Asking about how a specific part of the circuit works
Hi everyone. I am trying to make a weird VCO, but my main goal is to simulate frequency modulation. I have stumbled upon a few VCO builds, and I have come to this circuit which I was able to reduce to based on other people's builds. I understand what is happening in the Schmitt trigger part, but I have no idea what is happening in the presumable control voltage part. Like, how do the transistors come to play, and like...what is happening. I played with the simulation many times, and this form is the easiest that can be, with the output waveform that I'd want. I would legit appreciate any help. I do not understand why does it need a dual power supply, and why without the transistor part the frequency modulation does not work. Thank you for any help OTL