mp3PRO is an unmaintained proprietary audio compression codec that combines the MP3 audio format with the spectral band replication (SBR) compression method. At the time it was developed it could reduce the size of a stereo MP3 by as much as 50% while maintaining the same relative quality.The technology behind SBR was developed by the former Swedish company Coding Technologies AB (acquired by Dolby Laboratories in 2007) in the late 1990s.