Echogram: Echogram page
The Echogram page allows you to choose which type of echogram you wish to display. You can also control the TVG (Time Variable Gain). The ME70 can work with several different TVG compensation settings. The TVG (Time Variable Gain) compensation is designed to counteract the natural phenomena of geometric spread and absorption loss.
How to open
This dialog box is opened from the Active menu.
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Description
Each separate echogram view offered by the ME70 can show you a different echogram type. Which type to see is chosen on this Echogram page. The page also allows you to select which TVG curve to use for the chosen echogram.
The Compression and Expansion options are provided  to specify how sample data are converted to pixel data.
The setting(s) you choose will only be valid for the currently active echogram. Click in any echogram view to make it active. The active echogram view is identified with a thicker border. Select Apply to All if you wish to use the chosen settings on all the echograms of the same type.
Tip
You can also select TVG setting with the TVG function. The TVG (Time Varied Gain) function is located on the Active menu.
Details
TVG
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 TVG (Time Variable Gain) compensation is designed to counteract the natural phenomena of geometric spread and absorption loss.
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.
Select the TVG setting you want to use. Several TVG compensation settings are available.
•  Pr(No TVG)
TVG compensation is not implemented. This option is hardly ever used.
•  Sv (20 Log)
Volume backscattering strength
•  Sp (40 Log)
Point backscattering strength
Echogram Type
Use this function to select what kind of echogram you wish to see in the current (active) view.
•  Surface
A Surface echogram is mainly used when you wish to look at the entire water column starting from the sea surface and down to the sea bottom. Since this echogram is referenced to the sea surface, the sea bottom contour will vary with the actual depth. If you set up the Start Range and Range depths to place the sea bottom contour at the lower end of the echogram, you will have good opportunity to study the echoes from the water column.
In the surface echogram, all calculations are made from the sea surface and down to the detected sea bottom. Use this echogram type to obtain correct calculation of the biomass. It will also provide valid data for the Target Strength Histogram information field.
•  Bottom
A Bottom echogram is mainly used when you want to examine the echoes from fish close to the sea bottom. Since this echogram is referenced to the sea bottom, the sea surface will vary with the actual depth, while the bottom is drawn flat. This makes it easy to study the echoes from the sea bottom. You can investigate the sea bottom conditions and hardness, and detect fish.
The echogram is only drawn for pings that have a successful bottom detection.
Compression / Expansion
The Compression and Expansion options are provided  to specify how sample data are converted to pixel data.
Each ping consists a given number of data samples, where the number of samples is set by the current depth range. This number of samples does not necessarily match the number of vertical pixels in the echogram presentation. The data samples must therefore be compressed or expanded to fit the number of available pixels. In other echo sounders, this function is handled automatically without allowing you to control the process.
•  Compression
The Compression options describe the situation when the number of samples is higher than the number of pixels, and multiple samples are compressed into one pixel.
–  Mean: The mean of the samples is used as pixel value.
–  Peak: The peak value of the samples is used as pixel value.
•  Expansion
The Expansion function provides a mean to adjust the presentation when the number of samples is lower than the number of pixels used to display them. When this happens the sample values must be expanded to match the pixel values.
–  Interpolation: To adjust the presentation, the pixel values are interpolated between two sample values.
–  Copy: The expansion takes place by copying sample values into multiple pixel values.
Apply
Select Apply to save your settings without closing the dialog box.