## Multirate Digital Signal Processing Interview Questions and Answers:

1. What is multirate signal processing?

Ans. The theory of processing signals at different sampling rates is called multirate signal processing.

2. Define down sampling

Ans. Down sampling a sequence x(n) by a factor M is the process of picking every Mth sample and discarding the rest.

3. What is mean by upsampling?

Ans. Upsampling by a factor L is the process of inserting i – 1 zeros between two consecutive samples.

4. If the spectrum of a sequence x(n) is X(ejÏ‰),Â then what is the spectrum of a signal down sampled by a factor 2

Ans.

5. If the z-transform of a sequence x(n) is X(z) then What is the z-transform of a sequence down sampled by a factor M?

Ans.

6. If the z-transform of a sequence x(n) is X(z) then what is the z-transform of a sequence upsampled by a factor L

Ans.

7. What is the need for anti-aliasing filter prior to down sampling?

Ans. The spectra obtained after down sampling a signal by a factor M is the sum of all the uniformly shifted and stretched version of original spectrum scaled by a factor 1/M. If the original spectrum is not band limited to Ï€/M, then down sampling will cause aliasing. In order to avoid aliasing the signal x(n) is to be band limited to Â± Ï€/M. This can be done by filtering the signal x(n) with a lowpass filter with a cutoff frequency of Ï€/M. This filter is known as anti-aliasing filter.

8. What is the need for anti-imaging filter after upsampling a signal?

Ans. The frequency spectrum of upsampled signal with a factor L, contains (L – 1) additional images of the input spectrum. Since we are not interested in image spectra, a lowpass filter with a cutoff frequency Ï‰cÂ = Ï€/LÂ can be used after upsampler. This filter is known as anti-imaging filter.