TVG (Time Variable Gain)

TVG (Time Variable Gain) compensates for the loss of acoustic energy due to geometric spreading and absorption. This makes the targets with the same strength appear with the same intensity independent of their physical distance from the transducer.

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. 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 ST90 system, the TVG compensation is made using digital signal processing software.

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 acoustic 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. Each curve is identified with the equation X log TVG. The coefficient "X" is an integer. Each value for "X" will create a different slope with a different gain compensation. Typical values for "X" are 10 to 40.

The drawing illustrates the TVG principle: The gain at time (B) is larger than at time (A). The different settings determine how much gain the ST90 system will use as time elapses along the X-axis. A higher value for “X� will make the angle (C) steeper.

The ST90 system allows you to choose your own compensation. You can choose any value from 0 Log R to 40 Log R. If you choose 0 Log R the TVG compensation is disabled. Some experimentation may be required to find the compensations that fit your purposes for individual catch and environmental conditions. Typical time variable gain compensation values may be:

0 log R

TVG compensation is not implemented. This option is hardly ever used.

20 log R

This setting offers weaker amplification close to the bottom. It is commonly used to provide the best echoes for schools. The physical size of a school allows you to detect it even with reduced gain.

40 log R

This setting offers larger amplification closer to the bottom. It is commonly used to provide the best echoes from single fish.

Other options are provided when this function us used in an Inspection view. The echo presentation in the Inspection view resembles the echogram you see on an echo sounder. The Inspection view can be regarded as an "echo sounder" with an adjustable direction. In the ST90 system we use the phrase echogram to describe the information provided by the view.

No TVG

TVG compensation is not implemented. This option is hardly ever used.

School

This 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

This setting offers larger amplification closer to the bottom. It has been designed to provide the best echoes from single fish.

User

This setting allows you to define your own TVG curve. To choose your own TVG curve, use the dedicated function in the Echogram dialog box.