TVG function
TVG (Time Variable Gain) compensates for the loss of acoustic energy due to geometric spread and absorption. This makes the targets with the same strength appear with the same intensity independent of their physical distance from the transducer.
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How to open
This function is opened from the Active menu.
Description
By means of algorithms, the time variable gain compensation converts the echo presentation as a function of range. This makes the targets with the same strength appear with the same intensity independent of their physical distance from the transducer. In the ME70 system, the TVG compensation is made using digital signal processing software.
When an acoustic pulse is sent through the water, it will gradually lose its energy. The greater the distance between the transducer and the target(s), the greater the loss of energy. The loss of energy is explained with two factors.
•  Geometric spread: Once transmitted, the acoustic energy will spread out to form a circular beam. The width of this beam increases with the physical distance to the target(s).
•  Absorption loss: Depending on the salinity and temperature, the water will absorb some of the energy from the transmission. The absorption loss increases as the physical distance to the target(s) increases.
Both the geometric spread and the absorption will also have an effect on the returned echo signal. That is why we normally refer to these factors as the two-way transmission loss. TVG (Time Variable Gain) compensates for this loss of acoustic energy.
The TVG compensation is expressed as a logarithmic curve. You can choose from a selection of curves. Each curve has a different slope creating a different gain compensation. Each curve is identified with the equation X log TVG. The coefficient "X" is an integer. Typical values for "X" are 10 to 40.
Tip
You can select TVG using this function. You can also adjust the TVG setting in the Echogram dialog box. The TVG function is located on the Echogram page.
Details
TVG
Select the TVG setting you want to use. Several TVG compensation settings are available.
•  No TVG
TVG compensation is not implemented. This option is hardly ever used.
•  Sv (20 Log)
Volume backscattering strength
•  Sp (40 Log)
Point backscattering strength
Note
The setting you choose will only be valid for the currently active echogram. The active echogram view is identified with a thicker border. Normally, you must first click in the chosen echogram to activate it, and then choose the setting you wish to use.
Apply to All
Select this box to use the chosen setting on all the echograms of the same type.