% ffmpeg -i movie/VIDEO_TS/VTS_01_1.VOB -map 0:6 subtitles.ass
ffmpeg version 3.2.4 Copyright (c) 2000-2017 the FFmpeg developers
  built with Apple LLVM version 8.0.0 (clang-800.0.42.1)
  configuration: --prefix=/opt/local --enable-swscale --enable-avfilter --enable-avresample --enable-libmp3lame --enable-libvorbis --enable-libopus --enable-libtheora --enable-libschroedinger --enable-libopenjpeg --enable-libmodplug --enable-libvpx --enable-libsoxr --enable-libspeex --enable-libass --enable-libbluray --enable-lzma --enable-gnutls --enable-fontconfig --enable-libfreetype --enable-libfribidi --disable-indev=jack --disable-outdev=xv --enable-audiotoolbox --enable-sdl2 --mandir=/opt/local/share/man --enable-shared --enable-pthreads --cc=/usr/bin/clang --enable-vda --enable-videotoolbox --arch=x86_64 --enable-yasm --enable-libx265 --enable-gpl --enable-postproc --enable-libx264 --enable-libxvid --enable-nonfree --enable-libfdk-aac
  libavutil      55. 34.101 / 55. 34.101
  libavcodec     57. 64.101 / 57. 64.101
  libavformat    57. 56.101 / 57. 56.101
  libavdevice    57.  1.100 / 57.  1.100
  libavfilter     6. 65.100 /  6. 65.100
  libavresample   3.  1.  0 /  3.  1.  0
  libswscale      4.  2.100 /  4.  2.100
  libswresample   2.  3.100 /  2.  3.100
  libpostproc    54.  1.100 / 54.  1.100
Input #0, mpeg, from 'movie/VIDEO_TS/VTS_01_1.VOB':
  Duration: 00:16:00.54, start: 0.300300, bitrate: 8942 kb/s
    Stream #0:0[0x1bf]: Data: dvd_nav_packet
    Stream #0:1[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, top first), 720x480 [SAR 32:27 DAR 16:9], 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
    Stream #0:2[0x80]: Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s
    Stream #0:3[0x81]: Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s
    Stream #0:4[0x82]: Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s
    Stream #0:5[0x83]: Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s
    Stream #0:6[0x20]: Subtitle: dvd_subtitle
    Stream #0:7[0x21]: Subtitle: dvd_subtitle
    Stream #0:8[0x22]: Subtitle: dvd_subtitle
Output #0, ass, to 'subtitles.ass':
  Metadata:
    encoder         : Lavf57.56.101
    Stream #0:0: Subtitle: ass (ssa)
    Metadata:
      encoder         : Lavc57.64.101 ssa
Stream mapping:
  Stream #0:6 -> #0:0 (dvd_subtitle (dvdsub) -> ass (ssa))
Press [q] to stop, [?] for help
[ssa @ 0x7f8c17000c00] Only SUBTITLE_ASS type supported.
Subtitle encoding failed
Conversion failed!