The older detectors only had one tone. You had to interprete what the tone was saying by how it sounded. Did it crackle, did it only beep on a swing in one direction, was it really loud or faint, could you hold your detector off the ground several inches and still get a loud beep?
After practice, you knew what that monotone beep was telling you.
When I got my first detector that had a variable pitch to it, darn thing almost drove me crazy with high squeals and low braats....I switched it to single tone ID.
Every once in a while I'll switch it back to the variable ID...but still having a hard time getting used to it. But there again...where I work I have to listen for warning sirens, horns, bells....just jangles my nerves. Something soothing to me with a steady hum and beep of the same pitch.
And yes, a lot of times bottle caps, even in the good discriminating detectors, ring like a dollar and signal as such. My Vision has a specific setting for bottle cap rejection just because of their unique way of tricking machines.
Bottle caps ring somewhere around 94 on the VDI almost identical to silver dollars...quarters around 80...dimes 78...all very similar when your detector is varying in high pitched squeals...
Not to many other objects will fool you like a bottle cap, unless your looking for gold which rings in the same as pull tabs. People that find gold rings and such dig a lot of those tabs.
If you want to find silver dollars, you may have to dig quite a few beer caps.
Al