About Time Variable Gain

TVG (Time Variable Gain) is a method that compensate signal loss in the water. Basically, the aim is to have targets or sea bottom displayed in the same color on the echogram, whatever the distance from the sensor.

When the sounder sends pings, the deeper the target is, the more attenuated signals will be received and sent back. As a result, if the signal is too much attenuated, echoes (target strength) received from a target might not be as strong as they should be. TVG is here to compensate this effect. It uses a lower gain level when signals travel toward a target at a small distance and higher gain level when signals travel toward deeper targets. The end result is to compensate sounding attenuation and therefore to show a same target strength for a same target at different depths.

You can choose between three different TVG modes:

  • 20 log: use to focus on the bottom, footrope or a school of fish.
  • 40 log: use to focus on individual targets.
  • 30 log: compromise between the two others.

For example, if you want a good view of the footrope, select a TVG mode at 20 or 30 log. You can see on the images below that the footrope is clearer at 20 and 30 log.

If you want a good view of individual targets, you can see that with 40 log, targets in the water column are clearer.

20 log 30 log 40 log