Simrad ES80 Reference Manual page 417

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
ES80, 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.
• : Once transmitted, the acoustic energy will spread out to form a
Geometric spread
circular beam. The width of this beam increases with the physical distance to the
target(s).
• : Depending on the salinity and temperature, the water will absorb
Absorption loss
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
dialog box. The
Echogram
The ES80 allows you to use different TVG compensation for the echoes from the water
column than from the bottom. By making the bottom echoes individually adjustable,
you can fine tune the presentation from the bottom to investigate the bottom type and
conditions. The
Bottom TVG
adjust the
Bottom TVG
Details
TVG
Select the
TVG
available.
•
No TVG
TVG compensation is not implemented. This setting is hardly ever used.
406273/C
function is located on the
TVG
adjustment has no effect on fish close to the bottom. To
, open the
Echogram
setting you want to use. Several TVG compensation settings are
Functions and dialog boxes
Echogram
dialog box.
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