Incoming audio detection circuit for a speaker telephone

A circuit for use in a speaker telephone having a microphone amplifier, said circuit being used for detecting the presence of audio incoming from a telephone line while outgoing speech is being amplified by said microphone amplifier and transmitted to the telephone line, said circuit comprising:

• an impedance element connected between the telephone line and the output of said microphone amplifier,
• first rectifier means for rectifying without filtering the audio present at said telephone line,
• second rectifier means for rectifying without filtering the audio present at the output of said microphone amplifier, the effective conduction angle of said second rectifier means being greater than that of said first rectifier means, and
• comparator means for comparing the signal from said first and second rectifier means and for producing an output signal only when the rectified but unfiltered signal from said first rectifier means exceeds that from said second rectifier means, this condition being indicative of the presence of incoming audio, the operational mode of said speaker telephone being switched from outgoing speech transmission to incoming audio reception in response to said produced output signal.

In a speaker telephone of the type in which speech picked up by a microphone is amplified in a microphone amplification channel for transmission to a telephone line, and in which audio incoming from the telephone line is amplified for loudspeaker reproduction in a speaker amplification channel connected to the telephone line, the improvement for detecting audio incoming from the telephone line during outgoing speech transmission comprising:

• an impedance element connected between the output of said microphone amplification channel and said telephone line, said outgoing speech being transmitted via said impedance element,
• a first operational amplifier rectification circuit connected to rectify without filtering the signal present at said telephone line,
• a second operational amplifier rectification circuit connected to rectify without filtering the signal from the output of said microphone amplification channel, the effective angle of said second rectification circuit being greater than that of said first rectification circuit, and
• an operational amplifier comparator circuit receiving as inputs to be compared the outputs of said first and second rectification circuits, and providing an output signal only when the output of said first rectification circuit exceeds the output of said second rectification circuit, said provided output signal being used to switch said speaker telephone from the outgoing speech transmission mode to the incoming audio reproduction mode.