Convert 3GPP to AU — Free Online Tool
Convert 3GPP mobile video files to Sun AU audio format, extracting and re-encoding the AAC or MP3 audio track as uncompressed PCM (pcm_s16be) — a big-endian 16-bit raw audio stream compatible with Unix systems and legacy audio tools. This conversion strips the video entirely and produces a lossless-quality PCM representation of your 3GPP audio.
to
FFmpeg Command
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Estimated output:
Conversion Complete!
DownloadHow It Works
3GPP files typically contain a video stream (encoded with H.264/libx264) and an audio stream (encoded with AAC or MP3, both lossy codecs optimized for low-bitrate mobile delivery). During this conversion, FFmpeg discards the video stream entirely and decodes the compressed audio track, then re-encodes it as pcm_s16be — signed 16-bit big-endian PCM — wrapped in a Sun AU container. The AU format uses a minimal fixed-size header followed by raw PCM sample data, with no support for compression metadata or chapters. Because the source audio in 3GPP is typically compressed at a low bitrate (often 64k AAC), the resulting AU file will be uncompressed in encoding but limited in fidelity by whatever quality existed in the original mobile-optimized audio.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg command-line tool, which handles all media demuxing, decoding, and encoding. In the browser version of this tool, FFmpeg runs via WebAssembly (ffmpeg.wasm) without any server involvement. |
-i input.3gp
|
Specifies the input file in 3GPP format. FFmpeg demuxes the container, identifying the video stream (typically H.264) and the audio stream (typically AAC or MP3) stored inside the mobile-optimized 3GP container. |
-c:a pcm_s16be
|
Decodes the compressed AAC or MP3 audio from the 3GPP file and re-encodes it as signed 16-bit big-endian PCM — the native uncompressed audio encoding for the Sun AU format, matching the big-endian byte order of the original Sun SPARC architecture that defined this format. |
output.au
|
Specifies the output file with a .au extension, which tells FFmpeg to wrap the pcm_s16be audio data in a Sun AU container with a minimal 24-byte header. The video stream from the 3GPP input is automatically omitted because the AU format is audio-only. |
Common Use Cases
- Extracting voice call recordings or voice memos saved in 3GPP format on Android phones for use in Unix-based audio processing pipelines that expect AU or raw PCM input.
- Feeding 3GPP mobile audio into legacy Sun/Unix audio workstation software or tools like SoX that have native AU format support.
- Converting 3GPP audio clips captured on older 3G-era mobile devices into a format suitable for playback in early web browsers or Java applets that supported Sun AU natively.
- Archiving the audio content of short 3GPP video clips in an uncompressed, container-agnostic PCM format for long-term storage or forensic analysis.
- Preparing audio extracted from 3GPP recordings for use in academic or research signal processing tools that require raw big-endian PCM with an AU header.
- Stripping the video from a 3GPP file and producing a simple, headerless-adjacent AU file for integration into Unix shell script audio workflows.
Frequently Asked Questions
The AU file itself will be uncompressed PCM, but there is a generational quality loss involved because the source audio in 3GPP was already compressed — typically as AAC at 64kbps or MP3, both lossy codecs tuned for low-bitrate mobile streaming. The AU output faithfully represents the decoded version of that compressed audio, but any artifacts introduced by the original AAC or MP3 encoding cannot be recovered. Think of it as 'lossless capture of a lossy source' — the AU file will be larger but not higher quality than the 3GPP original.
3GPP files store audio as compressed AAC or MP3, which can achieve very small file sizes at the cost of some audio quality. The AU format in this conversion uses pcm_s16be — uncompressed 16-bit PCM — which stores every audio sample as raw data with no compression. A typical 3GPP file with 64kbps AAC audio will expand dramatically when converted to uncompressed PCM: roughly 1 minute of mono audio becomes around 5MB in AU format versus under 500KB in the original 3GPP.
Yes. FFmpeg reads the sample rate and channel layout from the 3GPP audio stream and writes those values into the AU file header without resampling or downmixing, unless you explicitly add flags like -ar or -ac to the command. The AU format's simple header stores sample rate, channel count, and encoding type, so these properties are preserved. If your 3GPP file had 8kHz mono audio — common for 3G voice recordings — the AU output will also be 8kHz mono.
Yes. The AU format supports several PCM variants: pcm_s8 (8-bit signed), pcm_u8 (8-bit unsigned), pcm_alaw (G.711 A-law, commonly used in telephony), and pcm_mulaw (G.711 μ-law). You can substitute any of these in the FFmpeg command by changing -c:a pcm_s16be to your preferred codec, for example: ffmpeg -i input.3gp -c:a pcm_mulaw output.au. The μ-law and A-law options are particularly useful if you are targeting telephony systems that originated alongside the AU format.
On Linux or macOS, you can loop over all 3GPP files in a directory using a shell one-liner: for f in *.3gp; do ffmpeg -i "$f" -c:a pcm_s16be "${f%.3gp}.au"; done. On Windows Command Prompt, use: for %f in (*.3gp) do ffmpeg -i "%f" -c:a pcm_s16be "%~nf.au". This applies the same audio extraction and PCM encoding to every file, outputting a matching AU file for each 3GPP source.
The Sun AU format has an extremely minimal header that contains only technical audio parameters — encoding type, sample rate, channel count, and a small annotation field. It does not support rich metadata tags like title, artist, or recording date. Any ID3-style or MP4 metadata atoms present in the 3GPP file will be silently dropped during this conversion. If preserving metadata matters, consider converting to a format like FLAC or WAV, which have robust metadata support.
Technical Notes
The Sun AU format (.au) is one of the oldest digital audio container formats, developed by Sun Microsystems for SunOS workstations and widely used in early Unix and Java environments. Its header is only 24 bytes minimum, encoding offset, data size, encoding type, sample rate, and channel count — making it trivially simple to parse but incapable of storing any metadata beyond these fields. In this conversion, the video stream from the 3GPP container is implicitly dropped because AU supports only audio; FFmpeg detects that no video codec is available for the output container and excludes it without requiring an explicit -vn flag. The default codec pcm_s16be uses big-endian byte ordering, which aligns with the format's Sun SPARC heritage — note that this differs from the little-endian PCM used in WAV files, so tools expecting WAV-style PCM may need byte-swapping. Because 3GPP was designed for 3G mobile networks with constrained bandwidth, its audio tracks are often recorded at reduced sample rates (8kHz or 16kHz for voice) and low bitrates, meaning the AU output, while technically uncompressed, will reflect the limited bandwidth of the original mobile audio capture.