Hi John,
I am somewhat familiar with the VCO concept on VLF's. In essence, as a target is approached, the audio level and the frequency of the tone will increase.
The amount of change in either is a function of the size and type of object approached. Those who have become very proficient on their detectors having this feature can distinguish one metal from another in many cases.
Obviously, this technique isn't foolproof, but it does have its merits. Generally, an iron object such as a washer or other small piece of iron will cause a much higher tone or frequency change than a similar sized non-ferrous object like a gold nugget or a gold ring, even though the volume change caused by the two objects may sound similar.
The reason is on a VLF, an iron object simply creates a stronger signal which thus causes a wider variation in pitch.
I have been experimenting with the concept with my PI. In my case, I have elected to basically use the level change out of my last opamp stage to change the audio tone generator.
The technique I used is crude to say the least since it presently causes the audio to go down, rather than the traditional up in frequency as a target is approached. However, my technique was quick and easy way to test the idea.
This rough form of experimenting did give me a better idea of what to expect and how effective it might be.
So far, it is difficult for me to tell differences in metals, but I haven't really done that much experimenting along those lines either. This type of experimenting will come later.
Instead, I was more concerned if the change in pitch would make it easier to determine a very weak target signal from any background noise.
I have determined that, at least for me, a change in the pitch, or frequency as it is commonly called, does make it much easier to distinguish a very weak signal. It also better defines a target, or in other words, makes it easier to hear or determine a buried object from other signals.
It is my guess that I will notice a difference in pitch change on similar sized objects when comparing an iron object to a non-ferrous one. However, this will be dependent upon the overall gain of the system and other factors which could easily mask the positive effects.
Reg