Vouvron,
Think for PI metal detector as if it is a camera with flash that takes pictures of the earth in darkness. The picture is large as coil size. In the attached figure is shown a square coil and a round coil.
If the camera does not move, it is sufficient to make only one picture to see what's underneath coil in place 1. Since each picture contains noise and interference, we should take several pictures of place 1, then averaging signals, and as result we will obtain a good picture of place 1 because noise and interference are suppressed.
As the camera moves, you should take a picture in the new location 2 over which search coil is passing. The faster you move, the more pictures you need in order not miss a target and to suppress noise and interference. There will be overlap of frames. At round head waving, we need more overlap as shown below.
Now imagine that in addition to your flash, the earth is illuminated by an another powerful flash, which lights up in the Philippines 60 times per second. You must comply with it because otherwise you will get beat effect. You will have in your video one or more slow-moving bright or dark stripes. To avoid strip moving, you have to do a synchronous 60, 120, 180, 240, etc. frames per second to get stationary stroboscopic effect. However there are PI machines that operates even at 3000 PPS (Pulses Per Second) or more because the increase in repetition frequency reduces interference and noise.
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