4 ms·
Many brilliant points. Particularly about Bessel filters instead of Butterworth. And also about lowering Fc and increasing Fs, rather than increasing order.
by diydsp 3y ago
Many brilliant points. Particularly about Bessel filters instead of Butterworth. And also about lowering Fc and increasing Fs, rather than increasing order. And also about considering time domain as well as frequency domain.
The dense interview question I use to assess this area of knowledge: "How do you choose the stopband attenuation of a filter?" You can assess a lot from the interviewee's response. Stopband attenuation, with respective to the input signal magnitude at the stopband frequency, or start of the stopband, is the most relevant term for determining the magnitude of noise in the sampled signal. And that is the upper bound on the performance of the downstream algorithm.
- patrakov 3y agoMy response would be: "to validate the chosen stopband attenuation, compare the unwanted signal or interference that passes through the filter with the noise that you already have or with other factors that would make this unwanted signal or interference unnoticeable".
- diydsp 3y agoThat's a good way of looking at it. The "unnoticeable" part is where more can be teased out: That is, to expect a specification for the desired S/N (signal-to-noise) ratio and bandwidth, and/or bit rate at which the downstream algorithm is expected to work. This doesn't have to be a perfect estimate/evaluation/measurement, but it helps to get the ballpark amount of computation power, delay, etc. needed to actually make the system work. All too often I see people react to a noisy signal, by "putting a filter on it," without knowing how much attenuation/gain/phase shift, etc. are actually needed to make the system work acceptably. E.g. You don't want to spend too long designing the perfect filter... And you want to know if there's any hope at all cleaning up the signal to make use of it! Good luck with your filtering projects!