MP4Box -info ./filename.m4v
#Media Info: Language "deu (deu)" - Type "subp:mp4s" - 904 samples
MP4Box -raw 9 ./filename.m4v
../build/bin/vobsub2srt filename_track9
#this makes filename_track9.srt from filename_track9.idx filename_track9.sub

#Media Info: Language "eng (eng)" - Type "sbtl:tx3g" - 1949 samples
MP4Box -srt 4 input.mp4  # take track 4 in sbtl:tx3g and save as .srt file