Muting circuit using common mode rejection of differential amplifier

• US 5,208,865 A
• Filed: 12/23/1991
• Issued: 05/04/1993
• Est. Priority Date: 12/23/1991
• Status: Expired due to Fees

• Current Assignee: Visteon Global Technologies Incorporated (Visteon Corporation)
• Original Assignee: Ford Motor Company
• Inventors: Friske, James W., Krochmal, Andrew C.
• Primary Examiner(s): Ng, Jin F.
• Assistant Examiner(s): Lefkowitz, Edward
• Application Number:
  US07/812,337
• Time in Patent Office: 498 Days
• Field of Search: 381/86, 381/107, 381/94, 455/221, 455/222, 455/212, 455/225
• US Class Current: 381/94.5
• CPC Class Codes: H03G 3/34 Muting amplifier when no signal is present or when only weak signals are present, or caused by the presence of noise signals, e.g. squelch systems
• CPC Class Codes: H03G 3/348 Muting in response to a mechanical action or to power supply variations, e.g. during tuning; Click removal circuits

Abstract

Differential signals are employed in a distributed audio system to improve noise immunity of the low level differential signals transmitted between a main unit and a power amplifier. The power amplifier may be in an auxiliary output unit which is locally muted by employing a field-effect transistor connected between the differential signals. When the field-effect transistor is turned on by a mute signal, the common mode rejection of the differential amplifier is employed to mute the amplifier output.

Claims

1. An audio system for a vehicle comprising:

a source of audio signals to be reproduced;

an audio processor coupled to said source and providing a low level differential output signal across a pair of output terminals;

main controller means coupled to said source and said audio processor for controlling audio parameters of said low level differential output signal;

primary output means coupled to said output terminals for amplifying said low level differential output signal and for reproducing sound in said vehicle; and

auxiliary output means coupled to said output terminals for reproducing sound audible in a portion of said vehicle, said auxiliary output means comprising;

a fixed gain differential amplifier having a pair of differential inputs coupled to receive said low level differential output signal;

muting means coupled to said pair of differential inputs for selectably connecting together said differential inputs in response to a mute signal; and

auxiliary controller means coupled to said muting means for generating said mute signal for a predetermined time when said audio processor is first activated.

2. The audio system of claim 1 wherein said muting means is comprised of a field-effect transistor connected between said differential inputs and receiving said mute signal at the gate thereof.

3. The audio system of claim 2 wherein said field-effect transistor is an enhancement-mode MOSFET.

4. The audio system of claim 1 wherein said muting means is comprised of a Pair of inverse-connected field-effect transistors connected between said differential inputs and receiving said mute signal at the gates thereof.

5. The audio system of claim 1 wherein said auxiliary output means comprises a headphone system, and wherein said auxiliary output means further includes a voltage divider for reducing the voltage of said low level differential output signal provided to said pair of differential inputs.

6. The audio system of claim 1 wherein said main controller means and said auxiliary controller means are coupled together to transmit control signals therebetween.

7. A mutable audio system comprising:

audio processor means for receiving input audio signals and providing a low level differential output signal across a pair of output terminals;

a fixed gain differential amplifier having a pair of differential inputs coupled to receive said low level differential output signal;

a pair of resistors, each respective resistor connected between a respective output terminal of said audio processor and a respective differential input;

a first field-effect transistor connected between said differential inputs and receiving a mute signal at the gate thereof; and

controller means coupled to said field-effect transistor for generating said mute signal when said audio system is to be muted.

8. The audio system of claim 7 wherein said first field-effect transistor is an enhancement-mode MOSFET.

9. The audio system of claim 7 further comprising a second field-effect transistor inverse connected with said first field-effect transistor and receiving said mute signal at the gate thereof.

10. The audio system of claim 9 wherein said first and second field-effect transistors are each an enhancement-mode MOSFET.

11. The audio system of claim 7 further comprising a speaker coupled to the output of said fixed gain differential amplifier.