Originally Posted by Frank
How does the upsampler know it is upsampling a 200Hz signal as opposed to a 2000Hz signal?
Quite simple actually: first the waveform is transformed from the time domain to the frequency domain using a fourier transform ... at least that is how Audio Remaster works ... however, you can transform calculations in the frequency domain back to a filter in the time domain, and that is how most resamplers work (an older version of Audio Remaster worked that way).

Originally Posted by Frank
I thought upsamplers simply calculated the approximate value of a sample between two real samples.
Only bad ones do ...