What is Digital Filter

Downsampling is the process of decreasing the sampling rate of signal by an integer. This process may introduce the unwanted spectrum replica called aliasing. To avoid aliasing the signal must be filtered by decimation filter prior downsampling. Decimation consists of filtering and downsampling. The most simple decimation filter is comb filter usually used in the first stage of decimation. However, comb filter does not provide a good aliasing rejection. This paper presents the methods for improving alias rejection of comb filters. The methods are based on comb zero rotation, cosine filters, Chebyshev polynomials, and cascade of combs with different parameters.