What does it cost to convert MP4 to WAV?

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

How large can the MP4 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 / MP4 to WAV

Convert MP4 to WAV

To convert MP4 to WAV, send one POST to /v1/audio-convert with the MP4 source and "to": "wav" in the body. Left out, "to" falls back to its default "mp3", so a WAV 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 MP4 to WAV 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.mp4",
  "to": "wav"
}

Response

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/pair-clip.mp4",
  "bytes": 11315,
  "source_container": "mp4",
  "format": "wav",
  "bitrate_kbps": null,
  "sample_rate": null,
  "audio_bytes": 64044,
  "audio_base64": "UklGRiT6AABXQVZFZm10IBAAAAABAAEAQB8AAIA+AAACABAAZGF0YQD6AAAA0ADc..."
}

Source file: av/pair-clip.mp4, 11 KB (11,315 bytes). 4 s MP4 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 mov_text subtitle stream with two cues. The recorded run returned 62.5 KB (64,044 bytes) of WAV, 5.7 times the source size, with the source read as the mp4 container. Decoded, the WAV in audio_base64 opens with 52 49 46 46 24 FA 00 00 57 41 56 45 (ASCII RIFF and WAVE). Recorded with ffmpeg 8.1.2.

In the sample itself, the ftyp box names major brand isom, minor version 512 and compatible brands isom, iso2 and mp41; the top-level boxes run ftyp, free, mdat and moov, so the moov index comes after the media data; mvhd counts 4,000 ticks at 1,000 per second, 4 s; the 3 tracks are vide with sample entry mp4v, soun with sample entry fLaC and sbtl with sample entry tx3g. In the returned WAV: the RIFF header counts 64,036 bytes after itself, and the fmt chunk, in its own field order, declares audio format 1 (integer PCM), 1 channel, 8,000 Hz, 16,000 bytes per second, a block align of 2 bytes and 16 bits per sample. The data chunk right after it holds 64,000 bytes of samples, 4 s at that rate.

03Format notes

MP4 as a source

MP4 belongs to the ISO base-media family, and Grist recognises it by the ftyp box type at bytes four to eight rather than by a file extension. QuickTime MOV and other files carrying the same box are sniffed as this one container, and the response names it mp4. The container can hold video, audio and text subtitle streams side by side, and each service takes one kind. audio-convert maps the first audio stream and drops the picture. video-thumbnail puts the seek before the input, so it happens at the demuxer and stays cheap in a large file, then decodes exactly one frame from the first video stream. subtitle-extract selects a text track by its zero-based position among subtitle streams. Each ffmpeg run strips container metadata and sets bit-exact flags, so outputs do not carry creation timestamps.

WAV as a target

WAV output is written with the pinned pcm_s16le codec, so every result holds signed little-endian PCM regardless of the source encoding. The WAV muxer fills in its RIFF chunk sizes when the file closes, so Grist points ffmpeg at a seekable temporary file inside the per-call directory and reads it back, rather than streaming to a pipe. A bitrate_kbps value is rejected for this lossless destination instead of being ignored. The optional sample_rate must come from the service's closed set; left out, ffmpeg chooses, normally keeping the source rate. Only the first audio stream is mapped, with video, metadata and chapters removed, and bit-exact flags make the bytes reproducible for the pinned ffmpeg. PCM from a lossy source contains the decoded samples at full size, without restoring detail the earlier codec discarded.

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 MP4 to WAV?: $0.003 per call, paid in USDC on Base via x402.
How large can the MP4 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.