BPMinus is a piece of software that allows musicians to control the tempo of their music without altering pitch and with minimal impact on sound quality. The technical term for this concept is called "time-stretching." BPMinus has a lot of other great features, like pitch correction and BPM analysis, but most people are using it for the time-stretching capabilities. BPMinus was created out of frustration with other time-stretching offerings on the market. They all seemed like they were designed in the early 1990s, featuring horrible user-interfaces that were unintuitive and ugly.
Development started with an emphasis on making a better time-stretcher than what was out there. We wanted better aesthetics, better control, better sound quality, and better ease-of-use. BPMinus has been developed in coordination with music teachers and students to ensure it provides the kind of assistance it was designed for.
If you simply speed up the playback your music, you end up increasing the pitch. Likewise, if you slow it down, the pitch decreases. It takes something a little extra to make your music maintain pitch while altering the tempo. In the digital audio world, this is done through something called a Phase Vocoder. Now, the phase vocoder concept isn't some brand new technology; it's been around for quite a while. However, implementing a simple phase vocoder and making something that sounds really great with music are two very different things. All around the world, there are brilliant engineers and researchers working very hard to make phase vocoders sound great and perform great. BPMinus didn't aim to reinvent the wheel with a new phase vocoder, but rather, bring together some of the best of this research in an easy-to-use application.
During BPMinus's development process, a number of different time-stretching technologies were implemented and put on trial. At the end of it all, we found there was not one single technology that sounded the best on all songs. Likewise, it was discovered that many people use the software differently. Some like to use the equalizer to really hone in on the mid-frequency sounds. Some like to downmix the stereo audio so both the left and right speaker sound the same. Some people are concerned about phase where others are concerned that the percussive sounds are crisp. With all these very specific needs for controlling sound, it became apparent that no single technology was going to suffice. Thus, BPMinus gives you a broad range to choose from:
Rubber Band by Breakfast Quay is the default phase vocoder of BPMinus. It's incredibly flexible with a great amount of adjustable parameters. RubberBand goes the extra mile to sound great at slow speeds thanks to its phase managements and handling of percussive transients.
DIRAC by The DSP Dimension is a very popular phase vocoder that offers great quality while consuming relatively little processor power. DIRAC is an industry standard being used by large industry forces like Steinberg, Prosoniq, and Autodesk/Discreet.
SoundTouch by Olli Parviainen is the time-stretching standard of the open-source community. Unlike the others, it is not actually a phase vocoder and does its processing in the time-domain, making it quicker and less CPU-intensive for real-time procesing. If you've ever used an open-source project with time-stretching, you're probably using SoundTouch. Popular open-source projects like Audacity are using SoundTouch.