Subtitles¶

Subtitles are first-class tracks in unshackle. When you download a title, subtitle tracks are parsed from the manifest alongside video and audio, downloaded, cleaned up, optionally converted to the format you want, and then either muxed into the final Matroska file or written out as sidecar files next to it. The sections below cover the supported formats, how tracks are normalized after download, how to select and filter subtitles, and every subtitle-related configuration option.

Most of this is controlled with a handful of dl flags and a small subtitle block in your configuration file. See Downloading for how subtitle flags fit into a full download command.

Supported formats¶

unshackle understands the following subtitle codecs. The Format column is the value shown in track listings; the Extension column is the file extension used on disk.

Codec (internal name)	Format	Extension	Notes
`SubRip`	SRT	`srt`	SubRip, the most widely compatible plain-text format
`SubStationAlpha`	SSA	`ssa`	SubStation Alpha (styled)
`SubStationAlphav4`	ASS	`ass`	Advanced SubStation Alpha (styled)
`TimedTextMarkupLang`	TTML	`ttml`	Timed Text Markup Language (IMSC/DFXP)
`WebVTT`	VTT	`vtt`	WebVTT
`SAMI`	SMI	`smi`	Synchronized Accessible Media Interchange
`MicroDVD`	SUB	`sub`	MicroDVD
`MPL2`	MPL2	`mpl2`	MPL2
`TMP`	TMP	`tmp`	TMP
`fTTML`	STPP	`stpp`	MPEG-DASH box-encapsulated TTML (IMSC1)
`fVTT`	WVTT	`wvtt`	MPEG-DASH box-encapsulated WebVTT

The two boxed formats convert automatically

fTTML (STPP) and fVTT (WVTT) are the fragmented, MP4-box-encapsulated forms that DASH and HLS deliver. You never keep these as-is: right after download they are unwrapped into their plain counterparts (fTTML becomes TTML and fVTT becomes WebVTT), so you always end up with an editable text subtitle. Note that this is a one-way, source-only relationship: you can convert from these boxed formats, but no backend can produce them as an output target, so they can never be a conversion destination.

What happens to a subtitle after download¶

A subtitle download does more than save a file. Once the bytes are on disk, unshackle runs format-appropriate clean-up to produce a single, valid file:

Encoding: non-boxed subtitles are forced to UTF-8, and stray &lrm; / &rlm; bidirectional entities are unescaped.
Segment merging: when a WebVTT or boxed subtitle arrives as many DASH/HLS segments, the segments are stitched back into one continuous document with correct timing.
WebVTT sanitization: merged WebVTT runs through a repair pipeline that clamps negative timestamps to 00:00:00.000, removes stray cue identifiers (like Q0, Q1) that confuse downstream parsers, and merges multi-line cues that a service split into separate overlapping cues. By default the original styling and layout are preserved (see preserve_formatting).
TTML for muxing: Matroska cannot carry TTML. If you do not request a specific output format, any plain TTML track is automatically converted to WebVTT before muxing (the closest lossless equivalent). Boxed fTTML is likewise unwrapped to TTML first.

You normally do not need to think about any of this. It just produces a clean file.

Selecting and filtering subtitles¶

Subtitle selection happens through dl flags. The most important is language selection.

Flag	Purpose
`-sl`, `--s-lang`	Language(s) wanted for subtitles. Defaults to `all`.
`--require-subs`	Require these languages to exist; if present, download all subtitles. Cannot be combined with `--s-lang`.
`-fs`, `--forced-subs`	Include forced subtitle tracks (excluded by default).
`--exact-lang`	Exact language matching, with no regional variants.
`-S`, `--subs-only`	Download only subtitle tracks.
`-ns`, `--no-subs`	Do not download subtitle tracks at all.
`--sub-format`	Output subtitle format / conversion target (see conversion).
`--skip-subtitle-errors`	Skip a subtitle that fails to download instead of aborting the whole title.

Choosing languages¶

By default (--s-lang all) unshackle keeps every subtitle language the service offers. To narrow it down, pass a comma-separated list of language tags:

English and Spanish subtitles only

unshackle dl --s-lang en,es EXAMPLE 81234567

Language matching is fuzzy by default: en will match en-US, en-GB, and similar variants. If a requested language is missing but others were found, unshackle continues with whatever remains and tells you which languages it dropped.

