Wonder what is really happening when you press the noise cancel button

The 28 frequencies transmitted by the Explorer are all produced from one variable frequency oscillator,and all 28 are multiples or sub multiples of this oscillator frequency. When noise cancel is selected,it steps the variable frequency oscillator through 11 frequencies (thus slightly changing all 28 frequencies by the same ratio). It records the received electrical noise at each step. At the end of this procedure, it selects the frequency that was the most quiet. It also remembers this frequency when you switch the Explorer off and sets it to that particular frequency when you turn it back on again. The receiver is designed to properly receive the whole range of possible transmitted frequencies,so depth,discrimination will not be affected by which frequency the noise cancel circuit selects. If the coil is moved during this procedure,what happens is that as you move the coil,it may change the noise it is picking up in that instant,therefore whatever frequency was being sampled at that instant will have noise' recorded against it,and it means that it won't use that particular frequency (even though that frequency may have been the quietest). The received electrical noise', referred to consists of electrical noise radiated by power lines,air conditioners, computers,other metal detectors. All electrical appliances radiate some electrical noise.