Adjusting The Tvg In The Echogram Dialog Box - Simrad ES80 Reference Manual

Simrad ES80 Reference Manual

Adjusting the TVG in the Echogram dialog box

The Time Varied Gain (TVG) can be defined in the
of the function on the
TVG
Context
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.
• : 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.
The TVG (Time Varible Gain) compensation is designed to counteract the natural
phenomena of geometric spread and absorption loss. In the ES80, the TVG compensation
is made using digital signal processing software.
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.
Several TVG compensation settings are available.
•
No TVG
TVG compensation is not implemented. This setting is hardly ever used.
•
School (20 log R)
This gain setting offers weaker amplification close to the bottom. It has been designed
to provide the best echoes for schools. The physical size of a school allows you to
detect it even with reduced gain.
•
Fish (40 log R)
This gain setting provides larger amplification closer to the bottom. It has been
designed to provide the best echoes from single fish.
•
Targets (40 log R)
This setting uses the same TVG amplification as
and compensation is implemented.
114
menu.
Active
dialog box, or by means
Echogram
, but additional filtering
Fish (40 log R)
406273/C