FreeBSD supports the (now old) Brooktree 848 series of video capture devices with the bktr(4) driver. I've owned one of these since the early 2000s, for recording VHS tapes and watching analog video in general.
FFmpeg supports the FreeBSD bktr driver, but for the longest time I couldn't figure out how to select the correct video input. Recording with FFmpeg always produced a video of the noise present at the (unused) coaxial input of my tuner card. There's no mention of input selection in the ffmpeg documentation. But a careful reading of libavdevice/bktr.c revealed that FFmpeg reads several (undocumented) environment variables to set options on the capture card. Notably, BKTR_DEV.
BKTR_DEV is used to select the analog video input on the tuner card. The value of BKTR_DEV is an index into an array defined at line ~83 of libavdevice/bktr.c. Some experimentation is needed to find the correct value; in my case it was '0'. The default is '1'.
In case this isn't completely obvious, a working bourne shell one-liner looks something like:
BKTR_DEV=0 ffmpeg -f bktr -i /dev/bktr0 -c:v libx264 -preset fast test.mkv
And that's it. I've written this up so I won't forget, and on the off chance it might help somebody else. Happy recording.