AGC Control for Panasonic Lumix FZ200
This is work in progress to produce a small device which will defeat the automatic gain control (AGC) of the FZ200 camera when using external microphones.
The AGC is responsible for keeping the audio recording levels at a point which is audible but not clipping causing distortion.
In most situations where the audio is constant this circuit performs well. However, in other situations like interviews, music recording and dictation there may be brief periods where there is no audio input. During this period the camera AGC will attempt to raise the background noise from the mic or its own pre-amp circuits to the normal level. You can here this as random hiss noise on the recorded audio track. As soon as the audio breaks this silence the camera AGC has to then quickly turn down the gain to get the audio back under control. The first second of this might be distorted as the AGC has a hysteresis lag.
I have been working on a small unit which will be battery powered and will allow you to plug an external stereo mic into it and then this unit plugs into the 2.5mm mic input socket of the camera.
In operation this unit generates an inaudible 20KHz signal of about 6-8 millivolts and is combined with the left hand channel of the audio. As this waveform is what the camera would expect to see for average recording at the right level no change in the AGC is made if the input volume decreases. It will still control the passages which exceed this recording waveform. As it is over the normal threshold for hearing the audio track does not contain this and cannot be heard.
The initial proof of concept uses a commercial waveform generator IC however my final design will be a much smaller unit using smaller and discrete electronic components. Here is the proof of concept at the moment
Here is the test of the unit in the proof of concept stage.