Mp3PRO
From Just Solve the File Format Problem
mp3PRO is an audio compression algorithm (or codec) that combines the MP3 audio format with spectral band replication (SBR) compression methods. It claims to achieve transparency at lower bitrates than MP3, resulting in a file nearly half the size of standard MP3
mp3PRO files were compatible with existing MP3 decoders, with the SBR data simply being ignored. However, this causes a significant reduction in audio quality as only the lower half of the original frequency range is available
