I’ve been using Audacity for a couple years and just joined the forum. I’m working with the Change Tempo effect on the stereo mix of a multi tracked analog synthesizer piece. It sounds to me that when I speed up the tempo at 20% or more that the sound quality degrades. I could be wrong though, so I’m wondering, can anyone explain to me if it is possible for Change Tempo to alter the sound quality in a negative way? A quick explanation on how Change Tempo actually achieves the tempo change and if the sound quality could degrade in that process would be incredibly helpful.
“Change Tempo” (and “Change Pitch”) are “time stretching” effects.
Normally, if you speed up or slow down some audio, then both the tempo and the pitch will change. This is what happens if you speed up or slow down a vinyl record or a cassette tape. The same happens with the “Change Speed” effect.
Changing tempo independently of pitch, or changing pitch independently of tempo, is much harder to do.
Audacity uses an algorithm called SoundTouch for this task (see the link for details).
Basically, what SoundTouch does is to split the sound into small fragments, then stretches and repositions those fragments and glues all of the pieces back together again. For small changes in pitch or tempo, the quality is usually reasonably good, though it is never perfect. For larger changes, the sound quality tends to suffer more.
An alternative algorithm is used in the “Sliding Time Scale / Pitch Shift” effect. This algorithm is called “Subband Sinusoidal Modeling Synthesis” (see: SBSMS). This algorithm is much more complex, and processing is slower, but the sound quality is generally better than SoundTouch.
This confirms what my ears were telling me…I knew it didn’t sound the same post Change Tempo. I’ll give the sliding time scale/pitch shift you mention a try. Thanks for the quick response!