I needed to make an AV-Cable for a monitor but the vendor did not supply an appropriate cable so I had to make my own cable.
The user manual stated that was an optional breakout-cable to connect a video signal, left- and right audio channel to the 3,5 mm headphone style jack. Unfortunately no info whatsoever on the pin out.
Fortunately this is not a huge problem when using ones brain a bit.
Know information. There are 3 signals + a ground connection makes 4 wires. So I sacrificed the headset from my old mobile phone and removed the leads from the plug. The I plugged the jack into the socket and now it was time to get my multimeter out and measure some stuff
First of I needed to find the ground pin so I set my multimeter to continuity test. and used the know ground pin of the monitors headphone jack to check which pin the ground pin is. I this case it was ring 1. Just for good measure I checked if the resistance between those two was zero. It was, so the ground pin was determined.
To find the video pins I left my multimeter set to the ohms range and measured the resistance between ground and and the other rings/tip. Between ground and the second pin I measured 74.9 Ohms which is surprisingly close to the 75 Ohms impedance of a FBAS video signal. So the video pin was determined as well.
Next where the 3rd ring and the tip (or ring 4). Both measured a resistance to ground of about 20 kilo Ohms which is a typical input resistance of an audio input. So the audio input was determined as well. Since my camera only delivers mono audio I did not care about which one was the left- and the right channel and connected both together to the audio output of my camera. I you need to know just touch the contacts individually with you finger and listen for the mains hum ;-)