Spider DSA User’s Manual
235
Meter. The Frequency weighting in the octave filters affects the results of all filter
bands.
Time or RPM based RMS Trace of the Octave Filters
The ANSI and IEC standard do not require storing the time history of the band
pass filter output. However, the user may be interested in viewing this
information. Using a Spider, the RMS histories of all the band pass filters are
stored in the RMS quantity. Below is the description of how the RMS history is
calculated.
The RMS history can be stored against one of two variables: Time or RPM.
Both the input and output of a digital filter are a series of data points. While it
requires excessive memory to keep all of the time data from all of the filters, it is
useful to keep the running RMS history of each filter’s output. The RMS time
histories are computed after the time weighting averaging operation as shown
below.
Figure 145. RMS time history calculation.
The Decimator allows the user to choose the length of time for the RMS data. For
example, given a buffer length of 1024, a Trace Update Time of 5 ms will keep
about 5 seconds of RMS history; if this update time is set to 5 seconds, it will
record 5000 seconds (1.4 hours) of RMS history.
If a cut is made across the Z axis, the resulting XY plane will be an octave
spectrum. If a cut is made across the X-axis (frequency), the result will be called a
Time Trace.
The Time Trace stores the history of the RMS of each filter output. The spacing
between two points in the Time Trace is called Trace Update Time, in seconds. On
the Spider, one Time Trace is allocated for each channel for display. Keep in mind
that this buffer of Time Trace is the output of a specific filter, the user can change
the center frequency of the filter for the Time Trace during run time. In other
words, this time trace display buffer will change its content completely if the user
switches the Time Trace Frequency.
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