Digital Filters (2): Digital Signal Processing

(...continued)

Like an analog filter, the digital filter is selected on the basis of which frequencies in the signal are to be retained and which are to be rejected. Low-pass filters, the most common type, are designed to allow signal components from 0 Hz (dc) to some nonzero passband frequency,

, to pass essentially unaltered. The filter does introduce a series of small positive and negative deviations from the actual signal in the passband. When this "ripple" exceeds a certain amount, typically 0.01 dB, it defines the passband frequency. For frequencies in the transition band between the passband frequency and higher stopband frequency, the signal is increasingly attenuated. When the attenuation reaches a certain level, typically in the vicinity of 95 dB, it defines the stopband frequency of the digital filter.

When using digital filters, the user should pay attention to both the stopband and the transition band. In some cases, particularly those with lower passband frequencies, the transition band may be as great as, or even greater than, the passband itself.

(continued...)

Page 7 of 24