Thinking about analog filters, most engineers arecomfortable in the time domain. For example, the operationof an RC lowpass filter can easily be envisioned as acapacitor charging and discharging through a resistor.
Likewise, it is easy to envision how a negative-feedbackactive filter uses phase shift as a function of frequency,which is a time domain operation.A digital filter is better conceptualized in the frequencydomain. The filter implementation simply performs aconvolution of the time domain impulse response and thesampled signal.
A filter is designed with a frequency domainimpulse response which is as close to the desired idealresponse as can be generated given the constraints of theimplementation. The frequency domain impulse response isthen transformed into a time domain impulse responsewhich is converted to the coefficients of the filter.
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