For strict matching, where es-419 matches only es-419 and never es-ES, add --exact-lang:

Only Latin-American Spanish, no other Spanish variants

unshackle dl --s-lang es-419 --exact-lang EXAMPLE 'https://www.example.com/...'

Requiring languages before downloading¶

--require-subs is a gate rather than a filter. It downloads all available subtitles, but only if every language you list is present; if any is missing the title errors out. This is useful for batch jobs where a title without your must-have language should fail loudly rather than silently produce a partial result.

Fail unless both English and French subs exist, then grab everything

unshackle dl --require-subs en,fr EXAMPLE B0ABCDEFGH

--require-subs and --s-lang are mutually exclusive

Because --require-subs forces downloading of all subtitle languages, it cannot be combined with --s-lang. Choose one or the other.

Forced subtitles¶

Forced tracks carry only the important on-screen signs and foreign-dialogue lines meant to display even when you are watching in your own language. They are excluded by default. Add -fs / --forced-subs to include them:

Include forced tracks alongside full subtitles

unshackle dl --s-lang en --forced-subs EXAMPLE 81234567

Track types: forced, SDH, and CC¶

Every subtitle track carries flags describing what kind of captions it holds. These flags drive selection, sort order, output filenames, and the Matroska track flags.

Forced: only signs and foreign dialogue; meant to play when the audio language matches. Off by default; enable with --forced-subs.
SDH (Subtitles for the Deaf and Hard-of-Hearing; also called HOH): a full transcript that includes both dialogue and non-speech sound cues.
CC (Closed Captions): captions originating from an EIA-608/708 broadcast stream, often uppercase with >>> speaker markers and sound cues.

SDH and CC are treated together for sorting and stripping

Within a language, unshackle orders subtitles from fewest to most captions: Forced → Normal → SDH/CC. For the purposes of hearing-impaired handling, SDH and CC tracks are grouped as the "most captions" variant.

A single track cannot be both CC and SDH, and a forced track cannot also be flagged CC or SDH. These combinations are rejected as invalid.

Closed captions embedded in video¶

Some services do not ship subtitles as separate tracks but instead embed EIA-608/708 captions inside the video stream. After a video track is downloaded and decrypted, unshackle probes it for these captions and, when found, extracts them into a SubRip (SRT) subtitle flagged as CC. This runs automatically unless you disabled subtitles (--no-subs) or video (--no-video / --video-only semantics), and it requires the CCExtractor tool on your PATH.

Automatic SDH stripping¶

When a language has an SDH track but no plain (non-SDH, non-forced) track, unshackle can generate a clean non-SDH version for you by stripping the hearing-impaired cues: sound effects, speaker labels, music notes, and similar. This is on by default and controlled by the strip_sdh config key.

The check is deliberately loose: if a close-enough plain track already exists (for example an en-GB non-SDH track when the SDH track is en-US), no stripped copy is created. When a stripped copy is created, the original SDH track is kept as well, so you end up with both.

The stripping engine itself is chosen by the sdh_method config key. The default (auto) prefers the subby cleaner for SRT, falls back to SubtitleEdit when it is installed, and otherwise uses the built-in filter-subs approach.

Why convert_before_strip exists

subby's SDH stripper only operates on SRT. Handed any other codec it returns silently without stripping anything, not an error but a no-op that leaves the SDH cues in place. This is the whole reason for convert_before_strip: non-SubtitleEdit engines need the subtitle converted to SRT first, or the hearing-impaired cues survive the strip untouched.

Converting subtitle formats¶

By default unshackle keeps each subtitle in its downloaded format (only auto-converting TTML to WebVTT for muxing, as noted above). To force a specific output format, use --sub-format:

Convert every subtitle to SRT

unshackle dl --s-lang en,es --sub-format srt EXAMPLE 81234567

--sub-format accepts the common format names and aliases, including srt, vtt, ssa, ass, and ttml. Pass original to explicitly keep the source format and skip all conversion:

Keep whatever the service delivered

unshackle dl --sub-format original EXAMPLE 'https://www.example.com/...'

Conversion only runs when the source format differs from the target, so --sub-format srt on a track that is already SRT is a no-op.

