Convert MKV to SRT (subtitle track)

To convert MKV to SRT (subtitle track), send one POST to /v1/subtitle-extract with the MKV source and "to": "srt" in the body. "srt" is the schema default of "to", so the field may also be left out. Each call costs $0.003 in USDC on Base via x402, with no account and no API key. The source file can be up to 200 MB.

Endpoint

POST /v1/subtitle-extract

Price

$0.003 per call

Service

subtitle-extract

Docs

/docs/subtitle-extract

01Limits

02Example

A real MKV to SRT run of subtitle-extract.

POST /v1/subtitle-extract
content-type: application/json
payment-signature: <base64 x402 payload>

{
  "url": "https://grist.tools/samples/av/pair-clip.mkv",
  "stream": 0,
  "to": "srt"
}

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/pair-clip.mkv",
  "bytes": 10531,
  "container": "matroska",
  "stream_index": 0,
  "format": "srt",
  "cue_count": 2,
  "subtitle": "1\n00:00:00,000 --> 00:00:01,750\nRain taps on the window.\n\n2\n00:00:02,250 --> 00:00:03,750\nA lamp glows in the hall.\n\n"
}

Source file: av/pair-clip.mkv, 10.3 KB (10,531 bytes). 4 s Matroska file made by ffmpeg: a 64x36 PNG video stream at 1 fps (a slate field with a small amber square moving right), a mono 8 kHz FLAC audio track of two 500 Hz triangle beeps, and an English SubRip subtitle stream with two cues. The request sets stream to 0. The recorded run returned 117 bytes of SRT, 1% of the source size, with the source read as the matroska container, stream index 0 and 2 cues. Recorded with ffmpeg 8.1.2.

In the sample itself, the EBML header names DocType matroska, version 4, readable by a version 2 parser; Segment Info sets a timestamp scale of 1,000,000 ns and a duration of 4,000 ticks, 4 s, with Lavf as the muxing application; the Tracks element lists codec IDs V_MS/VFW/FOURCC (video, language und), A_FLAC (audio, language und) and S_TEXT/UTF8 (subtitle, language eng). The returned text starts with cue number 1. Its 2 timing lines are 00:00:00,000 --> 00:00:01,750 and 00:00:02,250 --> 00:00:03,750, giving hours, minutes and seconds, and a comma before the milliseconds. Every cue has a number line above its timing, 1 and 2.

03Format notes

MKV as a source

Matroska files begin with the EBML magic 1A 45 DF A3, and WebM uses the same signature, so the sniffer treats both as a single container and reports it as matroska in the container field of each response. Matroska is built from tracks that ffmpeg can address independently. When audio is the target, the first audio track is decoded and any picture is ignored. For a still frame, the requested timestamp is sought at the demuxer level before one picture is decoded, with sound excluded. A subtitle track is chosen by its index counted among subtitle tracks only, starting from zero, and an index that matches nothing is refused rather than answered with an empty file. SubRip or WebVTT text written by ffmpeg comes back together with a cue count taken from the timing arrows in that text.

SRT as a target

SRT written by subtitle-convert numbers its cues sequentially from one and prints every timestamp as HH:MM:SS,mmm with a comma before the milliseconds, hours always included. When the source is WebVTT, each cue is rebuilt from its start, end and text, so cue identifiers, positioning settings and NOTE blocks do not survive. SubRip is also the default destination of subtitle-extract, which pulls a text subtitle stream out of a media container and writes it with ffmpeg's srt muxer instead of the JavaScript serialiser. That route refuses a stream that produces no text rather than returning an empty file, and it reports a cue count obtained by counting the timing arrows in the output. The two routes share the destination format but not the code that writes it. In the subtitle-convert file, each block stacks its index line, its timing line and its caption text, consecutive blocks are parted by one empty line, and one line feed closes the file.

Note: The container must carry a text subtitle track; image-based subtitles cannot be turned into text.

04Errors

When each is raised, and what it means for payment: /docs/subtitle-extract.

05Questions

What does it cost to convert MKV to SRT?

$0.003 per call, paid in USDC on Base via x402.

How large can the MKV file be?

Up to 200 MB (209,715,200 bytes). Past a limit the call answers too_large (413).

What happens if the conversion fails?

The tool answers with a typed JSON error from the errors table. Payment is settled only after the tool has produced its result; a call that fails inside the tool is never settled.

06Related

• Audio & video guide
• AVI to SRT
• MP4 to SRT
• Ogg to SRT
• WebVTT to SRT