Abstract:
A fish finder includes a transducer that repeatedly transmits ultrasonic pulse signals downward into the water and receives signals from the water, a first signal generating unit that generates a first signal on the basis of a reception signal at an identical depth caused by signals transmitted at least twice, and an interference detecting unit that detects an occurrence of interference on the basis of plural first signals at different depths.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fish finder that transmits and receives ultrasonic waves to find a shoal of fish in the water, and, more particularly to a fish finder that detects occurrence of interference caused by other ultrasonic devices and automatically performs interference removal. 
   2. Description of the Related Art 
   In general, a fish finder mounted on a fishing boat has a problem of interference caused by ultrasonic waves transmitted by other ultrasonic devices mounted on the fishing boat or other boats operating in sea areas near the fishing boat. 
   Conventionally, as means for solving this problem, a fish finder having an interference removal function is known. With this interference removal function, the fish finder compares, for each identical depth, reception signals obtained by transmission and reception performed plural times and selects a weakest signal to perform processing for interference removal. 
   When the interference removal function is used, there is an inconvenience that not only interference signals but also useful echoes including information on a shoal of fish or the like is controlled. Thus, when no interference occurs, the interference removal function should be turned off as much as possible. However, in the conventional apparatus, a user needs to judge at any time whether there exists interference and manually perform on/off operation for the interference removal function. As a result, the user is forced to perform troublesome operation. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a fish finder that can detect presence or absence of interference and automatically turn on and off an interference removal function without bothering a user. 
   According to the invention, there is provided a fish finder comprising a transducer that repeatedly transmits ultrasonic pulse signals downward into water and receives echo signals from the water, an A/D converter that converts the signal received by the transducer into a digital signal, a memory that stores the signal outputted from the A/D converter, an interference detecting unit that detects an occurrence of interference on the basis of a reception signal belonging to a predetermined depth range among reception signals stored in the memory, and an interference removing unit that when the interference detecting unit detects an occurrence of interference, outputs a signal obtained by applying predetermined interference removal processing to the reception signal. 
   According to the invention, there is provided an interference detection method in a fish finder comprising a step of repeatedly transmitting ultrasonic pulse signals downward into water and receiving signals from the water, a step of generating a first signal on the basis of reception signals at an identical depth resulting from a plurality of transmission signals, and a step of detecting an occurrence of interference on the basis of plural first signals at different depths. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagram for explaining an embodiment of the invention; 
       FIG. 2  is a diagram for explaining a structure of a memory  6 ; and 
       FIG. 3  is a flowchart for explaining operations of an interference removal function in the embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the invention will be explained with reference to  FIG. 1 . 
   In  FIG. 1 , a transducer  1  mounted on a ship bottom or the like is driven by an electric signal supplied from a transmitting unit  3  via a trap circuit  2  to transmit an ultrasonic pulse signal into the sea and receive echo of the ultrasonic pulse signal reflected by a target in the sea and outputs a reception signal to an amplifier  4  via the trap circuit  2 . The amplifier  4  amplifies the reception signal. An A/D converter  5  converts the amplified signal into a digital signal. A memory  6  sequentially stores reception signals converted into digital signals. 
   A structure of the memory  6  will be explained with reference to  FIG. 2 . 
   The memory  6  includes plural line memories that store data sequences of reception signals, which are obtained by one transmission, in an order of depths. The memory  6  stores data sequences of reception signals for several times of transmission. The memory  6  in  FIG. 2  includes line memories  6   a ,  6   b , and  6   c  of three rows and can store reception signals for transmission and reception for three times. In the memory  6 , the line memory  6   c  (D[0;0] to D[0;N]) stores a reception signal received this time (at a time), the line memory  6   b  (D[1;0] to D[1;N]) stores a reception signal received last time (a previous time), and the line memory  6   a  (D[2;0] to D[2;N]) stores a reception signal received before last (two times previous to the time). When a transmission and reception operation is performed anew, a data sequence of a reception signal obtained by the latest transmission is written in the line memory  6   c  on the right side. At the same time, data sequences of reception signals obtained by transmission before the last time are rewritten in the line memories while being shifted to the left by one row every time the a data sequence is rewritten. Oldest data sequence stored in the left line memory  6  is erased. 
