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Wideband frequency response graph

Wideband Frequency Response graph

The Wideband Frequency Response graph displays the frequency response of a selection or region on an active echogram. The graph can be linked to selections or regions, and will respect transects when they are used. The graph can be set up to display the frequency response for matching pings in specified multiple variables.

Note: Applied minimum or maximum thresholds, no-data, bad data, empty water values and some other sample value calculations in graph input variables may affect/degrade the wideband frequency response. The same warning is true for any upstream variables of operands used to detect single targets. See also: Effect of No data samples below.

The frequency response may be displayed for:

Sv pulse compressed wideband data
TS pulse compressed wideband data, with or without beam compensation
Single targets detected on pulse compressed wideband data, with or without beam compensation

To display the Wideband Frequency Response graph:

On the View menu, select Graph, Wideband Frequency Response.
On the graph, use right-click Graph Properties.
Configure the graph.
Make a selection or click a region on a pulse compressed wideband echogram.

Graph calculations

The output from the wideband frequency response graph is based on the computations in Andersen et al. (2023).

Echoview uses pulse compressed complex data to calculate the frequency response of selections or regions on echograms. The Fourier calculations are time consuming and, to maximize performance, the frequency response is calculated by processing contiguous samples in selected pings within the selection or region. Where a single target mask variable is specified, the frequency response centered around each single target within the selection or region is evaluated. Pings included in graph calculations have times that lie within the allowed time separation from reference variable pings.

Bad data samples and samples and targets within exclusion lines are not included in graph calculations.

Graph assumptions

The graph displays an error or the Wideband Frequency Response export is aborted (and a message is displayed) when any graph assumption fails.

The Wideband Frequency Response graph requires that:

Variables must exist and be available.
Variables have the correct data type.
All pings in the selection/region have the same calibration.
Reference variables are the correct data type for the selected graph configuration.
1. TS for TS variable configuration and Single target for (TS variable, single target variable) configuration.
2. Sv for Sv variable configuration.

Sv variable and TS variable configurations

The selection/region is sub-sampled as follows:

Each ping in the reference variable is analyzed. Pings in other selected variables closest to and within the Maximum acceptable time difference from reference (seconds) variable ping times, are also analyzed.
Identified pings with contiguous samples within the selection or region, generally output one frequency response per ping and per variable. A single frequency response is the average of adjacent and overlapping calculation windows evaluated over the samples in the ping. Windows overlap by 50%. The calculation Window size is restricted to a odd number of samples. Where the specified Window size is an even number of samples, the Window size is adjusted to the nearest odd sample number. No frequency response is displayed when the selection or region spans an even number of samples and the Window size is the same (vertical) size as the selection or region.
Identified pings with non-contiguous samples within a region may output a frequency response per set of contiguous samples.
When a variable does not have the samples to fill the Window size, that calculation is considered no data.
The range for TVG calculations, in power to Sv or power to TS calculations, is the range of the sample in the center of a calculation Window.

TS variable, single target variable configuration

Within the selection/region, single target pings is processed as follows:

The single target variable is automatically used as the frequency reference variable and all single target pings are used as reference pings. The TS pings closest to and within the Maximum acceptable time difference from reference (seconds) variable ping times are used for TS calculations. When a target ping-time is exactly between two TS ping-times, the left TS ping is used. Note that multiple single target pings may be matched to the same TS ping.
The single target range is used to calculate the TVG in the power to TS equation.
The single target range is used for the center of the frequency response calculation Window. In the case where there are not enough samples to fill the Window, the Window is padded with no data.
One frequency response is output for each single target.
You may choose to Apply beam compensation. The validity of frequency response values is affected by the evaluation of frequency dependent calibration values and by the limitations of the beam compensation equation. For further information refer to Simrad LOBE used by Simrad EK80 pulse compressed data.

Effect of No data samples

No data, bad data regions (No data or empty water) and thresholded samples are handled as the value (0 + 0i). Such sample values introduce edge noise and abrupt starts and stops to the data. FFT calculations handle such data poorly and the frequency response may behave unexpectedly. See also About transmitted pulses: Note.

Notes:

When frequency-dependent calibration settings are read from the data file, graph calculations use frequency-interpolated values. When frequency dependent calibration settings are absent, graph calculations use a nominal frequency.
Specified variables in the Filter variable list may have varying sampling thickness. In this case, the calculation Window may be different per specified variable.
Specified variables may contain only some of the ping-times that exist in the selection/region. When a Reference variable ping time or a single target ping time is outside the bounds of a variable in the Filter variable list, the calculation interval is treated as no data. As a result, no response is displayed and the export outputs -9.9e+37.
The resolution of the freqency axis can be specified.
Wideband Sv frequency response values include corrections for TransmittedPulseDuration and the calculation of frequency-dependent TwoWayBeamAngle.
At the top and bottom of a ping, the sample calculation Window may be clamped to the extent of the echogram. As a result, the actual Window size can be smaller than that specified under Graph or Export Properties. Under a smaller Window size, the FFT calculation is padded with samples of zero value and the autocorrelation is the same as for all other FFT calculation windows in the ping.
Wideband Frequency Response graph data may be exported.
See also Wideband Frequency Response Properties: Window size notes and Resolution of frequency axis (kHz) discussion.
The Wideband single target detection operator and the Wideband Frequency Response graph are under active development.

Graph linking

Initially, Graph menu Linking is set to Selection and Region. This enables the graph to update as the selection changes.

Graph Properties

Variables, beam compensation, (calculation) window size, graph resolution and averaging, and allowed time separation from reference pings may be specified using the Wideband Frequency Response Graph Properties dialog box.

Graph export

To export frequency response data from a Wideband Frequency Response graph:

Click on the Wideband Frequency Response graph.
On the Graph menu select Export Data.
Enter a Filename and click Save.