# Geopsy: Sampling Frequency

The selection of the frequency bands to be processed is driven by parameter inputs inside the Processing tab of the FK and MSPAC toolboxes. In the Frequency sampling layout group you can specify the minimum and the maximum (central) frequencies for processing. Between these two limits, the frequency axis will then be sampled according to the settings in the next line .

There are two options to be specified, namely Step and Number of Samples. For the Step parameter, there are two options to be selected from a drop box: Log or Linear. You might have guessed the meaning right away. When choosing Linear then the frequency axis is sampled linearly between the given limits and exactly Number of Samples will be distributed along the axis. Choosing Log accordingly samples the frequency axis logarithmically, i.e. frequency bands are close spaced at lower frequencies.

Note: the sampled frequencies are taken as center frequencies in the processing scheme. Therefore, it is necessary to specify additionally a bandwidth so that the processing can be performed in a finite but narrow frequency band.

• For FK and HRFK processing, the bandwidth parameter is chosen by the user and chosen as relative half-bandwidth in the FK-gridding layout group in the processing tab .
• For MSPAC, the bandwith is set to 0.1 by default and can not be modified.

The frequency limits for each frequency band are computed as ${\displaystyle [(1-bw)\cdot f_{c},(1+bw)\cdot f_{c}]}$ where ${\displaystyle f_{c}}$ is the central frequency and ${\displaystyle bw}$ is the bandwidth parameter.

Example: a setting of 0.10 for the bandwidth, a Linear spacing of 10 samples along the frequency axis from 1 Hz to 10 Hz will create the following frequency bands:

Frequency band creation using Step: Linear, Bandwidth: 0.10, From: 1 Hz, to: 10 Hz
Freq. band index Lower frequency limit Center frequency Upper frequency limit
1 0.9 Hz 1.0 Hz 1.1 Hz
2 1.8 Hz 2.0 Hz 2.2 Hz
3 2.7 Hz 3.0 Hz 3.3 Hz
4 3.6 Hz 4.0 Hz 4.4 Hz
5 4.5 Hz 5.0 Hz 5.5 Hz
6 5.4 Hz 6.0 Hz 6.6 Hz
7 6.3 Hz 7.0 Hz 7.7 Hz
8 7.2 Hz 8.0 Hz 8.8 Hz
9 8.1 Hz 9.0 Hz 9.9 Hz
10 9.0 Hz 10.0 Hz 11.0 Hz