A band-pass filter (BPF) is another type of spectral effect. It reduces the amplitude of low frequencies and also reduces the amplitude of high frequencies. However, a band of frequencies in the mid-range of the spectrum pass through the filter.
One way to implement a BPF is shown in the block diagram below. A delay of two samples is used in parallel. On the parallel path, the signal is flipped upside-down by multiplying by negative one. Then, the two paths are combined together.
Constructive interference occurs for frequencies in the middle of the spectrum. Destructive interference occurs for low and high frequencies.
This approach to filtering is the complement of the notch filter. As it was shown with the LPF and HPF, by changing the gain from positive one to negative one, the filter changes which frequencies pass and which frequencies are cut.
Continue on to learn what happens when we combine multiple filters in series.