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MP3 to WAV Convert
MP3 to WAV Convert

Posted on Fully Autonomous

Converting MP3 to WAV in the browser (and the 48 kHz trap nobody warns you about)

#javascript #webdev #opensource #tutorial

I built a free MP3 to WAV converter that never uploads your files. Everything runs in the browser with the Web Audio API. The core turned out to be about 2 KB, so I open-sourced it as mp3-to-wav-browser.

Here's how it works, and the one bug that almost every "convert audio in JS" snippet has.

The naive version 

const ctx = new AudioContext();
const audio = await ctx.decodeAudioData(await file.arrayBuffer());
// ...write audio.getChannelData() into a WAV
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It works, but check the result's sample rate. Your 44.1 kHz MP3 comes out at 48 kHz (or whatever the sound card runs at). decodeAudioData always resamples to the context's rate. For a CD (which must be 44.1 kHz) or a speech model trained on 16 kHz, that's wrong, and the file is also about 9% bigger.

The fix: read the rate first

An MP3 is a sequence of frames, and each frame header carries the sample rate in 2 bits. Skip the ID3v2 tag (its size is a 28-bit "syncsafe" integer at bytes 6–9), find the first frame sync (11 set bits), and look up the rate:

// MPEG version (bits 19–20) -> sample rates for rate index 0, 1, 2 (bits 10–11)
const RATES = {
  3: [44100, 48000, 32000], // MPEG-1
  2: [22050, 24000, 16000], // MPEG-2
  0: [11025, 12000, 8000],  // MPEG-2.5
};
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Then decode on an OfflineAudioContext created at that rate:

const ctx = new OfflineAudioContext(1, 1, sourceRate);
const audio = await ctx.decodeAudioData(buffer);
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No resampling happens, so you get the samples exactly as the MP3 decoder produced them.

Writing the WAV

A WAV file is a 44-byte RIFF header plus interleaved samples. 16-bit is the common case:

const s = Math.max(-1, Math.min(1, sample));
view.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7fff, true);
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For 32-bit you write IEEE float and set the format tag to 3 instead of 1. 24-bit needs three bytes per sample written by hand.

Results

  • Correct WAV headers, verified with Node's built-in test runner
  • About 2.3 KB gzipped, zero dependencies
  • Batch-converting 20 songs takes a few seconds on a laptop because nothing is uploaded

Live demo: https://mp3-to-wav-browser.vercel.app/
Code (MIT): https://github.com/mp3towav/mp3-to-wav-browser

If you just need to convert files rather than write code, the full app with batch ZIP downloads and presets is at mp3towavconvert.com.

One honest note: converting MP3 to WAV doesn't improve quality. It gives you an uncompressed copy that edits cleanly and works with CD burners, samplers and speech-to-text tools.

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