Convert AVI to SRT (subtitle track)

To convert AVI to SRT (subtitle track), send one POST to /v1/subtitle-extract with the AVI 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

No real AVI to SRT example: ffmpeg's AVI muxer writes no text subtitle stream (only DivX XSUB bitmaps), so there is no real AVI with captions to extract. The example below is the service's own.

The published example of subtitle-extract, verbatim. Its sample file may be in another source format than AVI. Send to: "srt" to get SRT.

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

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

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/subtitle-movie.mkv",
  "bytes": 1486,
  "container": "matroska",
  "stream_index": 0,
  "format": "srt",
  "cue_count": 2,
  "subtitle": "1\n00:00:00,500 --> 00:00:02,000\nThe kettle starts to hum.\n\n2\n00:00:02,500 --> 00:00:04,000\nSteam rises from the spout.\n\n"
}

03Format notes

AVI as a source

AVI is a RIFF container whose form type at byte eight is the four characters AVI followed by a space. Grist checks that form type, not the extension, before ffmpeg sees the file. Three of the conversion services on these pages accept it. audio-convert reads the first audio stream and ignores video; video-thumbnail seeks at the demuxer and decodes exactly one frame with audio disabled; subtitle-extract maps a subtitle stream by its zero-based index. That last route has a practical gap: ffmpeg's AVI muxer writes no text subtitle stream, only DivX XSUB bitmaps, so no real AVI with text captions could be produced for an example, and the AVI subtitle pages carry none. An AVI whose subtitle index matches nothing is refused rather than answered with an empty result. Each response reports the sniffed container as avi alongside the size of the fetched file.

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 AVI to SRT?

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

How large can the AVI 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
• MKV to SRT
• MP4 to SRT
• Ogg to SRT
• WebVTT to SRT