Join the microphone and audio lines from a headset for recording
What are the steps required to join the microphone and audio lines from a typical cell phone headset into one line so that the audio can be recorded by a standard voice recorder using the microphone and/or line-in input?


In this context "audio line" means a signal level &amp; output impedance intended to drive a typical pair of earphones. I'll assume mono rather than stereo.

I'd guess the processing steps may be not entirely unlike the following:


for mic-in (could be entirely passive?)


Attenuate the headphone signal to mic levels
mix audio signals
buffer output

for line-in (choose either pro or consumer level)


amplify the microphone signal to line-level.
amplify or attenuate earphone signal to line-level
mix audio signals
buffer output



By buffer output I mean set the appropriate output impedance.

For stereo, you'd typically split the mic signal and mix equally into left and right channels.

For recording conversations you might want separate automatic gain control (AGC) for each side of the conversation. Since conversations are mostly half-duplex I guess you mostly only need one AGC element.