Input Types : ComplexSpectrumSpectrumTimeFrequency
Output Types : ComplexSpectrumSpectrumTimeFrequency
Date : 04 Mar 2001
The user can also choose to smooth the edges of the frequency cutoff using a choice of window functions. The functions are those given in WindowFnc. These are useful if the filtered spectrum is to be inverted back to the time-domain: a sharp cutoff gives unwanted oscillations in the time-domain that are suppressed if the cutoff is smoothed.
The user can also choose to reduce the Nyquist
frequency of the output set to the upper-frequency cutoff. The Nyquist
frequency is the highest frequency that can be held in a spectral data
set. When the data set is inverted by FFT,
the time-domain result will have a sampling frequency that is twice the
Nyquist frequency. Thus, if the user reduces the Nyquist frequency when
filtering, then the time-domain inversion will be sampled at twice the
rate of the upper-frequency cutoff. This ensures that the time-domain set
contains no more samples than necessary.
The first slider chooses the frequency cutoff in Hz. All data at frequencies above this are set to zero.
The second line is a check box. Select this if you want narrow-band output. If it is not selected (the default) then the output spectral data set will have the same length as the input but the filtered data elements will be set to zero. If you choose narrow-band, then these zeros will not be present in the output: this saves storage. The FFT unit will still invert this properly.
The third choice is the window function that can be used to smooth the edges of the frequency cutoff.
The fourth choice is a check box. Select this if you want the Nyquist frequency of the output data set to be reduced to the frequency chosen in the slider. The output data set will be a full-bandwidth spectrum with this maximum frequency. See the discussion above for the meaning of the Nyquist frequency.