Down-converting styled subtitles is lossy

Converting styled SSA/ASS subtitles to SRT throws away positioning, colours, and italics. unshackle never does this automatically. It only happens when you explicitly ask for it via --sub-format. If you want to preserve styling, convert to a format that supports it (or keep original).

Note that this automatic protection guards only the default muxed track: there the conversion is skipped and the styled original is kept. It does not cover sidecars. A sidecar_format: srt will still lossily flatten styled subtitles, dropping positioning, colours, and styling, and a per-download --sub-format srt forces the muxed track to convert as well. To keep raw styled sidecars, set sidecar_format: original.

How the conversion backend is chosen¶

Under the hood unshackle picks the highest-fidelity converter that supports the source→target pair. Which converter runs depends on the conversion_method config key (default auto) and the formats involved:

Backend	Best at	Requires
`subtitleedit`	Highest fidelity; preserves positioning and italics	SubtitleEdit installed
`subby`	Native SRT output with clean-up	-
`pysubs2`	Best fidelity for styled SSA/ASS	- (pure Python, always available)
`pycaption`	Last-resort fallback; flattens styling	-

With conversion_method: auto, unshackle ranks these automatically per conversion. Setting it to a specific value pins that backend as the first choice, falling back to others only if the pin cannot handle the pair.

What each backend actually preserves¶

The table below comes from round-tripping a subtitle that carries italics, bold, underline, positioning, and colour through every backend. Each cell lists the styling that survives; — means the backend cannot handle that pair.

Conversion	`subtitleedit`	`pysubs2`	`subby`	`pycaption`
WebVTT → SRT	italic, bold, underline, position	italic, underline	italic, position	(strips all)
WebVTT → ASS	italic, bold, underline, position	italic, bold, underline	—	—
WebVTT → TTML	italic, bold, underline, position	italic, bold, underline	position	(strips all)
ASS/SSA → SRT	all (+ colour)	italic, underline	—	—
ASS/SSA → TTML	position only	italic, bold, underline	—	—
ASS/SSA → VTT	position only	italic, underline	—	—

Reading the table:

Keep the original: leaving --sub-format unset (or set to original) never round-trips the file, so every style survives. Only convert when your player cannot read the source format.
subtitleedit (SubtitleEdit / seconv): the best choice for anything → SRT. It is the only backend that carries colour, and it embeds {\an8} tags to preserve positioning. When writing TTML or WebVTT from ASS it flattens inline styling to plain text and keeps only positioning, so avoid it there.
pysubs2: keeps inline italic/underline on every pair, and bold except when writing SRT or WebVTT. It never carries positioning or colour. SSA/ASS is its native model, which makes it the best pick for SSA↔ASS.
subby: reads only WebVTT/fVTT/SAMI (never ASS) and is tuned for → SRT, where it uniquely converts WebVTT cue settings into {\an8} positioning. Its CommonIssuesFixer may also drop near-duplicate cues.
pycaption: strips all styling; last-resort fallback only.

What auto picks

auto prefers subtitleedit first when it is installed, then falls back to subby for → SRT and pysubs2 otherwise. The one exception comes from the table above: for ASS/SSA → TTML or WebVTT it picks pysubs2 even when SubtitleEdit is installed, since SubtitleEdit flattens inline styling on those pairs. Most installs do not ship SubtitleEdit, so in practice auto means subby (→ SRT) or pysubs2. Install SubtitleEdit / seconv if you need colour or the highest → SRT fidelity.

Config wins over a service's preference

A service may set a preferred_conversion_method on its own tracks (for example when it ships subtitles that a particular backend handles best). An explicit conversion_method in unshackle.yaml always overrides that per-track preference, so your config takes precedence. This only matters when a service ships a non-default preference; leave conversion_method at auto to let the service's hint stand.

Muxed vs. sidecar output¶

By default subtitles are muxed into the final .mkv. You can instead (or additionally) write them as separate sidecar files next to the video, controlled by the output_mode config key:

mux (default)sidecarboth

unshackle.yaml

subtitle:
  output_mode: mux

Subtitles are embedded in the Matroska file. Nothing is written separately.

unshackle.yaml

subtitle:
  output_mode: sidecar
  sidecar_format: srt

Subtitles are written as standalone files and not muxed into the video (when there is a video or audio track to sit beside).

unshackle.yaml

subtitle:
  output_mode: both
  sidecar_format: srt

Subtitles are muxed into the video and written as sidecar files.

