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.

 

Schematic for the working version of the FZ200 AGC Defeat Unit

 

The final circuit turned out to be more simplistic than I had originally thought.

I assumed that the input would need to be a sine wave to prevent harmonic distortion if a square or triangular wave was used for the input to the camera. In testing I found that the camera would still perform well with a square wave input so I decide to try both an astable multi-vibrator pair of transistors running at 20 KHz and a NE555 timer IC.

In the end I decide to use the more stable 555 IC as the frequency remained solid even during the battery run down.

Here is the schematic for the project.

The timing is done by R2-C2 giving 20KHz. The output from pin 3 is attenuated by R1-R3  to 7-8millivolts at the camera input after being coupled with the mic input and allowing for its impedance.

the right hand mic input goes straight to the camera input whilst the left hand input has the 20KHz signal injected into it.

If you wanted to get finer control of the AGC defeat voltage  input R3 could be substituted with a 27K resistor and a 100K potentiometer in parallel with it. The output would be taken to C3 form the wiper of the pot. This would allow the output to be set for 0V to about 10millivolts.

The unit is powered by a PP3 9v battery.

Here is the built unit. I used a stereo splitter cable from £/$ store to get the wired 3.5mm inline socket. The 2.5mm plug I wired but again it could be obtained from a 2.5mm to 3.5mm audio cable if you didn't want to have to solder one up.

The version 2 of this unit now features a trim pot (22K) in place of R3 ( the wiper connects to C3) so that you can adjust the level of the bias signal sent to the FZ200. Adjust to about 1/3 or the rotation as a starting point. Do a test recording with the unit on and adjust the trim pot until the background noise doesn't increase when you stop talking.

Modifying the Qutaway 5v to 9v Conversion cable for 8.4 Volts

 

This is the pcb from the Qutaway product. The case is easily removed by inserting a sharp blade at each corner and prising the two halve apart.

 

on the left is the input from the USB plug and on the right the output cable.

 

We are going to add a series diode in the red output lead to drop the 9v to 8.4v as shown below.

 

Note that the adaptor featured here will support video recording or burst mode with electronic shutter and single mode shooting with the mechanical shutter operative otherwise the camera demands too much current and resets when used with the FZ2000/2500, on the FZ1000 it is OK.

Here you can see the addition of the GP150 silicon diode (or a 1n54 series diode) connected in series with the red lead.

 

De-solder the red lead from the pcb, insert the diode with the anode (plain end marking) to the pcb and then connect the red cable to the cathode (ringed end) of the diode.

Ensure the diode is clear of the pcb when installed.

Re-fix the covers and add suitable marking to the unit to designate it now to be 8.4 volt output.

 

The completed unit.

 

Amazon link for module

 

Amazon UK http://amzn.to/2w34ZW5

Amazon USA http://amzn.to/2u1w52s