That might also be equivalent to one of the ARMA models, although I'd have to do the math to prove that (and to derive which one corresponds to what you're desc

That might also be equivalent to one of the ARMA models, although I'd have to do the math to prove that (and to derive which one corresponds to what you're describing). Edit: It seems that if you detrend the data, the "delta off the last delta", if I understand it correctly, is an MA(1) model. I think. Maybe. My math is wobbly.