Is M4A the same as AAC?

Not exactly. AAC is the codec — the compression algorithm that turns audio waveforms into compressed bits. M4A is the container — the file wrapper that holds those bits along with metadata, chapter markers, and (sometimes) album art. M4A almost always contains AAC audio, which is why people use the names interchangeably, but the same M4A container can also hold ALAC (Apple Lossless) or even raw PCM. Saying 'AAC file' is technically more precise than 'M4A file' when the codec is what matters.

Why does my OGG file play in VLC but not on my phone?

OGG is a container that can hold several different codecs — most commonly Vorbis (the original) and Opus (since 2012). Both use the .ogg extension and look identical from the filesystem. VLC ships with decoders for every common audio codec, so it plays both. Many phone music apps that advertise 'OGG support' actually only support OGG Vorbis and break on OGG Opus. The fix is to convert the file to MP3 or AAC, where the codec is unambiguous, using a tool like [OGG to MP3](/convert/ogg-to-mp3).

Does converting MP4 to MP3 change the video?

Yes — it discards the video entirely. MP3 is an audio-only codec and the MP3 file format has no provision for storing video frames. When a converter goes from MP4 to MP3, it reads the MP4 container, finds the audio stream (almost always AAC), decodes that audio to PCM, re-encodes to MP3, and writes the result. The video stream in the MP4 is ignored. If you need to keep both video and audio, convert to a different video container like MP4 with a different audio codec, or use a video editor.

Can I change a file's format by renaming the extension?

No. Changing the extension only changes how your operating system labels the file — it tells the OS which app to open it in by default. The actual encoded bytes inside remain whatever the original encoder produced. Renaming an AAC-in-M4A file to .mp3 produces a file with .mp3 on the end whose insides are still AAC. A player that trusts the extension will feed AAC data into an MP3 decoder, which will fail. True conversion requires decoding the source codec and re-encoding as the target.

Why is MP3 both a container and a codec?

MP3 is the unusual case where the codec and the container are essentially the same thing — the MPEG-1 Audio Layer III bitstream is laid out as a series of self-describing frames that can be played directly without an external wrapper. Every other modern format separates the two layers: AAC needs a container like M4A, MP4, ADTS, or LATM; Opus needs OGG or Matroska; FLAC has its own native container. MP3's frame structure is what makes it the only audio format you can split with a hex editor and have both halves still play.

Container vs Codec: The Most Confusing Thing in Audio

If you have ever tried to play an audio file and gotten a "format not supported" error from a player that, you swore, supported that exact format, the cause is almost always the gap between container and codec. They sound interchangeable. They are not. Understanding which is which explains why an .ogg file plays in VLC but not on your phone, why renaming .m4a to .mp3 produces garbled audio, and why Spotify can ingest WAV but not all WAV files.

The Bookshelf Analogy

Picture a bookshelf. The shelf itself is the container — it holds things, organizes them, exposes them to the reader. Whether the books are paperbacks, leather-bound first editions, or comic collections is a property of the books, not the shelf. The same shelf can hold any combination of book types.

In audio, the container (file format) is the shelf. The codec (encoding scheme) is the kind of book. A single container format can hold many different codecs, and a single codec can be wrapped in many different containers. The .mp3 case is unusual because there both names refer to the same thing — MP3 is both a codec (MPEG-1 Layer III) and the de facto container that wraps it. That coincidence trains people to assume container and codec are the same word for everything else, which they are not.

The Container Layer

A container is the file format that holds audio data, metadata, timing information, and sometimes multiple tracks. Common audio-bearing containers:

MP4 (.mp4) — the dominant modern multimedia container; can hold video, audio, subtitles, chapters
M4A (.m4a) — MP4 used for audio-only content; Apple's naming convention to distinguish audio-only files in a player UI
OGG (.ogg, .oga, .opus) — open-source container designed for streaming
MKV (.mkv) — Matroska; common for video with multiple language tracks
WAV (.wav) — a Microsoft RIFF container, almost always used for PCM
AIFF (.aiff, .aif) — Apple's equivalent to WAV, big-endian
FLAC (.flac) — FLAC's own native container with frame-level recovery
CAF (.caf) — Apple's Core Audio Format, no 4 GB size limit
WebM — a derivative of MKV used by web browsers

A container's job is to:

Identify what codec is inside (header magic bytes plus a codec ID)
Hold the encoded audio bitstream
Store metadata (artist, album, title, cover art, lyrics)
Define how a player should seek and stream
Carry timestamps so video and audio sync

