I have been trying to record some music I composed on my guitar,] using Audacity. I played some difficult passages slowly so I got clear notes and the recording sounds fine. But when I use the tempo change feature to bring the speed up (30% or so) to match the rest of the piece the result is “muddied” to the point of being unusable. Any hints on this. Maybe I should try changing the tempo many times, say 1 % at a time, or change pitch down and then use time change to bring it and the pitch back up, or maybe the tempo function will work better if the file is bass boosted or compressed or amplified first, or maybe I should superimpose the original bar by bar, or use a third party add-in etc etc. Suggestions?
The next version of Audacity has an option to “Use high quality stretching (slow)”. When available, you should use this - for small to moderate tempo changes it generally gives much better sound quality than the standard time-stretch algorithm.
In the mean time, you can use the “Sliding Time Scale / Pitch Shift” effect, which already uses the better quality algorithm (but is a bit more complicated to use): http://manual.audacityteam.org/man/sliding_time_scale_pitch_shift.html
Thanks for the quick reply Steve. Your suggestion works like a charm.