Your suspicion about subtitles is correct. It is burning them into the video
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] MDE: analyzing media item 34132
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] MDE: E1 - Un départ et des amis: Direct Play is disabled
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] MDE: E1 - Un départ et des amis: media must be transcoded in order to use the dash protocol
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] MDE: E1 - Un départ et des amis: selected subtitle cannot be converted to a compatible format, burning into video stream
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] MDE: E1 - Un départ et des amis: avoiding video remux due to burned subtitle stream
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] MDE: E1 - Un départ et des amis: no remuxable profile found, so video stream will be transcoded
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] MDE: Cannot direct stream video stream due to profile or setting limitations
Aug 14, 2023 13:48:34.214 [140185723509560] DEBUG - [Req#2cb/Transcode] Codecs: testing hevc (decoder) with hwdevice vaapi
Aug 14, 2023 13:39:37.279 [140111344708408] DEBUG - [Req#1094b0/Transcode] TPU: hardware transcoding: final decoder: vaapi, final encoder: vaapi
Aug 14, 2023 13:39:37.280 [140111324777272] DEBUG - We're going to try to auto-select an audio stream for account 1.
Aug 14, 2023 13:39:37.280 [140111324777272] DEBUG - Selecting best audio stream for part ID 36106 (language: fr)
Aug 14, 2023 13:39:37.280 [140111324777272] DEBUG - We're going to try to auto-select a subtitle.
Aug 14, 2023 13:39:37.280 [140111324777272] DEBUG - Subtitles: Found a candidate subtitle language [fr] for a foreign film
Aug 14, 2023 13:39:37.280 [140111324777272] DEBUG - Audio Stream: 103790, Subtitle Stream: 103792
Aug 14, 2023 13:39:37.280 [140111344708408] DEBUG - [Req#1094b0/Transcode/JobRunner] Job running: FFMPEG_EXTERNAL_LIBS='/config/Library/Application\ Support/Plex\ Media\ Server/Codecs/e51a01b-4528-linux-x86_64/' LIBVA_DRIVERS_PATH=/usr/lib/plexmediaserver/lib/dri XDG_CACHE_HOME="/config/Library/Application Support/Plex Media Server/Cache" XDG_DATA_HOME=/usr/lib/plexmediaserver/Resources X_PLEX_TOKEN=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx "/usr/lib/plexmediaserver/Plex Transcoder" -codec:0 hevc -hwaccel:0 vaapi -hwaccel_fallback_threshold:0 10 -hwaccel_device:0 vaapi -codec:1 ac3 -analyzeduration 20000000 -probesize 20000000 -i "/volume1/video/Animes/Hunter X Hunter (2011)/Hunter x Hunter (2011) - S01E01 - Departure x and x Friends.mkv" -map_inlineass 0:3 -filter_complex "[0:0]scale=w=1920:h=1080:force_divisible_by=4[0];[0]format=pix_fmts=nv12[1];[1]inlineass=font_scale=1.000000:font_path=/usr/lib/plexmediaserver/Resources/Fonts/NotoSans-Medium.otf:fontconfig_file=/usr/lib/plexmediaserver/Resources/fonts.conf:language=fr[2];[2]hwupload[3]" -map "[3]" -metadata:s:0 language=jpn -codec:0 h264_vaapi -b:0 20000k -r:0 23.975999999999999 -force_key_frames:0 "expr:gte(t,n_forced*1)" -filter_complex "[0:1] aresample=async=1:ochl='stereo':rematrix_maxval=0.000000dB:osr=48000[4]" -map "[4]" -metadata:s:1 language=fra -codec:1 aac -b:1 256k -f dash -seg_duration 1 -dash_segment_type mp4 -init_seg_name 'init-stream$RepresentationID$.m4s' -media_seg_name 'chunk-stream$RepresentationID$-$Number%05d$.m4s' -window_size 5 -delete_removed false -skip_to_segment 1 -time_delta 0.0625 -manifest_name "http://127.0.0.1:32400/video/:/transcode/session/g6av2tgkluo69hjvwhuk51tm/49fe525a-c24e-4204-96e6-0228b71d2511/manifest?X-Plex-Http-Pipeline=infinite" -avoid_negative_ts disabled -map_metadata -1 -map_chapters -1 dash -map 0:3 -f null -codec ass nullfile -start_at_zero -copyts -vsync cfr -init_hw_device vaapi=vaapi:/dev/dri/renderD128,driver=i965 -filter_hw_device vaapi -y -nostats -loglevel quiet -loglevel_plex error -progressurl http://127.0.0.1:32400/video/:/transcode/session/g6av2tgkluo69hjvwhuk51tm/49fe525a-c24e-4204-96e6-0228b71d2511/progress
Synology CPU don’t have enough speed/power to do that
Some playback devices will accept the ASS subtitles and some won’t.
In this case, it’s not able to play ASS as a 3rd stream so it’s going to burn in.
