The original Raspberry Pi’s had a dedicated composite video (yellow) connector and a stereo 1/8″ “headphone” jack. To save on space and cost, later versions of the Pi had only a single Tip Ring Ring Sleeve (T-R-R-S) 1/8″ jack, which is able to carry both audio and video. This one connector will give you Left / Right audio and Composite Video (Typically represented by Red, White, and Yellow RCA Cables)
Although there is no standard for how audio and video is separated on a T-R-R-S connector, most cables on the market follow a convention that is different than the Raspberry Pi. This can be a bit problematic.
Pi TRRS Connections
The pinout for the connector on the pi is shown below:
Notice that the ground is the second ring, and not the sleeve is the ground. This is so that if you insert a standard stereo 1/8″ plug into the Pi, you will have left and right audio with ground.
Typical T-R-R-S Connections
A typical breakout cable has the following pinout:
Notice that the ground is connected to the sleeve, and not the last ring.
If you are attempting to use a typical connector, All of the ground shields on your cable will be connected to the signal line of the Composite Video. This will not work.
If you wish to only get video from such a cable and you are not interested in any audio, leave the red and white connectors from your plug disconnected! Then reverse the polarity of your yellow wire. One simple way of doing this is by using alligator clips to connect the sleeve of one RCA connector to the tip of the other (and vice versa). Be careful not to short the pin and shield, however, or you will lose your signal. Below is a simple example: