How to record the video to sdcard?

I tried to modify the configuration file openhd-settings-1.txt of the air part. I changed the option “VIDEO_TMP=sdcard” . When I start the air/ground , video transmission works well. When I turn off the air part, no video recorded in the sdcard. The hardware is pi zero for the air, pi 4b for the ground. Software is 2.0.8