A practical example: I perform a DCT on a time series of discrete values that are spaced in time by 1/30 of a second. What frequencies each bin of this DCT represents? What is the formula to find that?
If I want to filter the DCT to remove all signal bins that correspond to frequencies below 0.4 Hz and above 4 Hz, and keep the values between these intact, how should the filter impulse be constructed?
I understand that in order to filter that DCT I have to to build a filter array that is a series of zeros in the bins I want to remove and ones in the bins I want to keep and multiply elements in each array by its correspondent in the other, zeroing the bins I don't want.
But I need to know the frequency each bin on the original DCT represents.