1.4 Frequency Limits Frequency band

1.4 Frequency Limits Frequency bands are analyzed beginning at the frequency specified by the FREQ_MIN parameter. The width of each band is specified by the FREQ_WIDTH parameter. The maximum frequency to process is specified by the FREQ_MAX parameter. Increment of frequency is specified by the FREQ_INC parameter. The frequency domain sampling interval is the Nyquist frequency divided by the number of mixed-radix samples in the FFT. This interval is not normally a multiple of the specified frequency parameters FREQ_MIN, FREQ_MAX, and FREQ_WIDTH, so the actual frequency bands processed do not correspond exactly to these values. Frequency bands are computed by starting with the frequency sample most closely corresponding to value of the FREQ_MIN parameter, and incrementing by the number of samples that most closely matches the FREQ_WIDTH parameter. If FIXED_STEP is selected in FREQ_INC, all frequency bands have the same width, this is the legacy mode that previous versions used; if OCTAVE is selected, width of frequency bands increases from low to high frequencies. The last band processed is the first one whose ending frequency exceeds the frequency of the FREQ_MAX parameter. For example, if the frequency sampling interval is 1.4 Hz/sample, FREQ_MIN = 0, FREQ_MAX = 20, FREQ_WIDTH = 5 Hz, and FREQ_INC is FIXED_STEP, frequency bands are 4 samples in width (5.6 Hz). The actual frequency bands processed are 0 to 4.2 Hz, 5.6 to 9.8 Hz, 11.2 to 15.4 Hz, and 16.8 to 21.0 Hz. The FFT lengths, and hence the frequency sampling intervals, are computed from the maximum length of each window across a gather. The FFT length used for a particular window is constant for that window across the entire gather, but can vary from window to window and from gather to gather. As a reference, the limits of the frequency bands used for the first gather of the data set are printed.