Playground

Speech to Text formats

Which files the Speech to Text page accepts, what the browser does to each format before upload, what a microphone take is saved as, and what the transcript is.

This page is the lookup table for what goes into the Speech to Text page and what comes out: the seven file formats it accepts, what your browser does to each one before it is sent, what a microphone take is saved as, the checks that run once the file has been sent, and what the transcript contains. For the limits and the credit rate, see Speech to Text limits and credits.

Accepted files

The page accepts a file by its extension. The file picker filters to these types; a file dropped on the drop area is checked the same way.

Format Extension Before upload
MP3 .mp3 Converted to WAV by the browser
WAV .wav Sent unchanged
FLAC .flac Sent unchanged
OGG .ogg Sent unchanged
M4A .m4a Converted to WAV by the browser
AAC .aac Converted to WAV by the browser
WEBM .webm Converted to WAV by the browser

Any other extension is refused before upload with Please upload one of: MP3, WAV, FLAC, OGG, M4A, AAC, WEBM. The badges under the drop area name four of the seven; all seven are accepted.

What the browser does before upload

A WAV, FLAC or OGG file is sent as you chose it. Every other accepted format is prepared in the browser first:

  1. The browser decodes the file.
  2. Stereo is mixed down to one channel; both channels are averaged, so a call recording with one quiet side keeps both voices.
  3. The audio is resampled to 16 kHz.
  4. The result is written as a 16-bit WAV, named after your file with a .wav extension, and that copy is sent.

What this means for you:

  • Playback uses the converted copy. The player on the transcript card, the play buttons in the Transcripts list and the audio kept in Assets all carry the 16 kHz mono WAV, not the file you chose. The name shown on the transcript card is still your original file name.
  • The 150 MB limit is checked twice: on your original file before conversion, and on the converted copy after it. One hour of 16 kHz mono WAV is about 115 MB, so a converted file inside the 1-hour limit always passes the second.
  • Preparation takes time. The status line shows Preparing audio… while the browser works. A long MP3 can hold there for a while before Uploading and its percentage appear.
  • A file the browser cannot decode is sent unchanged. The transcription then ends with a reason that starts with Audio could not be decoded and the charge is refunded. Export the recording again from the application that made it.

Microphone takes

Record captures the take in the page itself, as 16 kHz mono 16-bit WAV, whichever browser you use. It needs a browser with microphone capture and audio worklet support; otherwise the page says Your browser does not support audio recording.

What you record in Recorded as Extension of the take Before upload
Any supported browser 16 kHz mono WAV .wav Sent unchanged

The take is named recording- followed by a number, and it goes through the same checks as an uploaded file. A take stops on its own at 1 hour. Press Transcribe to send it.

Checks after upload

Once the file has been sent, more checks run before it is accepted and charged. Each refusal shows in the status line under the box, followed by a short code in square brackets, with a Try again button.

Check Rule Message
File type The file must be audio Unsupported file type: … Please upload an audio file., naming the type that was sent
Size 150 MB or less File too large
Length 1 hour or less, measured by decoding the audio Audio exceeds the 1-hour limit.
Readable The audio length can be measured Could not read the audio length. Try another file.

The server measures the length by decoding the audio, not from the file's header, and the credit charge uses that measured length. See Speech to Text limits and credits.

After acceptance, the audio is decoded. WAV, FLAC and OGG with Opus decode; a stereo file is averaged to mono; audio shorter than 50 milliseconds ends with a reason that starts with Audio is too short. Every failure after acceptance refunds the charge.

The transcript

Fact Value
Language Auto, Arabic or English, picked on the page. Auto lets the service decide, which is Arabic today.
Shape With Identify speakers on, the default: rows in the order they were said, each with its start time and its speaker. With it off: one block of text per file. No word timestamps and no confidence figures.
Shown on the page A transcript card with the file name, its length and the language, and three tabs: Text (the rows or the block), Subtitles (SRT) and JSON (the files themselves).
Saved as A TXT, SRT or JSON file, matching the open tab, named after your file with its extension replaced, from Download; Copy copies the open tab instead. SRT holds cues of at most 7 seconds, each starting with its speaker; a transcript in one block gives one cue for the whole file.

How to read the result view, and where the transcript is kept afterwards, is on Read a transcript.

Worked example

You drop memo.m4a, a 3-minute voice memo from your phone, on the drop area. The extension passes, the browser reads its length as 3:00, and you press Transcribe. The status line shows Preparing audio… while the memo is decoded, mixed to mono, resampled to 16 kHz and written as memo.wav, about 5.8 MB. It then shows Uploading with a percentage and switches to Transcribing.

When the transcript arrives, the transcript card is titled memo.m4a, the player plays the WAV copy, and Download on the Text tab saves memo.txt.

FAQ

My file is an MP3. Will it work?

Yes. The browser converts it to WAV before upload. If the conversion fails, the transcription reports it and the charge is refunded.

Why is the downloaded audio a WAV when I uploaded an M4A?

The page transcribes the converted copy and keeps that copy. Keep your original if you need it in its own format.

Why is my one-hour WAV refused when a one-hour MP3 is accepted?

The 150 MB limit applies to the file as you chose it. A one-hour MP3 is about 57 MB; a one-hour stereo 44.1 kHz WAV is far larger. Export the WAV as MP3, or as 16 kHz mono, before you upload it.

Can I get timestamps or speaker labels?

Yes. With Identify speakers on, which is the default, each row of the transcript shows its start time and its speaker, and the Subtitles (SRT) tab and its download carry both. There are no timestamps for single words. See Read a transcript.

Does a microphone take sound different after conversion?

The page records the take at 16 kHz mono, which is what transcription uses, so no conversion happens afterwards. The transcript is unaffected.