Pro Audio Converter Help Guides and reference for batch audio conversion on macOS
Help topics MP3

MP3

The ubiquitous lossy audio format — configured in every way you might need.

Pro Audio Converter can convert audio files to and from MP3 (.mp3), using the LAME encoder.

MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a digital audio encoding format using a form of lossy data compression. All patents covering MP3 expired in April 2017, making it fully free to use and distribute. It is a common audio format for consumer audio storage and a de facto standard of digital audio compression for the transfer and playback of music on digital audio players.

Encoding Options

Sample Rate — Pro Audio Converter automatically chooses the best sample rate for the job. If you have a specific reason to force a particular MP3 sample rate, you can choose any of the rates the MP3 specification allows.
Bitrate Mode

Constant Bitrate (CBR) — The default mode, and the most basic. The bitrate is the same for the whole file. Whether the passage is difficult or easy to encode, the encoder uses the same bitrate, so quality varies: complex parts are of lower quality than simple ones. The advantage is that the final file size won't change and can be accurately predicted.

Variable Bit Rate (VBR) — You choose the desired quality on a scale from 1 (lowest) to 10 (highest). The encoder tries to maintain the given quality throughout by choosing the optimal number of bits for each part of the music. You control quality directly, but file size is unpredictable.

Average Bit Rate (ABR) — You choose a target bitrate, and the encoder tries to maintain that as an average while using higher bitrates for passages that need more bits. The result is higher quality than CBR at a predictable average file size, so this mode is highly recommended over CBR.

Bitrate — With CBR, this is the constant bitrate to use. Higher bitrates give higher quality and larger files. With ABR, this is the average bitrate to aim for, allowing frames of different sizes.
VBR Quality — The VBR quality level. VBR quality ranges from 1 (lowest) to 10 (highest). On typical music you can expect a VBR quality of 5 to yield files averaging around 132 kbps, and 8 around 200 kbps.
Quality — Controls the quality of the encoder's algorithm. Bitrate is the main influence on quality, but for a given bitrate, the algorithm choice affects scalefactors and Huffman encoding (noise shaping).
  • 10: slowest and best possible version of all algorithms. Highest quality.
  • 8: recommended. 9 and 10 are slow and may not produce significantly higher quality.
  • 5: good speed, reasonable quality.
  • 3: very fast, ok quality. (Psychoacoustics are used for pre-echo and M/S, but no noise shaping is done.)
  • 1: disables almost all algorithms including the psy-model. Poor quality.
Stereo Mode

Auto — Stereo or Joint Stereo (mid/side stereo) is chosen on a frame-by-frame basis for stereo source files. In mid/side stereo, the mid (L+R) and side (L−R) channels are encoded, with more bits for the mid channel than the side. This effectively increases bandwidth for frames that do not have much stereo separation, yielding an overall higher quality file. Mono is chosen for mono source files. This setting is recommended for the highest quality.

Stereo — Left and Right are encoded as separate channels.

Joint Stereo — Forces Mid/Side encoding for every frame.

Mono — If the source file is stereo, the two channels are averaged into a mono signal. Mono source files are encoded as mono.