I have read this take me back to the Fishfinder COMPARE section.

Fishfinder and Sounder Module Introduction 

Fishfinders and Sounder Modules can show the bottom profile, the type of bottom, and the fish in the water column beneath the boat. They are indispensable for fishermen and widely used by most boaters to monitor the depth and bottom conditions.

Fishfinders have integrated displays while Sounder Modules must be attached to a separate display. In other words, Sounder Modules are the component in a modular marine electronics network between the depth transducer and the vessel’s displays. They power and control the transducer and feed bottom profile data to the network.

Integrated Fishfinders have 3 advantages-

• Integrated Fishfinders are simpler to install than a Sounder Module plus a display
• Integrated Fishfinders are more easily removed when not in use
• Integrated Fishfinders have a lower total cost than a Sounder Module plus a display.

But Sounder Module also offer advantages-

• Power- Sounder Modules are available with higher power than most integrated Fishfinders.
• Flexibility- You can select a Sounder module with the power you need, and large displays that fit your boat.
• Lower Noise- Since the Sounder Module can be mounted away from your other electronics it is less susceptible to noise pickup.

How Do Fishfinders and Sonar Modules Work?

The Fishfinder or Sounder Module sends a high frequency signal to a transducer mounted on your boat. The transducer can be mounted on the boat’s transom, through the hull, or bonded to the inside of the hull. Each pulse sent to the transducer is called a “Ping”. Think of the Fishfinder as a radio and the transducer as a speaker.

This Ping is broadcast by the transducer and travels through the water. It is reflected off the bottom or anything in its path and travels back to the transducer. The transducer, after transmitting the pulse, listens for this tiny signal to return. In other words, the speaker is now a microphone. The received signal is sent back to the processing unit. Since the Fishfinder knows when the ping was sent, when the echo was received, and the speed of sound through water, it can calculate the depth. By analyzing the returned signal it can also display any fish or other items in the water column between the transducer and the bottom.

Fishfinder Display-

The display is the most expensive component in the Fishfinder design, so it affects the price you pay. Most fishermen want the largest display their boat can accommodate. On smaller boats the Fishfinder is often installed on a mounting bracket. The width available at your helm determines the largest unit that can be installed. On larger boats the display is mounted in an overhead electronics box or flush mounted on the bridge, so Fishfinder height often determines the largest size that can be installed. Check the unit dimensions in the Guide to determine which Fishfinders will fit your boat.

Check the display’s brightness. If your Fishfinder is covered by a T-top or bimini cover, or you mostly fish in a cloudy climate, you can use a less expensive display than if you fish in Florida.

You can also select the display background color choice that works best for your conditions. For example, a white background while in bright sunlight, a blue background for evening use, and a nighttime pallet for night fishing or cruising

Sounder Module Display-.

The display is independent of the sounder module, but must be compatible. Although most marine networks are based on Ethernet, they are actually proprietary. You cannot attach a Sounder Module from one vendor to another vendor’s network or Display.

Since the Sounder Module is separate from the display it can be mounted in a convenient location isolated from noise, and be as large as necessary for the power it supplies. You can also choose the Module appropriate for your needs. A modular network provides flexibility and a path for expansion.

Power-

The Fishfinder or Sonar Module power determines how deep you can read, assuming all else is equal. Remember that the transducer changes from transmitting a high power pulse to receiving a very small signal. So the Fishfinder’s internal noise level is critical to preserving this signal. In other words, a well-designed, low-power Fishfinder or Sonar Module can read deeper and detect more fish than a poorly designed, high power one. Since a Sonar Module is independent of the display it can be mounted away from noise sources.

Manufacturers use different methods and units to report output power. Some use Watts RMS, others Watts Peak-to-Peak. Since the actual depth you can read depends upon the quality of the water beneath the boat, your transducer, and your installation, manufacturers do not quote the maximum depth their Fishfinders can read.

Ping Rate-

The Fishfinder’s ability to ping at many times per second will affect the apparent resolution. A low ping rate can be an indication of a low-cost design.

Choosing Your Fishfinder or Sonar Module-

If you are simply cruising, and not looking for fish, any Fishfinder or Sonar Module that can work at the depths you cruise should work well. Check the Guide carefully, pick one that has the features you need, install it and the transducer carefully, and enjoy.

If you will be fishing, check the Comparison guide for model that has the features that are important to you. Chose a model that supports the frequencies appropriate to your needs and that has a suitable transducer available.

If you want to connect your Fishfinder to other equipment check the interface specifications and choose one that can communicate with your other equipment. Choosing equipment from the same manufacturer can make this easier.

If you choose a Sonar Module select one from the same manufacturer as your displays.

Features-

• Many features are available on Fishfinders and Sonar Modules. Not all of them will be important to you. Most boaters simply use the automatic mode and are completely happy. Others want more sophisticated control. These features include-
• Depth Range Settings- automatically adjusts the range to the maximum depth being read
• Frequencies- automatically adjusts transducer frequency for optimum depth results
• Power- automatically adjusts the Fishfinder’s Gain
• Color Gain- adjusts the displayed colors to indicate depth
• Zoom- magnifies the bottom detail
• A-Scope shows the detail directly beneath the boat
• Bottom Lock- shows the data from the bottom up
• Scroll speed- adjusts the rate the data scrolls across the Fishfinder display
• Alarms for Shallow Water, Deep Water, Fish detected, and Temperature
• TVG (Time Variable Gain) or STC (Sensitivity Time Control) adjust the gain for the water depth
• Interference rejection- minimizes interference from other Fishfinders or noise sources
• Target Depth ID- shows the individual fish’s depth
• Metric / US Measurements- the ability to display depth in feet or meters
• Depth Offset- adjusts for the distance below water of your transducer
  

  I have read this take me back to the Fishfinder COMPARE section.