MPEG-4 Audio
Convert M4A audio to MP3, WAV, or OGG directly in your browser — no upload, no server, no quality loss from intermediate steps. FileDex uses FFmpeg WebAssembly to decode AAC or ALAC audio from the MPEG-4 container and re-encode locally.
Your files never leave your device
Common questions
What is the difference between M4A and MP4?
M4A and MP4 use the identical MPEG-4 Part 14 container format. The only difference is intent — .m4a signals audio-only content while .mp4 may contain video, audio, and subtitles. Renaming .m4a to .mp4 (or vice versa) does not change the file structure.
Can I convert M4A to MP3 without losing quality?
If the M4A contains AAC audio (lossy), converting to MP3 introduces generation loss — both are lossy codecs and re-encoding discards additional audio data. If the M4A contains ALAC (lossless), decoding to PCM is lossless, but the MP3 encoding step is lossy. Use the highest practical MP3 bitrate (VBR -q:a 2 or CBR 320 kbps) to minimize further degradation.
How do I tell if my M4A file contains AAC or ALAC?
Run ffprobe -v error -show_entries stream=codec_name input.m4a. AAC files report 'aac' as the codec, while Apple Lossless files report 'alac'. File size is also a strong indicator — a 4-minute ALAC track is typically 25-35 MB, while AAC at 256 kbps is around 7.5 MB.
Why can my Android phone play M4A but my car stereo cannot?
Most car stereos support MP3 and sometimes WMA, but lack AAC decoders required for M4A playback. Android has built-in AAC-LC support since version 1.0. Convert M4A to MP3 for car stereo compatibility.
Does FileDex upload my M4A file to a server?
No. FileDex converts M4A files entirely in your browser using FFmpeg compiled to WebAssembly. The audio data never leaves your device — all decoding and encoding happens locally in a sandboxed Web Worker.
What makes .M4A special
Stripped of video tracks, M4A is Apple's answer to a simple question: why ship a full MP4 container when the file holds only audio? The format uses the identical ISOBMFF (ISO Base Media File Format) box structure as MP4 but signals its audio-only nature through the ftyp box brand. "M4A " (with a trailing space, hex 4D 34 41 20) marks AAC-encoded files; "M4B " marks audiobooks carrying chapter markers and bookmark data. iTunes reads this brand byte to route downloads into Music or Books — swap the four bytes and the same file lands in a different library.
Continue reading — full technical deep dive
Internal structure
An M4A file contains the same box hierarchy as MP4: ftyp, moov, and mdat. The moov box holds all metadata — track headers, sample tables, timing info. The mdat box stores raw compressed audio frames. A stbl (sample table) box inside moov maps each audio frame to its byte offset in mdat, enabling sample-accurate seeking without scanning the entire file.
The default audio codec is AAC-LC (Low Complexity), though M4A also supports HE-AAC v1 (SBR), HE-AAC v2 (SBR + Parametric Stereo), and Apple Lossless (ALAC). When ALAC is used, the file is sometimes given an .m4a extension and sometimes .alac, but the container format is identical.
AAC-LC performance in M4A
| Bitrate | Stereo quality | Typical use |
|---|---|---|
| 96 kbit/s | Good for speech/podcasts | Podcast distribution |
| 128 kbit/s | Transparent for most listeners | iTunes Store default (pre-2023) |
| 256 kbit/s | Transparent | Apple Music AAC tier, iTunes Plus |
| ~900 kbit/s | Lossless (ALAC, 16-bit/44.1 kHz) | Apple Music Lossless tier |
Apple Music streams AAC at 256 kbit/s for its standard tier. The lossless tier ships ALAC inside the same M4A container, bumping file sizes roughly 3-4x.
Metadata: MP4 atoms vs. ID3
M4A does not use ID3 tags. Metadata lives in udta (user data) and ilst (item list) boxes inside moov. Common atoms: ©nam (title), ©ART (artist), ©alb (album), covr (cover art as embedded JPEG or PNG). Gapless playback data is stored in an iTunSMPB atom — a proprietary Apple extension that records encoder delay and padding samples so players can trim silence between tracks.
Compatibility and limitations
Every Apple device plays M4A natively. Android has supported AAC-in-M4A since API level 1. Windows plays M4A through the Media Foundation AAC decoder, included since Windows 7. Browser support via <audio> tags covers Chrome, Safari, Edge, and Firefox (since version 22).
