说明：  fft程序，最初在单片机c8051f020上实现，“fft.c”是核心程序，很短，程序中x_real[N]是输入序列，输出保存在x_real[N]和x_imag[N]中
(fft program, initially implemented on the MCU c8051f020, " fft.c" is the core program, very short, the program x_real [N] is the input sequence, the output stored in x_real [N] and x_imag [N] in)