Changed README and deleted one file.

04-FFT_DFT_and_Applications/spectrogram.ipynb

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-    "# Time-Frequency Analysis\n",
-    "## Moving window analysis\n",
-    "One way to analyse the time-varying frequency content of a signal is to\n",
-    "apply windows in the time domain to the signal and to calculate a Fourier spectrum\n",
-    "of the windowed part. The window marches along the signal with defined overlap creating\n",
-    "a series of Fourier spectra associated with the center times of the windows. The resulting amplitude\n",
-    "spectra are then plotted versus window center time. In more detail:\n",
-    "\n",
-    "1. Choose windowing functions: $w(t,t_m)$ with $t_m$ the center of the window.\n",
-    "2. Multiply windowing function with time series: $f_m(t) = f(t)w(t,t_m)$\n",
-    "3. Detrend the windowed signal.\n",
-    "4. Perform a DFT: $F_{km} = \\Delta t\\sum_{n=0}^N f_m(t)\\exp(-2\\pi i \\frac{kn}{N})$\n",
-    "    and calculate the absolute value, $|F_{km}|$.\n",
-    "5. Plot the resulting matrix: $|F_{km}|$ in the time-frequency plane."
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