Section 10. Processing Instructions
10-11
(speed and direction) representation of a wind vector. The power spectra results
indicate the amount of power at the different frequencies but do not contain any
phase information.
If desired, the original time varying signal can be reconstructed by taking the
Inverse Fourier Transform of either the real and imaginary or the magnitude and
phase results.
Programming
The FFT instruction is a Processing Instruction and will not output data to Final
Storage or any data storage device. FFT results are transferred to Final Storage by
setting the Output Flag and sampling the input locations that contain the FFT
results. In the example program in Section 8, the thirteenth to seventeenth
instructions illustrate one method of transferring the data from Input Storage to
Final Storage.
PAR.
DATA
NO.
TYPE
DESCRIPTION
01:
2
Log base 2 of N, where N is the number of samples in the
original data; e.g. this parameter is 11 for 2048 samples or 12
for 4096 samples. Maximum is 12 (for 4096 samples).
02:
2
FFT Option
x = No Taper
0x= Power Spectra
x = Taper
1x= Real & Imaginary
2x= Magnitude & Phase
03:
2
Log base 2 of the number of adjacent bins to average; e.g. this
parameter is 3 for averaging 8 adjacent bins.
04:
4
Input location containing the first value in the original time
series. After the instruction has executed, this location will
contain the first spectral bin or bin average of the result.
05:
FP
Multiplier for scaling FFT results
Input storage location
s
Greater than or equal to the number of values
required:
in the original time series data.
Intermediate storage
locations required:
2 + (0.125 * N)
Execution time
Real and imaginary = (.00065*N*LOG2 of N) + .58
in seconds:
Magnitude and phase = (.00077*N*LOG2 of N) +1.66
Power spectra
= (.00066*N*LOG2 of N) + .45
Parameter 1
is an integer from 1 to 13. The number 2, raised to the power of
parameter 1, is equal to the number of data values in the original time series data,
N. For example, if parameter 1=13, then N=8192. Maximum value for parameter
1 is 13 (8192).
Parameter 2
, the FFT Option, is a two-digit parameter. The first digit specifies
which results are to be generated and the second digit specifies whether or not a
‘taper’ is applied.
First Digit: A ‘0’ in the first digit specifies that the FFT result be expressed as the
power spectra. Bin averaging is allowed with this option. A ‘1
’
in the first digit
specifies that the FFT result be expressed as the real and imaginary components.
Bin averaging is not allowed with this option. A ‘2’ in the first digit specifies that
the FFT result be expressed as the magnitude and phase components. Bin
averaging is not allowed with this option.