   A seabed detecting unit  7  detects a seabed position for every transmission and reception on the basis of the data stored in the memory  6 . The detection of a seabed position is performed on the basis of, for example, whether intensity of an echo signal has exceeded a predetermined value set in advance. Information on the seabed position detected is sent to an interference detecting unit  8 . 
   The interference detecting unit  8  reads out the reception signal received this time and the reception signal received last time stored in the memory  6 . The interference detecting unit  8  judges presence or absence of interference on the basis of the reception signals and outputs information on presence or absence of interference to an interference removing unit. 
   Specifically, the judgment on presence or absence of interference in the interference detecting unit  8  is performed as described below. 
   The interference detecting unit  8  compares, for each same depth, the latest reception data sequence and the reception data sequence of the last time stored in the memory  6 . When the latest reception data is larger than the reception data received last time, the interference detecting unit  8  calculates a subtraction value obtained by subtracting the reception data of the last time from the latest reception data. (The subtraction value is one example of “a first signal” in claims.) The interference detecting unit  8  adds up subtraction values from data immediately after starting reception (or data after elapse of a predetermined time from start of reception) to data of a seabed position (or data of a position shallower than a seabed position by a predetermined depth) and sets the obtained addition value as Tk. As the seabed position, a shallower one of two seabed positions based on the reception signals received last time and this time, which are detected by the seabed detecting unit  7 , is adopted. 
   Although not described above to facilitate understanding, it is desirable to remove data of a portion corresponding to an oscillation line or tailing of the oscillation line (e.g., data immediately after start of reception such as D[0;0] to D[0;10]) from data to be used for the processing in the interference detecting unit  8 . The oscillation line is mainly caused by leakage of a part of a transmission signal to a reception circuit via the trap circuit  2 . 
   When the addition value Tk is larger than a threshold A corresponding to a detection range, the interference detecting unit  8  judges that interference has occurred. When the addition value Tk is smaller than the threshold A, the interference detecting unit  8  judges that interference has not occurred. As the threshold A, a value inputted by a user using predetermined input means or a value stored in storing means in association with a value of the detection range which is automatically set on the basis of a depth of the seabed is appropriately read out and used. 
   In addition, the interference detecting unit  8  judges intensity of interference removal processing (the intensity is represented as NL 1  and NL 2  in order from lowest intensity) that should be executed by an interference removing unit  9  and gives an instruction to the interference removing unit  9 . Specifically, when the interference detecting unit  8  detects interference once in a state in which there has been no interference, the interference detecting unit  8  judges that interference removal processing of the intensity NL 1  is appropriate and instructs the interference removing unit  9  to execute the interference removal processing of the intensity NL 1 . When the interference detecting unit  8  subsequently detects interference once more, the interference detecting unit  8  judges that interference removal processing of the intensity NL 2  is appropriate and instructs the interference removing unit  9  to execute the interference removal processing of the intensity NL 2 . When the interference detecting unit  8  detects interference further more, the interference detecting unit  8  judges to continue the interference removal processing of the intensity NL 2  is appropriate and instructs the interference removing unit  9  to continue to execute the interference removal processing of the intensity NL 2 . If interference is not detected any more, the interference detecting unit  8  gives the interference removing unit  9  an instruction that the interference removal processing does not have to be performed. 
   The interference removing unit  9  executes the interference removal processing for a reception signal on the basis of information on presence or absence of an occurrence of interference and information on intensity of the interference removal processing given by the interference detecting unit  8 . 
   The interference removal processing executed by the interference removing unit  9  will be explained. 
