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human
ELI5 How does one yellow RCA cable carry video?

RCA cables only seem to have one pin each. How can one cable carry color video, but stereo audio requires two whole cables? It doesn’t seem like the amount of data is comparable.
gpt
A RCA cable have two wires not just one. One is connects to the inne pin and the other to the outer ring. For all electrics connector you need two conductor. Often the like the 3.5 mm audio connector with 3 wires you have on common ground and a conductor for each audio channel.

The number of wire do not directly determine the amount of data it can carry. Look at the cable you use for Cable TV or over the air TV with two conductor but you can have multiple TV channels with sound and video.The amount of data depend more how you encode the data in the conductor.  So it is a trade off between complexity of electronic in a device versus number of conductors.

Stereo sound uses two pairs because you can then have very simple signals. You can have stereo sound in one pair if you add some electronic to encode it some way that you can later separate them.  But if you use two wires you can have signal in the form you get them from a microphone and can drive a speaker or head phones directly after just amplifying it without any complex electronic conversion. So two conductor pars is a cheap way to do stereo sound. The standard is not from TV but from audio equipment where you just have sound in two part and video copied that. One reason is so you could use audio amplifier you own for better sound the a TV can provide.

For video the signal you have is complex signal and you have complex electronics in the TV that can convert it into separate red, green and blue signal that is used to control the the screen. The signal is identical to how a a TV channel it transmitted analog over the air or in a cable TV network but is moved from a high frequency to low base frequency.  So the complex electronics to split the signal apart exited in all TV. So the simplest way to transfer a video signal was in a format at TV could decode.

The composite signal is how the over the air signal is after it is demodulated by the tuner in the TV. So to add a composite in just add a connector on the outside and something  that can change the input from the tuner to the outside plug.  The tuned have separate output for video and audio so there is not simple way to use only  on one wire with converting  it to a high frequency radio signal. To convert the signal will require stuff that cost more the two extra wires so you avoid it if you can But it was  way to get a VCR or game console into a TV before composite and SCART become standard and then you had sound and image in one wire pair.

There is other way it have been done and in Europe you hade the more complex SCART connector with more wires. It transmitted data  split into a red, green and blue and a sync signal and had the stereo sound in a single quire large 21 pins connector. The quality of the signal was a lot better and old computer that could use it as a output resulted in superior image quality the a single composite cable that was common in the US. The signal is quite close to how a analog VGA video output from a computer work, the image part is identical but SCART have difference sync. You could get some graphic card to output a SCART compatible signal.

So is is not the case that stereo require two pair because cable TV deliver multiple video and stereo audio channels in a singe pair. It is the case that you need less stuff to use to pars. The video is a more complex signal and you can split up version is 4 paris  like in SCART. So it is a trade off between complex electronics vs more conductors.