Click here to purchase the entire book in PDF format. Phase response So far, we've only been looking at the frequency response of a filter or equalizer. In other words, we've been looking at what the magnitude of the output of the filter would be if we send sine tones through it. If the filter has a gain of 6 dB at a certain frequency, then if we feed it a sine tone at that frequency, then the amplitude of the output will be 2 times the amplitude of the input (because a gain of 2 is the same as an increase of 6 dB). What we haven't looked at so far is any shift in phase (also known as phase distortion) that might be incurred by the filtering process. Any time there is a change in the frequency response in the signal, then there is an associated change in phase response that you may or may not want to worry about. That phase response is typically expressed as a shift (in degrees) for a given frequency. Positive phase shifts mean that the signal is delayed in phase whereas negative phase shifts indicate that the output is ahead of the input. ``The output is ahead of the input!?'' I hear you cry. ``How can the output be ahead of the input? Unless you've got one of those new digital filters that can see into the near future...'' Well, it's actually not as strange as it sounds. The thing to remember here is that we're talking about a sine wave - so don't think about using an equalizer to help your drummer get ahead of the beat... It doesn't mean that the whole signal comes out earlier than it went in. This is because we're not talking about negative delay - it's negative phase. Figure 6.18: The phase responses of bandpass 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. (Compare these curves to the plot in Figure 6.13) Figure 6.19: The phase responses of bandpass 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. (Compare these curves to the plot in Figures 6.13 and 6.18).