Click here to purchase the entire book in PDF format. Reciprocal Peak/Dip Filter Instead of building a filter where the cut and boost always maintain a constant Q, let's set about to build a filter that is symmetrical - that is to say that a matching boost and cut at the same centre frequency would result in the same shape. The nice thing about this design is that, if you take two such filters and connect them in series and set their parameters to be the same but opposite gains (for example, both with a centre frequency of 1 kHz and a Q of 2, but one has a boost of 6 dB and the other has a cut of 6 dB) then they'll cancel each other out and your output will be identical to your input. This also applies if you've equalized something while recording - assuming that you live in a perfect world, if you remember your original settings on the recorded EQ curve, you can undo what you've done by duplicating the settings and inverting the gain. As a result, we call this a reciprocal peak/dip filterfilter, reciprocal peak/dip. Figure 6.13: The frequency responses of various reciprocal peak/dip filters, all with a centre frequency of 1 kHz, gains of either 12 dB or -12 dB (depending on whether it's a boost or a cut) and various boost Q's. Black Q = 1. Green Q = 2. Blue Q = 4. Red Q = 8.