[srgssr] Add support for VTT subtitles extraction

[stissot]

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2021.01.16
• I've searched the bugtracker for similar site feature requests including closed ones

Description

The SRG/SSR websites (srf.ch, rts.ch, rsi.ch...) provide subtitles and close-captionning on many videos. The extraction of subtitles should be added to the existing srgssr extractor. They are using the WebVTT (VTT) format. Some episodes use clear text VTT segments, while newer episodes mostly the georestricted to Switzerland ones on rts.ch are serving as encrypted VTT stream using the Akamai method. Maybe this should be added to the separate rts extractor?

Example video with encrypted VTT served by Akamai (georestricted to CH):

• https://www.rts.ch/play/tv/doc-a-la-une/video/gluten-lennemi-public-?urn=urn:rts:video:11899965

Example videos with plain VTT (not georestricted):

• https://www.rts.ch/play/tv/les-documentaires-de-la-rts/video/la-suisse-de-nuit-hiver-33?urn=urn:rts:video:11837780
• https://www.srf.ch/play/tv/dok/video/nacht-in-der-schweiz?urn=urn:srf:video:4c60eac2-ec16-4fd1-ba14-2ef57e7b7581

[rkaravia]

Here is a workaround for getting subtitles for srf.ch, posting it here in case it is useful for someone else to download subtitles manually or to add this to the srgssr extractor.

youtube-dl --dump-json "https://www.srf.ch/play/tv/dok/video/nacht-in-der-schweiz-folge-3?urn=urn:srf:video:4c60eac2-ec16-4fd1-ba14-2ef57e7b7581" \
    | jq --raw-output .manifest_url \
    | xargs curl --silent \
    | grep -oP 'EXT-X-MEDIA:TYPE=SUBTITLES.*URI="\K([^"]*)' \
    | xargs ffmpeg subtitles.srt -i

Explanation

Get the JSON dump from youtube-dl:

youtube-dl --dump-json "https://www.srf.ch/play/tv/dok/video/nacht-in-der-schweiz-folge-3?urn=urn:srf:video:4c60eac2-ec16-4fd1-ba14-2ef57e7b7581"

Find the value of manifest_url at the root of the output. Download the file at manifest_url:

curl "https://hdvodsrforigin-f.akamaihd.net/i/vod/dok/2020/12/dok_20201218_113757_2195103_v_webcast_h264_,q40,q10,q20,q30,q50,q60,.mp4.csmil/master.m3u8?start=0.0&end=3001.2&caption=srf/ff81f027-5ece-4766-919e-679beefacc67/episode/de/vod/vod.m3u8:de:Deutsch:sdh&webvttbaseurl=www.srf.ch/subtitles&hdnts=exp=1627222906~acl=*~hmac=4c1d37ba20c663f105a4c2251241d9dd5c813ac45314933eb8ed2a9f44d31e26"

Find the line that contains EXT-X-MEDIA:TYPE=SUBTITLES:

#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="WebVTT",LANGUAGE="de",NAME="Deutsch",DEFAULT=NO,AUTOSELECT=YES,FORCED=NO,CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog, public.accessibility.describes-music-and-sound",URI="https://www.srf.ch/subtitles/srf/ff81f027-5ece-4766-919e-679beefacc67/episode/de/vod/vod.m3u8"

Download the subtitles using ffmpeg:

ffmpeg -i "https://www.srf.ch/subtitles/srf/ff81f027-5ece-4766-919e-679beefacc67/episode/de/vod/vod.m3u8" subtitles.srt