The Music Speed Changer is a miracle!
Can you imagine trying to play with the same track in all 12 keys before the age of digital transposers? Try playing any backing track you've downloaded with The Music Speed Changer. The app works great. You can also just use your computer. Try it with the I-V backing track!! I recommend that beginner improvisers really master the I-V backing track in one key first - your most familiar key; for me that was C... and THEN after you've "got it" learn to play along with it in all keys. What's your most familiar key signature?