I have understood the your requirement fully as experienced practitioner. The way I am planing and the same propose to accomplish this task by, Articulate the following parameters of the FFT function to analyse the signal and threshold criteria to brought reliable detection of your DTFM signaling digits.
Parameters: Sampling Frequency(Fs) and N-Points, in extant with, and requirement based Overlapping Time, Selectivity and Sensitivity with suitable scientific reasoning (or Mathematical Supporting Equations)
Its my first Bid in this environment. and I hope we will appreciate each other. looking forward and thankyou:).