Operator`s manual

SECTION 10. PROCESSING INSTRUCTIONS
10-7
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 memory or any
data storage device. FFT results are transferred
to Final Memory by setting the Output Flag and
sampling the input locations which contain the
FFT results. Referring to Table 5, the thirteenth
through the seventeenth instructions illustrate
one method of transferring the data from input
memory to final memory.
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 13 or 8192.
02: 2 FFT Option
X0 = No Taper 0X = Power
Spectra
X1 = 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 this program 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 locations: Greater than or equal
to the number of values in the original time
series data.
Intermediate storage locations: 2 + (.125 * N)
Execution time in seconds:
REAL AND IMAGINARY
= .00065*N*LOG2 of N + .58
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.
Second Digit: A "0" in the second digit specifies
that no taper be applied, whereas a "1" specifies
that a taper be applied. If the original data set is
not known to be periodic with an integral number
of periods in the data set, then it is necessary to
apply a taper to the beginning and end of the
data. The taper that is applied is a four term
Blackman-Harris as specified by Fredric J.
Harris, Proceedings of the IEEE, Vol. 66, No. 1,
January 1978. When the taper is used, the 21X
applies a correction by multiplying the results by
the ratio of the variance before the taper to the
variance after the taper.
The variance ratio is saved only when the power
spectra option is selected. The variance ratio is
saved in input location (N/2)+B, where B is the
location (Parameter 4) that contained the first
value of the original time series data.
PARAMETER 3 specifies how many of the
resulting spectral bins (if any) are to be
averaged. Bin averaging is only allowed when