What does it cost to convert MKV to AAC?

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

How large can the MKV file be?

Up to 100 MB (104,857,600 bytes), and at most 60 min (3,600 s) of media. 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.

catalogue / tools / Audio & video / MKV to AAC

Convert MKV to AAC

To convert MKV to AAC, send one POST to /v1/audio-convert with the MKV source and "to": "aac" in the body. Left out, "to" falls back to its default "mp3", so a AAC request has to send it. 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 100 MB and 60 min (3,600 s) long.

Endpoint: POST /v1/audio-convert
Price: $0.003 per call
Service: audio-convert
Docs: /docs/audio-convert

01Limits

Limit	Value
Largest source file	100 MB (104,857,600 bytes)
Longest input	60 min (3,600 s)
Timeout	94 s
Source formats	AIFF, AVI, FLAC, MKV, MP3, MP4, Ogg, WAV
Output formats	AAC, FLAC, MP3, Opus, WAV

02Example

A real MKV to AAC run of audio-convert.

Request

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

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

Response

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/pair-clip.mkv",
  "bytes": 10531,
  "source_container": "matroska",
  "format": "aac",
  "bitrate_kbps": null,
  "sample_rate": null,
  "audio_bytes": 11183,
  "audio_base64": "//FsQEV//AEQVCwkah2KhMuwwFjUFCWJ/0yl5qpSSvEt559nE+3Stuq5q1X//2HG..."
}

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 recorded run returned 10.9 KB (11,183 bytes) of AAC, 1.1 times the source size, with the source read as the matroska container. Decoded, the AAC in audio_base64 opens with FF F1 6C 40 45 7F FC 01 10 54 2C 24. 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). In the returned AAC: it opens with an ADTS frame header carrying no CRC: MPEG-4 AAC LC, 8,000 Hz, 1 channel, a first frame of 555 bytes, and buffer fullness 0x7FF, the value that marks a variable bitrate stream.

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.

AAC as a target

AAC output is a raw ADTS stream: ffmpeg's aac encoder writes through the adts muxer, and no MP4 container is built around it. Each ADTS frame carries its own header, and the stream is written straight to a pipe without a seekable file. The bitrate_kbps setting applies, but AAC has its own lower ceiling, because it is the slowest encoder of the set and that ceiling keeps a maximum-length input inside the encode deadline. Only the first audio stream is encoded; video, metadata and chapters are dropped, and bit-exact flags keep output reproducible for the pinned ffmpeg. sample_rate is optional, otherwise ffmpeg normally keeps the source rate. AAC is a destination only: a raw ADTS file would pass the MPEG frame-sync sniff, yet nothing proves ffmpeg would receive the right demuxer, so it is not accepted as a source.

04Errors

Code	HTTP
`invalid_input`	`400`
`blocked_target`	`403`
`unreachable_target`	`424`
`upstream_timeout`	`424`
`unsupported_content_type`	`415`
`too_large`	`413`
`unprocessable`	`422`
`internal`	`500`

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

05Questions

What does it cost to convert MKV to AAC?: $0.003 per call, paid in USDC on Base via x402.
How large can the MKV file be?: Up to 100 MB (104,857,600 bytes), and at most 60 min (3,600 s) of media. 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.