steve wrote:
As a workaround, apply half of the amount of pitch shift twice.

*[...]*

Thanks for this tip!

I did some math and found that this approximation gets better the more times you apply the effect*. All the way to infinity! Of course making the number of iterations too high is not practical, and can even introduce error from dividing the amount of semitones desired by this huge number (I'm guessing, I didn't test).

For raising the frequency from unchanged to an octave above, here is the maximum frequency error (that occurs in the middle of the audio) I calculated for each amount of iterations:

1) 6.4%

2) 3.5%

3) 2.1%

4) 2.1%

10) 0.7%

∞) 0.0%

For comparison, two consecutive semitones have their frequencies 5.6% apart.

These errors should be the same for any initial and final pitch shift that are one octave apart, since the result would only be shifted by a certain amount of semitones.

However, the percentage gets higher if the difference between the inicial and final pitch shift increases, and vice versa. Intuitively, if you raise a sound by only 1 semitone with this effect, you wouldn't expect to have a 6.4% error anywhere, because that's more than 1 semitone.

**Conclusion**: Applying the effect "Sliding Time Scale/Pitch Shift" twice or three times should give a good approximation for a linear semitones slide in most cases.

Notice: Please double check my results if you intend to use them.
* I'm assuming that the effect turns the signal f(t) 0<=t<=1 into (1+rt)f(t) when "inicial pitch shift" is set to 0 and "final" is set so the frequency at t=1 is multiplied by r+1. Assuming also that a linear semitones slide (desired) would turn f(t) into 2^t*f(t). The multiplications here are done in the frequencies.