1st, only gives the function value the signal in certain not continual instants, becomes the discrete time signal. It may come from the simulated signal sampling, or is the time and the scope not continual discrete signal. When discrete signal analysis and processing, usually presses certain priority order to arrange in the time not a continual number of classes set, calls it the sequence. 2, two different sequences may correspond the same Z transformation, but the convergence domain is not same, therefore, to cause the sequence and the Z transformation is 11 correspondences, while gives the sequence Z transformation, must assign its convergence domain. 3, the random signal cannot use the precise mathematics relationship description the signal, but its change obedience statistics rule, may use the probability statistical property to describe. Each vice minister who does according to the time course the time observation record is called the sample function immediately to the signal. 4, random signal's time domain and the frequency range characteristic may use the statistical method to conduct the research, its peak-to-peak value characteristic uses the signal the average value, the mean square value and the probability density function expression. The signal time domain characteristic may also use the autocorrelation function to describe, the frequency range characteristic may use the power spectral density function to describe