BSA Flow Software Installation and User's Guide - CSI

Aliasing
30%
20%
10%
0%
Loss of information due to resampling
1 2 3 4 5 6 7 8 9 10
Resampling rate expressed as "... times the average datarate of the original dataset"
Figure 7-84 : Several particle arrivals between two resamples will cause
loss of information, since only the last particle is recorded.
Figure 7-84 shows the likelihood of having more than one true sample in the
time between two resamples. Increasing the resampling frequency, will
reduce the width of each time-slot, and obviously reduce the risk of having
more than one true sample in it. Unfortunately it also increases the
probability of no true samples at all (not shown), and furthermore increase
the total amount of data, requiring additional storage capacity and increased
calculation time. Please note that due to the random nature of particle
arrivals, you can never completely avoid loss of data: Even with a
resampling rate of 10 times the average data rate, approximately 0.5% of the
time-slots will contain two (or more) true samples of which all but the last
one will be lost. At the same time the total amount of data will have
increased tenfold, but approximately 90,5% of these new samples will be
identical to the previous one, since no particles have arrived in the meantime
to provide new information on the flow investigated.
Aliasing is a problem in any periodic sampling procedure: If an oscillating
signal is sampled less than twice per cycle, it will produce the same samples
as an artificial (aliased) signal with lower frequency.
Knowing only the sample-values, later analysis will be unable to distinguish
between the true and the aliased signal, and consequently the high frequency
signal will contribute to the calculations as if the frequency was actually
lower.
The phenomenon is illustrated in Figure 7-85, where the true signal is
sampled 1.6 times per cycle.
True Sample
Aliased signal
Figure 7-85: Sampling less than twice per cycle cause aliasing.
Nyquist criterion For any sampling interval ∆t there is a special frequency fc, known as the
Nyquist critical frequency, given by:
7-128 BSA Flow Software: Reference guide