Slowing down in Phrase Trainer 2 works in real-time which means you can slowdown/speedup music as it is being played.
Click the 'plus' button to increase the tempo (speed) of the song (or any audio file) or the 'minus' button to decrease the tempo. The same can be done using your computer keyboard 'Home' and 'End' keys. Or click speed indicator to choose from a popup menu.
Click 'On' button to enable/disable processing of the audio.
Speed/tempo can be changed from - 50% (one half speed) to 200% (double speed) without changing the pitch.
All is done in real-time with MP3, Wave, Wma,Ogg, FLAC , midi and AAC/M4A files on your computer.
That's nice but can changed songs/audio files be saved?
Yes they can, there is an 'Save file as' function in the main menu under 'File' submenu where you can choose between a number of export options (wav,mp3,ogg,wma):
Why is there slowdown limit of 50% ?
Because slowing down of audio introduces sound degradation and above 50% it really does not sound good any more, besides slowing more than 50% does not really make sense for most of the music or human voice.
But why the degradation occurs?
The same reason as for videos/images - you can not zoom in without introducing some pixelization, and more you zoom in the uglier the picture .
Curiouser and curiouser! How is it done actually if not a secret?
Well it used to be a secret but not any more...The Internet completely took the mystery out of it.
So in short:
Speed up twice - just remove every second audio sample.
Speed up a third -remove every third.
Slow down - add some more samples (and this is the reason for degradation because we are adding something that can only be guessed at because it doesn't really exist. Interpolation they call it in science :)