Click here to purchase the entire book in PDF format. Peak Filter There is a variation on the bandpass filter that, instead of attenuating all frequencies outside the passband, the filter typically leaves them at a gain of 0 dB. This kind of filter can be seen in the plot of an in Figure 6.6 and is called a peaking filter or peak filter. Notice now that, rather than attenuating all unwanted frequencies, the filter can be thought of as simply applying a known gain in the passband. The further away you get from the passband, the less the signal is affected. Notice, however, that we still measure the bandwidth using the two points that are 3 dB down from the peak of the curve. Figure 6.6: The frequency response of a peaking filter with a centre frequency of 1 kHz, a Q of 4, and a gain of 12 dB in a typical equalizer. Figure 6.7: The frequency responses of peaking filters with a various centre frequencies, a Q of 4, and a gain of 12 dB in a typical equalizer. Blue $f_{c}$ = 250 Hz. Red $f_{c}$ = 500 Hz. Green $f_{c}$ = 1000 Hz. Black $f_{c}$ = 2000 Hz. Figure 6.8: The frequency responses of peaking filters with a centre frequency of 1 kHz, various Q's, and a gain of 12 dB in a typical equalizer. Black Q = 1. Green Q = 2. Blue Q = 4. Red Q = 8. Figure 6.9: The frequency responses of peaking filters with a centre frequency of 1 kHz, a Q of 4, and various gains from 0 dB to 12 dB in a typical equalizer. Yellow gain = 0 dB. Red gain = 3 dB. Green gain = 6 dB. Blue gain = 9 dB. Black gain = 12 dB. In most instances of these kinds of filters, it is also possible to attenuate the same frequency band, as is shown in Figure 6.10. Since the practical implementation of this filter allows you to boost and attenuate, they are commonly known of as peak/dip filters. Figure 6.10: The frequency responses of bandreject filters with a centre frequency of 1 kHz, a Q of 4, and various gains from -12 dB to 0 dB in a typical equalizer. Yellow gain = 0 dB. Red gain = -3 dB. Green gain = -6 dB. Blue gain = -9 dB. Black gain = -12 dB.