MPEG-1/2/2½ audio layer I / II / III streams
- These files can't always be auto-detected reliably so be sure to use any of the above file extensions.
- The MPEG-version number (1, 2, or the unofficial "2½") only indicates the allowed sample rates.
- The MPEG-layer number (I, II, or III) indicates the compression algorithm that is used (with layer I the least complex, and layer III the most complex algorithm).
- Bit rate formats: When writing, you have 3 different output format selections to choose from, each selecting a different encoding bit-rate allocation strategy:
Note that some MP3 decoders only support constant bit rate files. Also note that you need Lame_Enc.dll version 3.92 or later to use average bit rate encoding. Also, version 3.94 or later is required for writing 8/11/12 kHz files.
- Constant Bit Rate: This is the simplest strategy and always uses the same amount of bits per time unit.
- Average Bit Rate: This let's you (indirectly) specify the total size of the output file (file size = audio length * bit rate). The encoder then 'portions out' the total amount of available bits in variable amounts to different parts of the audio clip in such a manner as it thinks will produce the best overall audio quality.
- Variable Bit Rate: This let's you specify a 'quality level' and the encoder then varies the bit rate so as to always achieve at least that given quality, without any concerns about what the total file size may end up being. This is the optimal way to compress the audio!
- Meta info: It's not an official part of any of the MPEG specifications, but there's several de-facto 'standards' for adding meta-info structures to .MP2 and .MP3 files. Here's how that is handled:
- 'ID3 v1 meta info tags', 'ID2 v2 meta info tags' and 'APE meta info tags' are all read if present.
- Awave Studio: 'ID3 v1 meta info tags' and/or 'APE meta info tags' can be written - select what you prefer on the 'Format options' tab in the program settings.
- Awave Studio: 'ID3 v1 meta info tags' and/or 'APE meta info tags' can be written - select what you prefer in the 'More options' dialog.
- ACDR: 'ID3 v1 meta info tags' are written.
- More options: For Awave Studio/Awave Audio you can also select if you want stereo MP2/MP3 files to be created in 'joint stereo' instead of 'normal' stereo format. The 'joint stereo' mode usually gives better compression performance, but some claim it will damage important phase-coherence in Dolby ProLogic encoded material.
For Awave Audio you furthermore select if you want energy levels to be stored in the 'frame aux-field'. This is not so common, but it is required by e.g. Digigram hardware.
- A .MUS (Musifile MPEG audio layer II stream) file is the same as a .MP2 file.
- 16 bit mono / stereo, 48, 44.1, 32, 24, 22.05, 16, 12, 11.025, 8 kHz, compresses from 8 to 448 kbit/s.