20.6.5 DAS Spectra data

The DAS Spectrum object stores one spectrum or multiple spectra calculated from the raw DAS data recordings. The spectra are typically calculated as N-point fast Fourier transforms (FFTs) along a window of length FilterWindowSize Raw DAS sample. The FilterWindowSize equals the FFT size N and is the number of output points from the discrete Fourier transform (DFT) calculation.

For each locus in the array of loci specified by StartLocus and NumberOfLoci, the FFT calculation produces FilterWindowSize data points. FFT calculations can be repeated by shifting the filter-window over the raw DAS samples. The windows can be overlapping and the number of samples that overlap is specified by the FilterWindowOverlap attribute. This means that the spectrum values are stored in a 3D array [time:locus:FFT-value]. Figure 20.6.5-1 shows an example of 4 overlapping windows in the spectra object in the example file.

Figure 20.6.5-1 Spectra[0] attributes in H5 sample file part1.h5.

Figure 20.6.5-2 shows one of the FFT sub-arrays (FFT index 4 out of 32) for the 4 overlapping windows (index 0 to 3 on the vertical axis) calculated for the 5 loci (index 0 to 4 on the horizontal axis).

Figure 20.6.5-2 DAS FFT spectra for the spectra data sub-array for one loci (index 3) for the 3 overlapping windows (rows: index 0 to 21) calculated at different times.