TVG function

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.

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 ST90 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 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.

Details

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 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.

Additional information for the technically minded

When a sound pulse is travelling from the transducer through the water it will undergo geometrical spreading. The sound intensity falls off with the range squared (1/r² where r=range). The same applies to the echo from a target in the water column; sound intensity falls off with 1/r². The total two-way spreading is therefore:

1/r² x 1/r² = 1/r⁴

To remove this effect, the received echo must be compensated according to r⁴. In logarithmic terms this is 10 log r⁴ or 40 log r. This compensation applies to single targets.

When a school of fish covers the whole acoustic beam cross section, the received echo will increase with range simply because the acoustic beam cross section increases with range. The received echo increases according to r², where r is now the radius of the acoustic beam. This introduces an r² into the-two way spreading (r²/r⁴) so that the school compensation becomes r², or in logarithmic terms 20 log r.

In addition to the geometrical spreading, sound energy is absorbed in water. Absorption is higher in salt than in fresh water and increases with the operating frequency. The received echo must also be compensated according to the absorption.

The combined compensation for the geometrical spreading and absorption is the TVG