Free online ULAW to AAC converter

What the ULAW format is

ULAW, also written as μ-law or G.711 μ-law, stores logarithmically compressed 8-bit audio samples. It is common in telephone systems, VoIP applications, SIP/RTP streams, call-recording software, PBX systems such as Asterisk, and audio extracted from telephony APIs.

Most ULAW recordings use an 8 kHz sample rate, one channel, and a data rate of 64 kbps. A raw file may contain only sample bytes, without a header describing its sample rate, channel count, or encoding. The .ulaw extension therefore does not always provide enough information for automatic detection.

What the AAC format is

AAC is a lossy audio codec designed for smaller files and broad playback support. It is used by phones, web browsers, streaming platforms, media players, video editors, and container formats such as M4A and MP4.

AAC normally provides better perceived quality than ULAW at comparable or lower bitrates, but it cannot restore quality removed by the original telephony encoding. An M4A file containing AAC is generally a better choice than an elementary .aac stream when the destination expects a normal, metadata-capable audio file.

How to actually convert ULAW to AAC

You can convert the file for free on the 101convert.com website without installing software. Select the ULAW file, choose AAC or M4A as the output format, start the conversion, and download the result. For repeated conversions or files whose technical parameters are unknown, FFmpeg provides more control.

Convert a raw ULAW file with FFmpeg

Install FFmpeg, then run this command for the common telephony configuration:

ffmpeg -f mulaw -ar 8000 -ac 1 -i input.ulaw -c:a aac -b:a 96k output.m4a

-f mulaw identifies the raw μ-law encoding, -ar 8000 supplies the sample rate, -ac 1 specifies mono audio, and -b:a 96k sets the AAC bitrate. Replace input.ulaw and output.m4a with the actual filenames.

If the recording uses a different sample rate or channel count, replace those values, for example -ar 16000 -ac 1. Do not use a guessed setting if timing or pitch is wrong; obtain the parameters from the recording system that produced the file.

Convert a ULAW WAV file

Some files use a WAV container even though their audio codec is μ-law. Such files should be opened through the container rather than forced through the raw demuxer:

ffmpeg -i input.wav -c:a aac -b:a 96k output.m4a

FFmpeg reads the WAV header and normally detects the μ-law codec, sample rate, and channel count automatically. Use ffprobe input.wav or ffprobe input.ulaw to inspect the detected stream before converting.

Quality, compatibility, and limitations

ULAW is already a lossy, narrowband telephony format. Converting it to AAC is another lossy encoding step, so increasing the AAC bitrate cannot recover missing speech frequencies or detail. For ordinary voice recordings, 64–96 kbps AAC is usually sufficient; use a higher bitrate only when the source contains wider-band audio or the destination requires it.

Raw ULAW files have no dependable self-description. An incorrect sample rate changes playback speed and pitch, while an incorrect channel count changes how the bytes are interpreted. The standard 8 kHz mono assumptions apply to many G.711 telephone recordings but not every file with a .ulaw extension.

Choose .m4a for compatibility with general-purpose players and editors. Use an elementary .aac file only when a particular application requests AAC in ADTS format:

ffmpeg -f mulaw -ar 8000 -ac 1 -i input.ulaw -c:a aac -b:a 96k -f adts output.aac