Improving comb aliasing rejection using filters with stepped triangular impulse response

Decreasing the sampling rate by an integer is called decimation. Decimation finds applications in audio signal processing, Analog/Digital (A/D) converters, digital receivers, among others. The signal must be filtered by a digital filter, before decimation, to prevent aliasing, which may deteriorate decimated signal. This filter is called anti-aliasing or decimation filter. The simplest decimation filter is a comb filter which has all coefficients equal to unity, and consequently does not require multipliers for its implementation. This filter is usually used in the first stage of decimation. The aliasing occur in the bands around comb zeros, called folding bands. Consequently, comb filter naturally provides the aliasing rejection in folding bands. However, this attenuation is not enough in many applications, and must be increased. The principal goal is keeping comb low complexity, while improving its aliasing rejection. In this paper we propose novel method based on filter with stepped triangular impulse response (STIR). It is shown that the STIR filters are multiplier less filters and have all zeros on unit circle. It is elaborated how to choose the STIR filter parameters to get an improved comb aliasing rejection and have the same parameters of design, as the original comb filter. To this end, comb decimation factor is presented as the product of two integers, which are also the parameters of the STIR filter. The magnitude response of the proposed decimator is compared with that of the original comb to show the benefit of the proposed method.