The 4Kopen board has support for analog CVBS output via a 3.5mm TRRS jack. This connector provides composite video, and stereo audio - e.g: Yellow, White and Red RCA connectors.

This can be enabled by running the commands below:

media-ctl -l '"Main-VID":1->"analog_hdout0":0[0]'
media-ctl -l '"Main-VID":1->"analog_sdout0":0[1]'
media-ctl -l '"Main-PIP":1->"analog_hdout0":0[0]'
media-ctl -l '"Main-PIP":1->"analog_sdout0":0[1]'

This will present framebuffer 1 (/dev/fb1) on the CVBS output, with the video planes visible as well.

You will also need to alter the graphics plane's transparency - either by drawing a mask, or by setting it to fully transparent:

stfbset -f /dev/fb1 -a 0

To decode video, you must use the stdisplaysink element, and specify plane-name="Aux-GDP2", as follows:

gst-launch-1.0 videotestsrc ! videoconvert ! stdisplaysink plane-name="Aux-GDP2"

Or:

gst-launch-1.0 filesrc location=Sintel.2010.720p.mkv ! matroskademux name=x \
    x.video_0 ! stvideo ! stdisplaysink plane-name="Aux-GDP2"