About BioSonics Visual Analyzer
BioSonics Visual Analyzer is an analysis package which supports the display of echograms, echo integration and single target detection. A comparison of results between Visual Analyzer and Echoview may reveal differences. Echoview Software can supply EV files, data files and integration results from Echoview and Visual Analyzer that illustrate the differences. These are discussed below:
Echo integration
There are several reasons Visual Analyzer calculates different mean Sv values to Echoview. We have compared Visual Analyzer with Echoview in detail and identified the following sources of discrepancy. If these are correctly addressed Visual Analyzer and Echoview will produce the same result.
Calibration offset
Visual Analyzer 4.0.2 erroneously rounds the values of receive sensitivity (RS) and source level (SL) to integers when reading data files created with older versions of Visual Acquisition (contact BioSonics for precise software version numbers). These values are used in converting recorded received power measurements to volume backscattering strength (Sv). The values used by Visual Analyzer are recorded in the analysis report it produces (a .csv format file).
Echoview reads the receive sensitivity (RS) and source level (SL) from the data file and uses these to convert the recorded power readings to Sv values. If your version of Visual Analyzer is erroneously reading these, a calibration offset must be applied to Echoview to reproduce this error in Visual Analyzer.
The magnitude of the necessary offset is the magnitude of the rounding error established as follows:
Calibration Offset = RSEV + SLEV - (RSVA + SLVA)
Where:
CalibrationOffset is the value displayed on the Calibration page of the Variable Properties dialog box for your Sv variable.
RSEV is the Receive sensitivity read from the data file and is displayed on the Details dialog box.
SLEV is the Source level read from the data file and is displayed on the Details dialog box.
RSVA is the Receive sensitivity read from the Visual Analyzer analysis report file.
SLVA is the Source level read from the Visual Analyzer analysis report file.
Note: This calibration offset is only used to reproduce an error in Visual Analyzer. It should not be used in analyzing data. The recommended value is 0.
Absorption coefficient
The Echoview Sonar calculator can calculate the absorption coefficient based upon temperature and salinity according to Francois & Garrison 1982. We observe small differences in Visual Analyzer's calculation of absorption coefficient based upon the same temperature and salinity. The difference does not appear to be significant. If comparing results between Echoview and Visual Analyzer however you must calibrate Echoview with the value Visual Analyzer is using, as recorded in the analysis report it produces.
Sound speed
The Echoview Sonar calculator can calculate sound speed based upon temperature and salinity according to Mackenzie 1981 or LeRoy 1969. We observe small differences in Visual Analyzer's calculation of sound speed based upon the same temperature and salinity. The difference does not appear to be significant. If comparing results between Echoview and Visual Analyzer however you must calibrate Echoview with the value Visual Analyzer is using, as recorded in the analysis report it produces.
Thresholds
Echoview applies thresholds during analysis by setting the value of every sample at or below the threshold value to 0 m2/m3 (integration is executed in the linear domain). Visual Analyzer applies thresholds differently. This may cause slight differences in integration result if thresholds are applied. They are not expected to be significant. If comparing results between Echoview and Visual Analyzer however you must disable the threshold in Echoview on the Data page of the Variable Properties dialog box and set Visual Analyzer's data threshold to its minimum value (-150 dB in version 4.0.2).
Please consult with BioSonics regarding the details of their thresholding method.
Residual rounding
With the calibration offset, absorption coefficient and sound speed considerations taken into account, there may still be a small difference in results due to internal rounding of results during calculation. Any such residual difference should not be significant. In a thorough comparison we found a residual difference between the Echoview integration and Visual Analyzer integration of 0.07%. With a data threshold of -50 dB re m-1 applied, the difference rose to 0.5%.
Single target detection
There are several reasons Visual Analyzer reports different results to Echoview for single targets. In consultation with BioSonics we have determined the following.
Target detection
Visual Analyzer uses a different technique for identifying which pulses in the return signal are identified as single targets. Echoview's algorithm is documented in detail in this help file (see Single target detection algorithms), the exact algorithm implemented in Visual Analyzer is unavailable to us.
Target TS and angles
Visual Analyzer has a configurable "End Point Criteria" which is the functional equivalent of Echoview's pulse length determination level which is used to identify which samples lie within the pulse and how long the pulse is.
Visual Analyzer uses the average of the center 50% of samples to determine the TS and angles of the single target. Echoview uses the TS and angles of the peak sample (the sample with the highest TS). This will lead to differences in the properties of single targets.
A worked example
A worked example including exported analysis results from both Visual Analyzer and Echoview and a supporting document is available for download at:
https://support.echoview.com/downloads/HelpFile/EV-VA_Comparison.zip