Now I’m wondering. Raspberry Pi has CSI video input for the camera. Surely it should be easy to connect a composite video to CSI adapter to this input to record video from a composite video source, should it not?
Well, as it turns out, unfortunately it is quite hard, mainly due to the lack of market interest.
20180818/DuckDuckGo composite video to csi camera input
Heree we see that there were plans by Raspberry Pi Foundation to officially release such a board.
20180818/https://www.raspberrypi.org/forums/viewtopic.php?p=209166
20180818/DuckDuckGo raspberry pi camera board composite video input
20180818/DuckDuckGo raspberry pi camera composite video input board hat
Here we see chatting on StackExchange by many people who do not really know what they are talking about. This is only one out of several such similar question articles.
20180818/https://raspberrypi.stackexchange.com/questions/13450/can-i-use-the-composite-video-connecor-to-send-video-into-the-raspberry-pi
Now we see there is this third party that produced this PiCapture adapter that does exactly what you wanted. Technically it is a HAT that fits on top of select Raspberry Pi models, Raspberry Pi Zero not included in this list, unless you use adapters for the GPIO pin dimensions.
20180818/https://www.electronicsweekly.com/blogs/gadget-master/raspberry-pi-gadget-master/picapture-captures-video-raspberry-pi-2016-09/
Indeed, they still are in business and have a site that is alive and well.
20180818/https://lintestsystems.com/
20180818/DuckDuckGo picapture
Unfortunately, upon closer analysis through this review, all the colors are a little bit off from this PiCapture board. Sure, you can correct for this in software, especially via GPU acceleration before encoding to H.264 video, I just need to figure out how to program that, though, and that will take time.
20180818/https://www.raspberrypi.org/magpi/picapture-hd1-review/
I can’t access this page because of a server error, but it looks interesting if I could access it.
broken/https://www.element14.com/community/thread/46092/l/connect-analog-cctv-to-rpi-2?displayFullThread=true
This is an analog-to-digital board, but it doesn’t have enough sampling bandwidth to convert full video. Nope, use it more for motors and mechanical sensors.
20180818/https://www.tindie.com/products/cburgess129/10-bit-adcvms-board-hat-for-raspberry-pi-v21/
This is how you easily mount composite video onto a Raspberry Pi Zero.
20180818/http://www.mrhobbytronics.com/raspberry-pi-video-adapter/
Now I saw mention of another third party adapter named Auvidea. This one only converts HDMI, though.
20180818/DuckDuckGo Auvidea hdmi input raspberry pi
20180818/https://www.raspberrypi.org/forums/viewtopic.php?p=458852
Now this is unfortunate. So you know the official Raspberry Pi Foundation camera video input board that was planned? It turns out that they basically shelved that idea, due to not knowing market demand for it. So third party sources are your only option for your idea.
20180818/https://www.raspberrypi.org/forums/viewtopic.php?f=43&t=24660&start=275#p465911
20180818/https://auvidea.com/
20180818/https://raspberrypi.stackexchange.com/questions/63653/auvidea-b102-and-rpi3-b
Now down this list we have a long list of failed crowdsourcing projects to go and do likewise.
20180818/https://www.geeky-gadgets.com/raspberry-pi-hdmi-input-allows-hd-recording-video-14-03-2014/
20180818/https://www.kickstarter.com/projects/1419380698/connect-your-hd-camcorder-to-your-raspberry-pi?ref=discovery
20180818/https://www.kicktraq.com/projects/1419380698/connect-your-hd-camcorder-to-your-raspberry-pi/