Frequency "selection" is automatically selected via the Frequency Scan selection for the most quiet frequency. The user has no control of which Detection Frequency is used.
The Discrimination adjustment is user controlled and adjusts the timing period of the return signal that is looked at.
The square wave is a complex waveform is a made up of a series of sine waves. The square wave contains a fundamental and a series of ODD HARMONICS; that is harmonics which are odd number multiples of the fundamental (x3 x5 x7 etc.) These are called the 3rd harmonic, 5th harmonic etc.
I have to believe, from my electronics background, the ATX and Infinium both have the amplitude of the odd harmonics frequencies (in multiple groups) set to different "power" factors which will alter the shape of the pulse and thus reduce some of the mineralized soil factors.
The pulse is not just a simple on-off (0 or 1) state. I would also say that the Detection Frequency is actually a Repetition Rate (pulses per second) as opposed to a frequency (cycles per second). See below diagram.
From Garrett:
Detection Frequency 730 pulses per second, adjustable
MULTI-FREQUENCY (PULSE INDUCTION)
Detector that divides its power into multiple frequencies to overcome the effects of mineralized soil and saltwater content. Discrimination and Target ID capabilities are limited.Detection Frequency
Frequency Scan: Use this function to obtain the quietest operating frequency, Operating near power lines, other detectors, fluorescent lights, etc. may cause interference. To check, hold searchcoil stationary away from any metal and listen for signal interference (i,e, noisy operation),