Hi John,
The different names for tuning have evolved as metal detectors evolved. Originally, there was a manual tune, which basically allowed the operator set the threshold level. This is the level of the audio you hear with nothing influencing the detector. Then came Pushbutton tuning (PB). Because metal detectors have a tendency to drift in audio level, manufacturers sort of separated the threshold tuning from other circuitry and added more. The result was, once you set the threshold audio level, if it changed for any other reason, you could push the PB and the audio level would quickly return to your original setting.
If what ever caused the audio to change in the first place disappeared, the audio would then change again, and the operator would have to push the button again, to reset the audio to the original level.
Autotune is the next part of the evolution. It is a feature where the audio will always try to readjust itself back to the level you initially set. If you set the level for just a slight sound, and a target came into the field of the searchcoil, you would get a sudden increase, but then the audio would diminish back to the preset threshold level.
For every action there is an opposite reaction, so once the audio had compensated for a target and the target was then removed, the audio would immediately go silent and the autotune would have to tune itself back to the original setting.
Some manufacturers have made the autotune rate adjustable. This feature is more popular on certain coin and nugget hunting detectors. What an adjustable autotune feature allows the operator to do is to adjust just how fast the audio readjusts itself back to the preset value. Normally, this feature is unnessary except when the detector is going to be used in harsh environments where everything is changing.
A faster autotune normally results in depth loss, but helps keep the audio threshold steadier or at a more constant level.
In areas such as nugget hunting country, the ground conditions constantly change and the terrain is not level. As such, as a person swept the search coil over the ground, he or she would have to constantly change the threshold level, and the ground balance to keep the audio level constant. This was very difficult to do, and it was very difficult to try to figure out what was a target and what was ground noise.
The autotune feature basically takes care of that for you and tries to compensate for the continuous variations by trying to maintain a constant audio level. It also compensates for slight audio changes that would normally occur as the result of internal electrical changes because of component drift.
In short, autotune makes life easier for the operator when operating in less than perfect conditions. On the downside, autotune makes it somewhat harder to pinpoint an object, especially for a beginner, because if you stop the coil over an object, the detector will tune it out or seem to disappear.
Also, autotune will reduce the sensitivity some, simply because it is trying to tune out slight changes. As such, if an operator were to do a bench test, they would see a reduction of sensitivity. However, in the field, the autotune feature can increase depth, simply because it always keeps the threshold at a level where small changes can be heard.
I hope this helps.
Reg
