Bandpass filter on noisy sinusoid that predicts turning points Is there an application where it is required to predict turning-points of a signal that
is like a sinusoid with constant frequency, variable amplitude and some noise (Gaussian additive noise)? The amplitude of the noise should be less than 10% of the amplitude of the original sinusoid every "oscillation".

Reason I ask is that I think I found an interesting causal filter for this very specific
situation, but I have no imagination for the moment on how it could be usefull.

My causal band-pass filter turns earlier than the turning point of the clean sinusoid, 
so it's like the filter has a negative lag. And the computation time is short, less than my code using FFT (in R: statistics software).

I'm a hobiest in DSP so please excuse my ignorance. My knowledge about filters comes mainly
from finance (econometrics).

To the man with a hammer, the entire world looks like a nail..

Thanks for any suggestions.
Assuming your change of amplitude is very slow when compared to the frequency of the sinusoid, this problem can be posed as finding frequency of a noisy sinusoid. You can take small windows of the signal and then take it's fourier transform. Since we know that the amplitude is almost constant and that there is only one sinusoid, we can look for two peaks in the DFT of the signal with some side lobes(Due to windowing). Your goal would be to find those two peaks, which would reduce the error energy in your estimate. Once you have the frequency and the phase information, finding turning points is very easy.

Hope this helps.