Time-Domain and Frequency-Domain
What we have in this byte array is signal recorded in the time domain. The time-domain signal represents the amplitude change of the signal over time.
In the early 1800s, Jean-Baptiste Joseph Fourier made the remarkable discovery that any signal in the time domain is equivalent to the sum of some (possibly infinite) number of simple sinusoidal signals, given that each component sinusoid has a certain frequency, amplitude, and phase. The series of sinusoids that together form the original time-domain signal is known as its Fourier series.
In other words, it is possible to represent any time domain signal by simply giving the set of frequencies, amplitudes, and phases corresponding to each sinusoid that makes up the signal. This representation of the signal is known as the frequency domain.
In some ways, the frequency domain acts as a type of fingerprint or signature for the time-domain signal, providing a static representation of a dynamic signal.
时域与频域
The following animation demonstrates the Fourier series of a 1 Hz square wave, and how an (approximate) square wave can be generated out of sinusoidal components. The signal is shown in the time domain above, and the frequency domain below.
频域
References:
https://www.toptal.com/algorithms/shazam-it-music-processing-fingerprinting-and-recognition
