Hi John,
As far as I know, the only time channel changing is done is when you start getting interference from other detectors near you, or from electronic devices such as power lines. You hit NOISECANCEL and the machine takes 30 seconds to check several different channels to see which one is the least noisy and then it operates on that channel. You can hit the noisecancel button, or you can choose which channel you want to use by choosing them manually. For example, my buddy Richard got too close to me with his new explorer and my explorer started going crazy with extra noises. I hit noisecancel and 30 seconds later, we were both operating with no interference.
With respect to the 28 frequencies, if you look on page 7 of the manual, the text says that the machine analyzes the target by simultaneously emitting 28 frequencies of electromagnetic radiation out from the seach coil. The 28 frequencies hit the target and return to the coil. The machine analyzes all the responses from the target reacting to the 28 frequencies. I have not read anywhere in the text that you can control which frequency or frequencies that are being emitted at a given time. The text says that the advantage of the explorer is that it uses 28 frequencies all at the same time instead of using a smaller number of frequencies.
Mike
