Hello, Suppose I have a discrete function of a perfect cosine wave. So if I will do a DCT on this function I will get one non zero coefficient which corresponds to the perfect cosine wave, and the rest will be zero. Now I have a pass filter, which filters out anything with a frequency which is different from the original cosine wave. If I will do this filter on the DCT I did to the cosine wave, then no coefficient should change. Now, suppose I have a second function which is also a perfect cosine wave of the same frequency as the cosine wave in the first function, but with a different phase. So the DCT of the second function will give me many non zero coefficients. If will pass the same filter I did on the first function DCT, then I will loose many coefficient and the result will be some wave which is weaker then the second function original cosine wave. Is that true? Basicaly I am trying to find the "obvious" frequency of a discrete wave. Lets say I have a pure triangle wave. Doing DCT on it will produce a lot of coefficients of different frequencies, but how do I discover the obvious frequency of the triangle wave from these coefficients? Thanks in advance.