Audio peripheral mixer circuit and method for noise reduction

• US 5,440,643 A
• Filed: 05/13/1993
• Issued: 08/08/1995
• Est. Priority Date: 05/13/1993
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Heyl, Lawrence F., Wilson, David F.
• Primary Examiner(s): Kuntz, Curtis
• Assistant Examiner(s): TRAN, SINH N
• Application Number:
  US08/062,422
• Time in Patent Office: 817 Days
• Field of Search: 381/94, 381/119, 381/106, 381/56, 333/14
• US Class Current: 381/119
• 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: H04H 60/04 Studio equipment; Interconnection of studios

Abstract

A circuit and method for mixing signals from audio sources provide noise reduction by employing a noise gate to selectively attenuate one of the source signals. The noise gate has a low gain state and a unity gain state. The noise gate switches to its unity gain state when a control signal reaches a predefined threshold voltage. The control signal is generated by rectifying and integrating the noisy source audio signal, so that the noise gate enters the unity gain state only when a useful sound occurs. Thus, the noise in the combined audio signal is reduced, without attenuating useful components of the source signals.

Claims

1. A circuit for mixing a first audio signal with a second audio signal, comprising:

a control signal generator coupled to receive the first audio signal and having an output, the control signal generator generating at its output a control signal having a first state corresponding to the first audio signal not lower than a threshold value and a second state corresponding to the first audio signal lower than the threshold value;

a selective attenuator coupled to receive the first audio signal and the control signal, the selective attenuator producing at its output an attenuated first audio signal to reduce noise in response to the control signal in the first state and a substantially unattenuated first audio signal in response to the control signal in the second state; and

a mixing circuit coupled to receive the output signal from the selective attenuator and the second audio signal, the mixing circuit combining the selective attenuator output with the second audio signal, wherein the second audio signal is substantially unattenuated.

2. The circuit of claim 1, wherein:

the control signal has an attack time representing responsiveness to an increase in the amplitude of the first audio signal, and a decay time representing responsiveness to a decrease in the amplitude of the first audio signal; and

the attack time is of lesser duration than the decay time.

3. The circuit of claim 2, wherein:

the attack time is less than 100 msec; and

the decay time is greater than 100 msec.

4. The circuit of claim 1, wherein the selective attenuator includes a noise gate comprising a shunt switch selectively coupling the first audio signal through a resistive path to a ground.

5. The circuit of claim 4, wherein the control signal generator comprises:

a rectifier coupled to receive the first audio signal, for rectifying the first audio signal; and

an integrator coupled to the rectifier, for integrating the rectified signal to produce the control signal.

6. The circuit of claim 5, wherein the control signal generator further comprises a voltage divider coupled to the rectifier, for biasing the rectifier.

7. A circuit for providing selective attenuation of a first set of audio signals to reduce noise, the first set including a plurality of audio signals, and for mixing the selectively attenuated first set of audio signals with a second set of audio signals, the circuit comprising:

a summer for generating a sum signal representing an algebraic sum of the plurality of audio signals;

a rectifier coupled to receive the sum signal, for rectifying the sum signal;

a voltage divider coupled to the rectifier, for biasing the rectifier;

an integrator coupled to the rectifier, for integrating the rectified sum signal to produce a control signal;

a noise gate coupled to receive the first set of audio signals, for selectively attenuating the first set of audio signals responsive to the control signal; and

a mixing amplifier coupled to the noise gate and coupled to receive the second set of audio signals, for combining the selectively attenuated first set of audio signals with the second set of audio signals, wherein the second set of audio signals is substantially unattenuated.

8. A method of selectively attenuating a plurality of audio signals to reduce noise, and mixing the selectively attenuated plurality of audio signals with at least one substantially unattenuated audio signals, the method comprising the steps of:

generating a sum signal representing an algebraic sum of the plurality of audio signals; and

rectifying the sum signal;

integrating the rectified sum signal to produce a control signal indicating one of two states;

responsive to the control signal indicating a first of the two states, attenuating the plurality of audio signals; and

combining the selectively attenuated plurality of audio signals with at least one substantially unattenuated audio signal.