[BioC] vsn on log2 transformed data?

[Kamila Naxerova]

Hi!

Could anyone explain to me *why* I cannot apply vsn to log2 transformed 
data? "...may not satisfy the requirements of the 
multiplicative-additive noise model" is a little hard to grasp for 
someone without a rigorous statistical background and the vignette and 
the original publication did not enlighten me very much. Does this 
really mean I cannot use vsn on a data set that contains log2 
transformed ratios from a dual channel experiment?

Thanks for any help!!!
Kamila