Analog video recording principles

[old_tv_nut]

It's the mathematics of what the TIME waveform of the FM signal looks like, and how it is composed of a combination of pure sine waves that individually have a constant frequency and amplitude (the spectrum).

With the extreme case where the luma consists of a high-frequency alternating stripe pattern having peaks and troughs and a certain average level, the TIME waveform of the FM signal shows the FM carrier frequency changing rapidly and repeatedly from low frequency (wide cycles) to high frequency (narrow cycles) representing the troughs and peaks of the luma waveform. Fourier analysis shows that this TIME waveform of the FM signal can be obtained by adding a sine wave with the (constant) average frequency to sine waves of the sideband frequencies. The plot of how much of the average frequency sine wave is there and how much of the sideband sine wave is there is the frequency domain plot of the signal.