14-8 Configuring a Measurement Model 831 Manual
The minimum required count value is determined by the
Frequency Span settings as illustrated in Table 14-4. The
minimum count prevents data being gathered faster than it
can be stored into flash memory.
Table 14-4 Minimum Count
Resolution = Span / Lines
FFT Time = 1 / Resolution Narrow frequency resolutions take longer to gather, so the
response is slower. For example, when configured for 100
Hz span and 6400 lines, each FFT record is 64 seconds long.
Using 67% overlap allows the subsequent frequency
spectrum updates to occur at approximately 1/3 the time of
the initial update. These nominal times are given in Table
14-5.
Table 14-5 Nominal Calculation Time (seconds) with 67% Overlap
The actual amount of overlap can vary based on processor
load. Varying the amount of overlap allows the Model-831
to process all incoming sound samples even when the
processor is busy (i.e., storing a file). Variations in the
Frequency
Span
Minimum
Count
20 kHz 20
10 kHz 10
5 kHz 5
2 kHz 2
1 kHz 1
500 Hz 1
200 Hz 1
100 Hz 1
Frequency Span (Hz)
100 200 500 1000 2000 5000 10000 20000
400
1.32 0.66 0.264 0.132 0.066 0.0264 0.0132 0.0066
800
2.64 1.32 0.528 0.264 0.132 0.0528 0.0264 0.0132
1600
5.28 2.64 1.06 0.528 0.264 0.1056 0.0528 0.0264
3200
10.56 5.28 2.11 1.06 0.528 0.2112 0.1056 0.0528
Lines
6400
21.12 10.56 4.22 2.11 1.06 0.4224 0.2112 0.1056
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