Free online ULAW to FLAC converter

What the ULAW format is

ULAW usually means G.711 μ-law, an 8-bit companded audio encoding used mainly for telephone speech. It is common in PBX and voicemail systems, SIP and VoIP recordings, legacy Unix audio tools, speech datasets, and telephony APIs; Asterisk, FFmpeg, SoX, and similar software can create or process it.

A file with a .ulaw extension is often raw, headerless data. Unlike WAV, it may not store its sample rate, channel count, or encoding details. The usual telephony parameters are 8,000 Hz, mono, 8-bit μ-law, but the source system's documentation should take precedence.

What the FLAC format is

FLAC is a lossless format for linear PCM audio. It reduces the size of PCM without discarding additional information, supports metadata such as artist and recording date, and is accepted by many media players, editors, archival systems, and command-line tools.

FLAC does not restore detail lost when audio was encoded as μ-law. It stores the decoded μ-law samples as PCM, usually at the original 8 kHz telephony rate unless you explicitly resample them.

How to convert ULAW to FLAC

FFmpeg is suitable for both raw ULAW files and WAV files containing μ-law audio. Install FFmpeg, open a terminal in the directory containing the recording, and use the parameters that match the source.

For a raw, 8 kHz mono file, run:

ffmpeg -f mulaw -ar 8000 -ac 1 -i input.ulaw output.flac

Replace 8000 and 1 if the recording uses a different sample rate or number of channels. FFmpeg detects the output format from the .flac extension.

If the input is a WAV file that already contains μ-law audio, do not force the raw input demuxer:

ffmpeg -i input.wav output.flac

To preserve or add basic metadata, use options such as:

ffmpeg -f mulaw -ar 8000 -ac 1 -i input.ulaw -metadata title="Call recording" output.flac

For a no-installation conversion, you can use the free converter on 101convert.com. Upload the ULAW file, select FLAC, verify the detected sample rate and channels, then download the result; raw files may require manual parameter selection.

Quality and compatibility limitations

μ-law is already a lossy speech codec, so ULAW-to-FLAC conversion is not a lossless recovery of the original microphone signal. FLAC prevents another lossy generation and may be larger than a compressed speech codec, but it cannot improve bandwidth, clarity, or frequency response that the μ-law source lacks.

If the converted file sounds too fast, too slow, noisy, or distorted, the raw input parameters are probably wrong. A mismatched sample rate changes playback speed, while an incorrect channel count changes interpretation of the byte stream. Keep the original ULAW file and record the parameters used for conversion because raw ULAW normally has no embedded technical metadata.