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two values: 25.6 kHz or 51.2 kHz. The NI 4551 has two input
channels, and the NI 4552 has four input channels. The higher the
number of channels used, the more difficult it is for the processor to
keep up in real-time. With an NI 4552, the more input channels that
you use for fractional-octave analysis, the lower the highest center
frequency analyzed must be in order for real-time processing to be
possible. The NI 4552 is unable to perform 1/3-octave analysis on all
four channels at the default sampling rate, even with the highest center
frequency set to 4 kHz. If you use three or more channels, use
Sampling Rate
process all four channels with a high center frequency of up to 10 kHz.
•
Fractional-octave mode (1/1, 1/3 or 1/12), frequency range, and
averaging mode—1/12-octave analysis requires the most processing
and 1/1-octave the least. A higher center frequency requires more
processing than a lower one. In general, impulse-exponential
averaging requires more processing than the other exponential
averaging modes. Linear averaging and peak hold averaging are the
least computationally intensive of the averaging modes.
•
Number of level measurements—The higher the number of level
measurements per channel, the more processing time required.
•
Weighting (A-, B-, or C-weighting)—A-weighting is most
computationally intensive, and C-weighting the least.
•
Simultaneous streaming to disk—If your instrument is configured for
simultaneous streaming to disk, there is less processor time available
for performing octave and level measurements.
Sometimes, you must eliminate one or more of these factors to get real-time
performance. For example, you can usually get real-time fractional-octave
analysis at higher center frequencies by choosing fewer channels and
limiting, or eliminating, simultaneous level measurements or streaming
I/O.
to set the sampling rate to 25.6 kHz, so you can
6-37
Chapter 6
Advanced Concepts
Set OLM
NI-DSA Software User Manual
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