In practice, the real input quantity vit is not known, as only v0t can be
measured. This output voltage, however, has suffered from the loss of information
contained in Hjω. No appreciable transmission errors could occur, if
at least the amplitude frequency response Hω D jHjωj would be constant
within a frequency range, in which the line or continuous frequency spectra,
Vik or Vijω, cannot be neglected. Thus the computation of the spectra of
an estimated input quantity is a very efficient tool to judge the necessary
frequency range or bandwidth of our measuring system and its individual
components.