Sidecar files are named after the output file with the language and flags encoded in the name, so players and media managers can match them automatically:

Show.S01E01.1080p.WEB.en.srt
Show.S01E01.1080p.WEB.en.forced.srt
Show.S01E01.1080p.WEB.en.sdh.srt

The pattern is {base}.{language}[.forced][.sdh].{extension}. The sidecar file format is set by sidecar_format; use a format name like srt or vtt, or original to keep each subtitle in its downloaded format.

Fonts for styled subtitles¶

Styled SSA/ASS subtitles reference specific fonts by name. When unshackle muxes such a subtitle it scans it for the fonts it uses and attaches the matching font files to the Matroska output so the styling renders correctly on playback. If a referenced font is not available locally, unshackle logs the missing font names and suggests installing the relevant font packages.

Subtitle configuration¶

All of the behaviour above can be defaulted in the subtitle block of your configuration file:

unshackle.yaml

subtitle:
  strip_sdh: true
  preserve_formatting: true
  conversion_method: auto
  sdh_method: auto
  convert_before_strip: true
  output_mode: mux
  sidecar_format: srt

Key	Default	Values	Purpose
`strip_sdh`	`true`	`true` / `false`	Auto-generate a stripped non-SDH copy when a language has only an SDH track.
`preserve_formatting`	`true`	`true` / `false`	Keep original WebVTT styling and layout during sanitization. When `false`, WebVTT is re-encoded more aggressively (merging identical cues, dropping empty ones).
`conversion_method`	`auto`	`auto`, `subby`, `pysubs2`, `subtitleedit`, `pycaption`	Which conversion backend to prefer. `auto` ranks them per conversion.
`sdh_method`	`auto`	`auto`, `subby`, `subtitleedit`, `filter-subs`	Engine used to strip hearing-impaired cues.
`convert_before_strip`	`true`	`true` / `false`	Convert a subtitle to SRT before stripping SDH (for non-SubtitleEdit engines).
`output_mode`	`mux`	`mux`, `sidecar`, `both`	Whether subtitles are embedded, written as sidecars, or both.
`sidecar_format`	`srt`	a format name (e.g. `srt`, `vtt`) or `original`	Format used for sidecar files.

SubtitleEdit is optional but recommended

Several high-fidelity paths (the best-quality conversions and the SubtitleEdit SDH stripper) only activate when the SubtitleEdit CLI is installed and on your PATH. See Installation for how to add it. Without it, unshackle falls back to its built-in Python backends.

Developer reference¶

Audience

This section is for people extending unshackle or writing services. Everyday users can stop here.

The subtitle model lives in unshackle.core.tracks.Subtitle. A few internals worth knowing when working with subtitle tracks in code:

Flags and validation: Subtitle(codec=..., cc=..., sdh=..., forced=...). A track cannot be both cc and sdh, and forced cannot be combined with either; both raise ValueError at construction.
convert(codec, *, forced=False): converts the downloaded file in place through the backend chain in subtitle_convert.run_conversion. forced=True (what --sub-format sets) is the only way lossy styled down-converts such as ASS→SRT are permitted. Raises NotImplementedError if no backend supports the pair and RuntimeError if all attempts fail.
strip_hearing_impaired(): removes SDH cues using the engine selected by config.subtitle["sdh_method"].
reverse_rtl(): fixes right-to-left sentence-ending positioning via SubtitleEdit (requires the binary).
parse(data, codec): the central parser, returning a pycaption.CaptionSet; it handles the boxed DASH formats, SAMI, and the WebVTT repair helpers.
extract_fonts(text): returns the set of font names referenced by an ASS/SSA subtitle, used to decide which fonts to attach at mux time.

The conversion backends (SubtitleEditBackend, SubbyBackend, Pysubs2Backend, PycaptionBackend) each implement a small protocol (is_available(), can_convert(), rank(), convert()) and are tried in rank order, lowest rank first. See the Tracks API reference for the full model.