Audio mix this function is used when the pd 170 is used as a player. Now lets assume that you put the lapel mic in the 1st xlr and the 2nd one has ambient. If you put the switch on Ch then the both tracks of audio will be recorded separately on the two channels. AGC = automatic gain control. Here the camera controls the levels of the audio. The camera pushes the levels higher and lower depending on the levels generated by the subject. if you put this on off the you have to set the levels manually. Initial PD 150s had a hiss problem while using external mics using the agc turned off.