The Codec Layer

A codec (encoder/decoder) is the algorithm that turns audio waveforms into compressed bits, and back again. It is the actual compression scheme. Common audio codecs:

MP3 (MPEG-1 Audio Layer III) — 1993, universal compatibility, lossy
AAC (Advanced Audio Coding) — successor to MP3, used in M4A and most MP4 audio, lossy
Vorbis — open-source lossy, common in OGG containers
Opus — IETF-standardized lossy, dominant in WebRTC and Discord
FLAC — open-source lossless, runs about 50% the size of WAV with bit-exact recovery
ALAC (Apple Lossless) — Apple's lossless equivalent to FLAC
PCM — raw uncompressed samples; what comes out of the analog-to-digital converter
AC-3 (Dolby Digital) — multi-channel for film and TV
DTS — competitor to AC-3, also for surround
WMA — Microsoft's family, lossy and lossless variants

Why the Same Container Holds Different Codecs

Most containers are codec-agnostic by design. An MP4 wrapper can hold AAC, MP3, AC-3, ALAC, or even raw PCM. An OGG container can hold Vorbis, Opus, FLAC, or Speex. A WAV container almost always holds PCM but the spec also permits ADPCM, μ-law, and even MP3. Matroska/MKV can hold practically every audio codec ever made.

This flexibility is what makes "supports MP4" a meaningless claim from a device. The device may decode the MP4 wrapper but lack a decoder for the AAC, AC-3, or ALAC inside.

The OGG Trap

OGG is the canonical example of container-codec confusion. The .ogg extension can wrap:

OGG Vorbis (the original use, 2002 onward)
OGG Opus (since 2012, increasingly common on Wikipedia and streaming)
OGG FLAC (rare but valid)
OGG Speex (legacy speech codec)

A music player advertising "OGG support" may have meant Vorbis only. Open the same .ogg file in VLC and it plays — VLC has Opus, Vorbis, FLAC, and Speex decoders bundled. Open it in an older Android music app and you may get silence. The fix is to convert with a tool like OGG to MP3 so the codec layer is unambiguous.

M4A vs MP4 — Same Container, Different Naming Convention

M4A files and audio-only MP4 files are byte-for-byte identical containers. Apple introduced the .m4a extension in 2003 with iTunes 4 specifically so users could tell audio-only files apart from video files in the Finder and in player UIs. Most software treats them interchangeably. Some (older Windows Media Player, some car infotainment systems) reject .mp4 audio because they assume MP4 means video — renaming to .m4a "fixes" it without touching the data.

This is also why renaming .m4a to .mp4 is a valid lossless operation, but renaming .m4a to .mp3 is not — the second swaps the implied codec from AAC to MP3 and the file becomes corrupted from the player's perspective.

Why Renaming Never Converts

Changing a file extension changes only the OS-level hint about what to do with the file. The bytes inside remain whatever the encoder originally wrote. Renaming an AAC-in-M4A file to .mp3 does not magically transcode the AAC bitstream into MP3. A player that trusts the extension will try to feed AAC bytes into an MP3 decoder, fail, and either silently produce garbage or refuse to open the file.

Real conversion requires:

1. Reading the container to find the codec 2. Decoding the codec back to PCM samples 3. Encoding those PCM samples with the new codec 4. Writing the result into the new container

That four-step process is what every audio converter does. Tools like M4A to MP3 and MP4 to MP3 handle all four steps automatically.

Diagnosing "Unsupported Format" Errors

When a player rejects a file, work the layers from outside in:

1. Check the extension. Does the player officially support this extension? 2. Check the codec inside. Use 'ffprobe filename' (FFmpeg) or open in VLC, then Tools > Codec Information. Note the audio codec. 3. Check the player's codec list. Player documentation usually lists supported codecs separately from supported containers. 4. Convert to remove ambiguity. When in doubt, convert to MP3 (universal codec, universal container) and the playback issue almost always evaporates.

Format Recommendations Reconsidered

Once you internalize the container vs codec split, format recommendations become more precise:

Maximum compatibility: MP3 codec in MP3 container (the rare case where they are the same)
Apple ecosystem: AAC codec in M4A container
Lossless archive: FLAC codec in FLAC container, or ALAC in M4A
Pro audio editing: PCM codec in WAV or AIFF container
Web and VoIP: Opus codec in OGG container or in WebM
Streaming distribution: AAC codec in MP4 container for music; Opus in WebM for chat

For codec-specific deep dives, see what is audio codec, what is audio container, and OGG Vorbis vs MP3. For the practical question of how containers affect file size, see why is my audio file so large.