applying said intelligence plus thermal and atmospheric noise signals to a first input of a differential output circuit and to a controllable gain circuit, the output of said controllable gain circuit is connected to a second input of said differential output circuit;

controlling the gain of said controllable gain circuit to be proportional to the amplitude of the dc control signal from said low pass filter to thereby cause the thermal and atmospheric noise only signal to the second input of the differential output circuit when such noise only signal is present to be essentially equal in amplitude to the thermal and atmospheric noise signal at the first input thereof to cause cancellation of the thermal and atmospheric noise only signal in the output circuit; and

controlling the gain of said controllable gain circuit proportional to the amplitude of the dc control signal from said low pass filter to thereby cause the intelligence plus thermal and atmospheric noise signal to the second input of the differential output circuit when such intelligence plus thermal and atmospheric noise signal is present to differ in amplitude from the intelligence plus thermal and atmospheric noise signal at the first input thereof to cause an intelligence plus thermal and atmospheric noise signal to be present at the output of said communication system.

11. A thermal and atmospheric noise reduction system for connection to the audio of a radio receiver or the like, in which audio signals therefrom include short periods of thermal and atmospheric noise only, for reduction of such noise during such periods comprising:

a signal input for receiving alternately an audio plus thermal and atmospheric noise signal and a thermal and atmospheric noise only signal;

a differential output circuit having a first input connected to said signal input, and a second input;

a variable gain circuit having an input connected to said signal input and having an output connected to said second input of said differential output circuit, said variable gain circuit controllable by a dc control signal; and

an envelope detector connected to said signal input for producing a slowly varying dc control signal proportional to the envelope of the audio and thermal and atmospheric noise signals at said signal input, said dc control signal applied to said variable gain circuit to control the gain thereof, said variable gain circuit controlled to produce an output signal at said second input of said differential output circuit essentially equal in amplitude and phase to a signal at said first input thereof when a thermal and atmospheric noise only signal is present at said signal input and to produce an output signal at said second input of said differential output circuit differing in amplitude from a signal at said first input thereof when an audio plus thermal and atmospheric noise signal is present at said signal input.

12. A thermal and atmospheric noise reduction system for connection to the output of a radio receiver or the like, in which desired signals therefrom include short periods of thermal and atmospheric noise only, for reduction of such noise during such periods comprising:

a signal input for receiving alternately a desired signal plus thermal and atmospheric noise signals and thermal and atmospheric noise only signals;

an output circuit means having a first input connected to said signal input for receiving input signals therefrom, a second input for receiving modified said input signals, and an output;

amplifier circuit means for amplifying signals at said signal input and having an input connected to said signal input and having an output connected to said second input of said output circuit means, said amplifier circuit means having its gain controllable by a dc control signal; and

detector means connected to said signal input for producing a slowly varying dc control signal proportional to the envelope of the desired signal and thermal and atmospheric noise signals at said signal input, said dc control signal applied to said amplifier circuit means to control the gain thereof, said amplifier circuit means controlled to produce said modified input signals at said second input of said output circuit means in opposing relationship in amplitude and phase to said input signals at said first input thereof when thermal and atmospheric noise only signals are present at said signal input for cancelling said noise only signals at said output circuit means output, and to produce an output signal at said second input of said output circuit means differing in amplitude from a signal at said first input thereof when a desired signal plus thermal and atmospheric noise signal is present at said signal input.