Patent ID: 8160272

Claim:
An audio output circuit, comprising: a plurality of audio sources having outputs maintained at unequal dc voltage offsets when driven with respective audio signals; an audio signal selection circuit electrically coupled to the outputs of said plurality of audio sources, said audio signal selection circuit configured to route an audio signal from a selected one of said plurality of audio sources to an output thereof; and a port attenuation circuit electrically coupled to the output of said audio signal selection circuit, said port attenuation circuit configured to convert an abrupt dc voltage offset transition between a pair of audio signals received in sequence from said audio signal selection circuit into a more gradual transition that meets a pop disturbance threshold, said port attenuation circuit comprising: a port attenuation control circuit configured to generate an active ramp enable signal during an operation to change the selected one of said plurality of audio sources output by said audio signal selection circuit; and an attenuation switch network responsive to the ramp enable signal and an audio signal that is output from said audio signal selection circuit; wherein the abrupt dc voltage offset transition exceeds the pop disturbance threshold; wherein said port attenuation circuit is configured to convert the abrupt dc voltage offset transition into a more gradual transition that meets a pop disturbance threshold by performing ramp-to-mute and ramp-from-mute operations in sequence in said attenuation switch network, said ramp-to-mute and ramp-from-mute operations comprising ramping a first non-zero dc voltage offset associated with a first audio signal to a mute condition and then ramping a dc voltage associated with the mute condition to a second non-zero dc voltage offset associated with a second audio signal, which is unequal to the first non-zero dc offset voltage; wherein said attenuation switch network comprises a series string of resistors having a first terminal configured to receive the audio signal output from said audio signal selection circuit during the ramp-to-mute and ramp-from-mute operations and a second terminal electrically coupled a reference voltage; and wherein the dc voltage associated with the mute condition is unequal to the first and second non-zero dc voltage offsets.