The main limitation is tooling assumptions. Some audio editors still treat M4A as a video format and reject it on import. Batch processing scripts built for MP3/WAV pipelines may need explicit M4A handling. The container overhead is minimal — roughly 1–3 KB for the moov box on a typical 4-minute track — so there is no meaningful size penalty versus raw AAC in ADTS framing.
When to use M4A over MP3
M4A with AAC-LC at 128 kbit/s matches MP3 quality at 192 kbit/s. For podcast distribution, 96 kbit/s AAC in M4A cuts bandwidth nearly in half compared to 128 kbit/s MP3 with no perceptible loss. Choose MP3 only when targeting hardware that lacks AAC decoding — an increasingly rare scenario. FileDex converts between M4A, MP3, FLAC, WAV, and other audio formats entirely in the browser with no server upload.
.M4A compared to alternatives
| Formats | Criteria | Winner |
|---|---|---|
| .M4A (AAC) vs .MP3 | Audio quality at 128 kbps AAC-LC in M4A uses improved MDCT windowing and temporal noise shaping, producing 20-30% better perceived quality than MP3 at the same bitrate. The difference is most audible below 128 kbps. | M4A (AAC) wins |
| .M4A (AAC) vs .OGG VORBIS | Browser support OGG Vorbis has native support in Chrome, Firefox, and Edge without codec licensing. M4A/AAC playback in Firefox on Linux historically required GStreamer AAC plugins, though modern distributions include them. | OGG VORBIS wins |
| .M4A (ALAC) vs .FLAC | Cross-platform support FLAC is natively supported on Windows, Linux, Android, and most hardware players. ALAC-in-M4A is primarily supported on Apple platforms — Android and Linux support depends on third-party decoders. | FLAC wins |
| .M4A vs .WAV | File size (3-minute song) AAC at 256 kbps produces roughly 5.7 MB for a 3-minute track, while WAV at 16-bit 44.1 kHz requires approximately 30 MB — a 5:1 ratio with perceptually transparent quality from AAC. | M4A wins |
Convert .M4A to...
Technical reference
- MIME Type
audio/mp4- Magic Bytes
00 00 00 xx 66 74 79 70ftyp box with M4A or mp42 brand.- Developer
- Apple Inc. / ISO
- Year Introduced
- 2001
- Open Standard
- Yes
ftyp box with M4A or mp42 brand.
Binary Structure
M4A files use the ISO Base Media File Format (ISOBMFF), identical to MP4 but restricted to audio streams. The file begins with a size prefix (4 bytes big-endian) followed by the ftyp box containing the brand identifier 'M4A ' (4D 34 41 20). The moov box holds all metadata: mvhd (movie header with timescale and duration), trak (one per audio stream), and udta/meta (iTunes metadata atoms). Each trak contains mdia → minf → stbl, which indexes sample offsets, sizes, and timing via stts, stsc, stsz, and stco/co64 boxes. The actual compressed audio frames reside in one or more mdat boxes. The moov box position is critical — if moov follows mdat (common after recording), players must download the entire file before playback starts. The -movflags +faststart flag in FFmpeg relocates moov before mdat for streaming.
| Offset | Length | Field | Example | Description |
|---|---|---|---|---|
0x00 | 4 bytes | Box Size | 00 00 00 20 | Size of the ftyp box in bytes (big-endian). Value 0x20 = 32 bytes for a standard M4A ftyp. |
0x04 | 4 bytes | Box Type | 66 74 79 70 (ftyp) | FourCC identifying this as a File Type box — present in all ISOBMFF files. |
0x08 | 4 bytes | Major Brand | 4D 34 41 20 (M4A ) | Identifies this as an audio-only MPEG-4 file. iTunes uses this brand for AAC purchases. |
0x0C | 4 bytes | Minor Version | 00 00 00 00 | Version of the major brand specification. Typically zero for M4A files. |
0x10 | variable | Compatible Brands | 69 73 6F 6D 4D 34 41 20 (isom M4A ) | List of compatible brands — isom (ISO Base Media), mp42, M4A are common entries. |
Attack Vectors
- Malformed moov box with crafted stco offsets pointing outside file boundaries can trigger out-of-bounds reads in parsers that do not validate sample offsets against file size
- Embedded cover art (covr atom) containing a malicious JPEG/PNG payload exploits image decoder vulnerabilities in players that auto-render artwork
- Deeply nested box structures with recursive or circular references can cause stack overflow in recursive ISOBMFF parsers
Mitigation: FileDex processes M4A files entirely in-browser using FFmpeg WebAssembly sandboxed in a Web Worker. No file data leaves the device. FFmpeg's mature ISOBMFF parser handles malformed boxes gracefully.