  For the white noise, the spectral density is constant, and the past
(restriction to (-\infty,0)) is independent from the future (restriction to
(0,+\infty)). If the spectral density is not too far from being constant, then
dependence between the past and the future can be eliminated by an equivalent
measure change; that is called an off-white noise. I derive from well-known
results a necessary and sufficient condition for a spectral density to describe
an off-white noise.
