The automatic fader drops at the background music while the narration is in place. The control input through RIO, a preset audio level control, into an emitter-follower buffer stage CQI). The buffer provides high input impedance and ensures that the source impedance is low enough to drive the rectifier and smoothing circuit, consisting of DI, D2, and C5. The smoothed output drives a simple LED circuit. LD and R8 form an IR input pad through which the output is fed through C6 and C7 to the output jack.
The output to the emitter of QI couples making this C4 and R5. R5 and R7 are a passive mixer. With 200 mV or less at the entrance, there is enough voltage on C5 to tum Q2. More than 200 mV, Q2 will turn on a boundary, and the LED is energized. This brings down the resistance of the LDR, and signal loss due to increased attenuation. Increase the input of 350 mV rms, and you get a signal reduction greater than 20 dB.