Free online ULAW to OPUS converter

What the ULAW format is

ULAW, also called μ-law or G.711 μ-law, stores audio as 8-bit logarithmically encoded samples. It is common in telephone systems, VoIP recordings, SIP and PBX software such as Asterisk, and some speech APIs. Raw files may use extensions such as .ulaw, .mulaw, or .mu; a WAV file can also contain μ-law audio while using the .wav extension.

Telephone ULAW is normally mono at 8,000 Hz, although raw files do not contain a header, so the correct sample rate and channel count may need to come from the recording system that created them.

What the OPUS format is

Opus is a lossy audio codec usually stored in an Ogg container with the .opus extension. It is designed for speech, music, VoIP, WebRTC, messaging, and streaming. Compared with G.711 μ-law, Opus generally provides smaller files at similar perceived speech quality and supports a wider range of sample rates, channels, and bitrates.

Opus is a suitable target when the receiving application explicitly accepts .opus. It is not suitable for workflows that require bit-exact G.711 audio or uncompressed editing masters.

How to actually convert ULAW to OPUS

Using FFmpeg

FFmpeg is the most reliable desktop option for raw ULAW because it lets you specify missing input parameters. For standard mono 8 kHz telephony audio, run:

ffmpeg -f mulaw -ar 8000 -ac 1 -i input.ulaw -c:a libopus -b:a 24k output.opus

Replace input.ulaw with the source filename. Use a different -ar value if the source system used another sample rate, and use -ac 2 for stereo. The mulaw input format tells FFmpeg that the file contains raw μ-law bytes rather than a self-describing container.

If the source is a WAV file containing μ-law audio, let FFmpeg read the WAV header instead:

ffmpeg -i input.wav -c:a libopus -b:a 24k output.opus

For a command-line alternative, opusenc can encode an intermediate WAV, but FFmpeg avoids that extra conversion step. You can also use the free converter on 101convert.com without installing software; raw ULAW still requires the correct sample rate and channel settings when the service requests them.

Quality and compatibility

Both ULAW and Opus are lossy formats, so converting cannot restore detail discarded during the original μ-law encoding. For speech, 24k to 32k is usually a practical Opus bitrate; increasing it does not recover missing source quality.

Keep the source at its native sample rate unless the target application requires another one. Resampling 8 kHz telephone audio to 48 kHz does not add information, although Opus commonly uses a 48 kHz internal operating rate. Verify that the receiving program accepts Ogg Opus rather than raw Opus packets, because output.opus normally means an Ogg Opus file.