中文说明：这个
English Description：
The procedure processes an input signal using a first difference filter. The output is formed as follows. y[n] = x[n] - a * x[n-1] The preemphasis factor pre is zero for no filtering and equal to one for a full first difference. The filter memory Fmem is used to store the previous input value. Before processing any data, it is typically set to zero. On return, Fmem is equal to the last input data element processed. For block processing of a signal, the value of Fmem from one call is suitable as input to the next call. Parameters