[1] This works, fundamentally, by discarding the higher half of the frequency range and algorithmically replicating that information while decoding.
[2][3] It was originally claimed that mp3PRO files were compatible with existing MP3 decoders, and that the SBR data could simply be ignored.
The reality was that MP3 players lacking specific mp3PRO decoding capability experienced a significant reduction in audio quality when playing mp3PRO files as only the lower half of the original frequency range is available.
Some versions of the outdated MusicMatch Jukebox player (which will still run even on Windows 10 x64) were able to decode and encode this format, too.
Thomson Demo mp3PRO Player/Encoder is an unmaintained proprietary freeware media software program originally released by Thomson (now Technicolor SA) and Coding Technologies (acquired by Dolby Laboratories in 2007) which converts Waveform Audio File Format (WAV) files to mp3PRO.