Sound are digitalized in computers as an archive with its intensity amplitude at the given time. I want to ask a very simple question about frequency filtering. How it's done, if there's no frequency information stored in the digital audio? It's all intensity. I cannot understand.
An set of digitized intensity samples that varies in time produces a waveform (just graph the data). If the waveform shows any periodicity, that will be an obvious frequency. Even if it doesn't, Fourier theory says any (non-pathological) waveform can be decomposed into a set (perhaps infinite) of sinewaves. And sinewaves are describe as having a frequency. Thus frequency information is implicit.
And this frequency information can be changed by certain filtering processes or algorithms many different ways.
Even if the digitized samples don't vary at all (a set of constants), that is the same as DC, or a frequency of zero (0 Hz).