BSA Flow Software Installation and User's Guide - CSI

Blocking Using the FFT-approach to estimate spectra, the raw samples need only
cover a time-span corresponding to half the reciprocal of the desired
frequency resolution. If more samples are available several “raw” estimates
will be calculated, and the final spectrum estimate is the average of all these
“raw” estimates.
Since the raw samples are divided in consecutive blocks of equal duration,
this approach is referred to as blocking, and the accuracy of the estimated
autospectral density function increases with the number of blocks.
The duration of each block and the number of blocks is determined as:
T
b
N
=
⋅ f
est
2 max
N
b
T
≤
T
tot
b
-where Tb is the duration of the block, Nest is the number of spectral
estimates, fmax is the desired maximum frequency, Nb is the number of
blocks, and Ttot is the total duration of the experiment.
(The number of blocks is rounded towards the nearest lower integer value).
To get as many blocks as possible within the limited duration of the
experiment, short blocks are desirable. Unfortunately the frequencyresolution
∆f of the calculated spectrum is inversely proportional with the
duration Tb of the blocks, so the settings chosen will be a compromise
between resolution and accuracy:
∆ f
fmax
1
= =
N 2 ⋅ T
est b
There is a lower limit to the duration of each block, since obviously it must
contain a reasonable amount of raw data, in order to get meaningful results:
It is thus required that each block should contain at least 16 raw samples.
Upon completion of the spectrum-calculation the number of blocks included
is shown in the notification pane at the bottom of the screen (Select the
Status-tab if several tabs are available).
The message 0 blocks processed indicate that the software was unable to
perform the calculation requested due to insufficient data. This may happen
if the maximum frequency and number of samples requested yield a block
length that is either too short or too long. If the block length is too short,
there may not be 16 raw samples within each block as required, and
obviously the longest block allowed correspond to the duration of the
experiment itself, treating the whole experiment as one single block.
Filter type If the necessary resolution and maximum frequency combined with the
duration of the experiment yields a very limited number of blocks, the
calculated spectrum-estimates may not be very accurate. The problem will
show as a noisy spectrum with relatively large differences between estimates
at neighbouring frequencies, and to mitigate this you can use a filter to
smooth the calculated spectrum. (In the context of spectra filtering is
sometimes referred to as frequency smoothing). The filter will replace each
spectral estimate with a weighted average of raw estimates calculated at
neighbouring frequencies. A multitude of filters exist, but they differ mainly
in subtle differences in the weighting schemes used, and only a limited
number is supported by the spectrum object. The property Filter type may
thus be selected as No filter, Box, Hanning or Papoulis, where the default
value No filter obviously mean that no filtering is performed.
BSA Flow Software:Options and Add-ons 6-55