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Fish Finders and Depth Sounders

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In broad terms, depth sounders and fish finders are the same - both are forms of active sonar. Sonar uses four basic components, a display unit, transducer, transmitter and receiver. Sonar begins with a transmitter which emits an electrical impulse. This goes to a transducer. The transducer, like an antenna, converts this impulse into focused sound waves shot into the water. The sound frequency is inaudible to humans and fish. As the sound wave bounces off objects it reflects back and is heard by the same transducer. The transducer converts the sound back to an electrical signal and sends it on to the receiver. The receiver amplifies the return signal or "echo", and passes it along to the display unit. A microprocessor within the display calculates the time lapse between the transmitted signal and echo return to determine distance to the object. The result is the readout on display. The whole process repeats itself several times each second.The principle of operation is the same in both depth sounders and fish finders.

So what's the difference? A depth sounder displays a number for depth whereas a fishfinder has a scrolling timeline graphic. The transducer is basically the same for depth sounder and fishfinder. Add a specialized microprocessor to a depth sounder (which can calculate in a time line display) and it becomes a crude fishfinder. This is true in principle, but in reality there is much more complexity involved in modern fishfinders. One major difference, the transducer in a fishfinder is engineered to reflect fish. This is tricky because fish are mostly water based. Most sonar frequencies pass through a fish without registering. The fishfinder emits sonar frequencies tuned specifically to detect the air within fish swim bladders. Many incorporate dual frequency transducers for various depth ranges. The two frequencies are optimized, 192 or 200 KHz for shallow depths and 50 KHz for deeper depths up to 1000 ft. Add multiple sonars reporting back to one display, and the picture gains dimension. These are "Multi-beam fishfinders," which means they use two or more transducers. With multiple sonar readings, the fishfinder has a parallax to create dimensional images. Thus multi-beam transducers are capable of creating a 3 dimensional readout. Rather than a spike in depth, the display shows bottom structure such as fallen trees and shipwrecks. Taken another step, newer multibeam fishfinders will scan sideways. This function is like an undersea radar. It scans 360 degrees around the boat from straight down to horizontal in search of fish and structure. Such fishfinders paint a virtual picture of the bottom.

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