⁷⁴FM is short for Frequency Modulation; the information then is encapsulated in the frequency deviation from a center frequency Creating FM is not very difficult if you use an oscillator with an electronically controllable oscillation frequency that hence creates $f_{\mathit{osc}}(v_{\mathit{in}})$. Detecting FM is more complicated. One way is reusing the transmitter’s voltage controlled oscillator in the feedback path around a stage that has a transfer function $A=v_{\mathit{out}}(\mathrm{\Delta }f)$ with high gain. The system transfer is then approximately the reciprocal of the transfer of the feedback path, $v_{\mathit{out}}\approx f_{\mathit{osc}}^{-1}(f_{\mathit{in}})$ and hence $v_{\mathit{out}}=v_{\mathit{in}}$.