BSA Flow Software Installation and User's Guide - CSI

1,2
1,0
0,8
0,6
0,4
0,2
Weight factor w
Box
Hanning
Papoulis
0,0
τ / T
0,0 0,2 0,4 0,6 0,8 1,0 1,2
Figure 6-53 Weighting factors used for filtering
Filter width The property Filter width determines when the lag-window reaches zero and
should be set to a value between 0 and 1. The value relates to the maximum
lag-time of the correlation estimate calculated by Fourier-transformation of
the spectrum: Filter width = 1.0 means that the lag-window reaches zero at
the maximum lag-time, Filter width = 0.5 means that it reaches zero at half
the maximum lag, and so on. Filter width = 0.0 is not allowed, since this
would produce correlation and spectrum estimates equal zero for all lags and
frequencies.
In general large filter widths will smooth the spectrum less than small
widths, and the Box filter with Filter width = 1.0 actually correspond to no
filter at all. Depending on the quality of the raw data, the number of spectral
estimates and the maximum frequency selected, there is also a lower limit for
the filter width, but no general values can be given;
In most cases a filter width in the range 0.1 → 1.0 will work fine, and
sometimes even a filter width of 0.01 may be OK.
Peak smearing Please be aware that filtering will smooth true peaks as well as noise peaks
in the spectrum. Any peak will be broadened, and its height reduced, and in
severe cases a true peak may not even be recognised after filtering. (See
Figure 6-54).
It is therefore strongly recommended that you take a look at the spectrum
calculated without filters before you decide whether or not to use a filter.
If you are in doubt as to whether or not a peak in the calculated spectrum is a
noise peak, try increasing and/or decreasing the maximum frequency
somewhat. Noise peaks are random by nature, and even such slight changes
will affect them considerably. A true peak on the other hand should remain
more or less unaffected.
BSA Flow Software:Options and Add-ons 6-57