Onwa KF-667 - Manual (Page 6)

PRINCIPLE OF OPERATION
This Color Video Sounder determines the distance between its transducer and underwater
objects such as fish, lake bottom or seabed and displays the results on a 5.6-inch color screen.
It does this by utilizing the fact that an ultrasonic wave transmitted through water travels at a
nearly constant speed of 4800 feet (1500 meters) per second. When a sound wave strikes an
underwater object such as fish or sea bottom, part of the sound wave is reflected back toward
the source. Thus by calculating the time difference between the transmission of a sound wave
and the reception of the reflected sound wave, the depth to the object can be determined. In a
sense an echo sounder can be thought of as being an extremely sophisticated and quick timer,
since it is capable of resolving time differences shorter than one thousandth of a second.
The entire process begins in the display unit. Transmitter power is sent to the transducer as a
short pulse of electrical energy. The electrical signal produced by the transmitter is converted into
an ultrasonic signal by the transducer and transmitted into the water. Any reflected signals from
intervening objects (such as a fish school) are received by the transducer and converted back
into an electrical signal. It is then amplified in the amplifier section, and finally, displayed on the
screen.
The picture displayed by the Color Video Sounder is made up of a series of vertical scan lines,
one for each transmission, Each line represents a snapshot of what has occurred beneath the
boat. The series of snapshots are accumulated side by side across the screen, and the resulting
contours of the bottom and fish between the bottom and surface are displayed. The amount of
history of objects that have passed beneath the boat over a series of transmission varies from
less than a minute to a few minutes, depending on how you adjust the unit.
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