2. Low-pass filter (Fig. 2.23) 
Fig. 2.23. Low-—pass filter. 
When w, = 0 for a band-pass filter, 
IG@)? = (2.124) 
0 lal > wo 
g(t) = a (2.125) 
3. High-—pass filter (Fig. 2.24) 
Wo 0 Wo —-> 
Fig. 2.24. High—pass filter. 
When w,— © for a band-pass filter, 
0 lal S wo 
IG@)!? = (2.126) 
1 lol > wo. 
Also for digital processes, we can express the digital filtering by 
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