Free online ADTS to FLAC converter

Some audio editors, archival workflows, and playback devices accept FLAC but not AAC audio wrapped in ADTS headers. Converting the stream produces a widely supported, lossless FLAC file, although it cannot restore quality already discarded by the original AAC compression.

What the ADTS format is

ADTS, or Audio Data Transport Stream, is a sequence of AAC audio frames, with a small header attached to each frame. The headers provide information such as sample rate, channel configuration, frame length, and synchronization data, allowing a decoder to process the stream without a separate container index.

ADTS is commonly stored with the .aac extension and sometimes with .adts. It appears in radio and broadcast workflows, transport streams, device recordings, and AAC exports made by tools such as FFmpeg, FAAC, and fdkaac. It is an audio stream format rather than a general-purpose container like MP4, so it normally carries AAC audio without metadata, cover art, or multiple tracks.

What the FLAC format is

FLAC is a lossless audio codec and file format. It reduces the size of uncompressed PCM audio while preserving every decoded sample, and it supports metadata such as artist, album, title, comments, and embedded artwork.

FLAC is useful for editing, long-term storage, playback on compatible music software, and sharing files when WAV-sized PCM is unnecessary. Its compression is typically smaller than WAV, but it is not smaller than a lossy AAC stream. FLAC also cannot recover detail removed when the source was encoded as AAC.

How to actually convert ADTS to FLAC

FFmpeg is the most reliable desktop method because it detects ADTS framing and decodes AAC directly.

1. Install FFmpeg from a trusted package source for your operating system.
2. Place the source file in a convenient folder. The extension may be .aac, .adts, or another name even when the contents are ADTS.
3. Run this command in a terminal from that folder:
   ffmpeg -i input.aac output.flac
4. Replace input.aac with the actual source filename and output.flac with the desired destination name.

For a source named recording.adts, use:

ffmpeg -i recording.adts recording.flac

FFmpeg chooses FLAC encoding automatically from the output extension. To specify it explicitly, use ffmpeg -i input.aac -c:a flac output.flac. The default FLAC compression level is suitable for most conversions; levels from 0 to 8 change encoding speed and file size, not audio quality. For example, ffmpeg -i input.aac -c:a flac -compression_level 8 output.flac creates a smaller file at the cost of more encoding time.

To preserve or add basic metadata, use options such as -metadata artist=Name and -metadata title=Track. ADTS usually contains no metadata to transfer. You can also convert the file for free on the 101convert.com website without installing software.

Quality, compatibility, and limitations

The conversion decodes lossy AAC and re-encodes the decoded samples as lossless FLAC. It does not improve the source quality, remove AAC artifacts, or recreate missing frequencies. The resulting FLAC may be larger than the ADTS file because FLAC preserves the decoded audio rather than applying another lossy compression step.

Check the input before conversion with ffprobe input.aac. This reports the codec, sample rate, channel count, duration, and other stream properties. If FFmpeg reports invalid data, the file may be truncated, encrypted, mislabeled, or not actually an ADTS AAC stream. ADTS normally carries one AAC program; multiple tracks, chapters, and rich metadata require a container such as M4A or Matroska before conversion.