Digitize a VHS tape with FFmpeg and get a live preview
The tee muxer is suitable for this.

ffmpeg \
-f v4l2 -standard PAL -thread_queue_size 2048 -i /dev/video1 \
-f alsa -thread_queue_size 2048 -i hw:2,0 \
-map 0 -map 1 \
-vf yadif=1 \
-vcodec libx264 -preset superfast -crf 23 -flags +global_header \
-acodec libmp3lame -b:a 128k -ac 2 -ar 48000 \
-f tee "vhs_output.mov|[f=nut:onfail=ignore]pipe:1" | ffplay -