   In the weak interference removal processing (NL 1 ) executed by the interference removing unit  9 , the interference removing unit  9  selects, for each identical depth, a signal with lower intensity out of reception signals for two times (reception signals received this time and last time) and outputs the signal. On the other hand, in the intense interference removal processing (NL 2 ) executed by the interference removing unit  9 , the interference removing unit  9  selects, for each identical depth, a signal with lowest intensity out of reception signals for three times (reception signals received this time, last time, and before last) and outputs the signal. When the interference removal processing is unnecessary, the interference removing unit  9  directly outputs the received reception signal which is inputted this time. 
   A display processing unit  10  processes a signal outputted by the interference removing unit  9  and generates a display signal. A not-shown display unit displays images of a shoal of fish and the seabed. 
   Operations in the interference detecting unit  8  and the interference removing unit  9  according to this embodiment will be explained with reference to a flowchart in  FIG. 3 . In this flowchart, a variable b is a parameter concerning “whether interference was detected last time” stored in the interference detecting unit  8 . That is, “b=0” indicates that the interference detecting unit  8  did not detect interference last time and “b=1” indicates that the interference detecting unit  8  detected interference last time. Data stored in the line memories  6   a ,  6   b , and  6   c  are represented as D[2; i], D[1;i], and D[0;i], respectively, using a parameter i concerning a depth. Tk is a parameter representing an integral value of intensity and A is a threshold for judging presence or absence of interference. 
   (Step 1) In the interference detecting unit  8 , initial values are set as b=0, i=−1, and Tk=0. 
   (Step 2) The interference detecting unit  8  adds 1 to i. 
   (Step 3) The interference detecting unit  8  compares intensity of the reception signal D[0;1] received this time and intensity of the reception signal D[1:i] received last time at an identical depth. 
   (Step 4) When D[0;i] is larger than D[1;i] in step 3, the interference detecting unit  8  adds a value obtained by subtracting D[1:i] from D[0;i] to the parameter Tk representing an intensity integral value. 
   (Step 5) The interference detecting unit  8  judges whether is a value i 0  corresponding to a seabed position. The value i 0 , which is information on a seabed position, has been inputted to the interference detecting unit  8  from the seabed detecting unit  7 . 
   (Step 6) The interference detecting unit  8  judges whether Tk is larger than the predetermined threshold A determined on the basis of a detection range. 
   (Step 7) The interference detecting unit  8  judges, on the basis of a value of b, whether interference was detected last time. 
   (Step 8) The interference removing unit  9  executes the weak interference removal processing of NL 1 . The interference removing unit  9  outputs a smaller signal out of D[0;i] and D[1:i] for each i to the display processing unit  10  as a signal subjected to interference removal. 
   (Step 9) The interference detecting unit  6  changes the value of b to 1. The interference detecting unit  8  changes Tk to 0 and i to −1 and returns to step 2. 
   (step 10) The interference removing unit  9  executes the intense interference removal processing of NL 2 . The interference removing unit  9  outputs a signal of smallest one of D[0;i], D[1;i], and D[2;i] for each i to the display processing unit  10  as a signal subjected to interference removal. 
   (Step 11) The interference detecting unit  8  changes Tk to 0 and i to −1 and returns to step 2. 
   (Step 12) The interference removing unit  9  directly outputs the reception signal D[0;i] received this time to the display processing unit  10 . The interference detecting unit  8  changes Tk to 0 and i to −1 and returns to step 2. 
   The threshold A is determined taking into account the number of data N of a reception signal, a water depth value, and the like. 
   In the above explanation, intensity of the interference removal processing is represented as two ranks, NL 1  and NL 2 . However, as intensity of the interference removal processing, one rank or three or more ranks may be adopted. 
   When the seabed is not detected in the seabed detecting unit  7 , the interference removal function in the interference removing unit  9  may be unconditionally turned on. Intensity of interference removal in this case may be NL 1  or NL 2 .