Frequency and time representation of signals. Spectrum. Modulation
The camera's autofocus ruins everything, but it's better than nothing.
Less confusing explanations on this topic can be found here:
1) Signals. Time Representation: • Лекция 2. Сигналы. Временное представлен...
2) Frequency Representation of Signals. Spectra. Bandwidth - • Лекция 3. Частотное представление сигнало...
3) Signal Coding, Modulation, and Manipulation - • Лекция 8. Кодирование, модуляция и манипул...