Click here to purchase the entire book in PDF format. Equalizers Unlike its counterpart from the days of long-distance phone calls, a modern equalizer is a device that is capable of attenuating and boosting frequencies according to the desire and expertise of the user. There are four basic types of equalizers, but we'll have to talk about a couple of issues before getting into the nitty-gritty. An equalizer typically consists of a collection of filters, each of which permits you to control one or more of three things: the gain, centre frequency and Q of the filter. There are some minor differences in these filters from the ones we discussed above, but we'll sort that out before moving on. Also, the filters in the equalizer may be connected in parallel or in series, depending on the type of equalizer and the manufacturer. To begin with, as we'll see, a filter in an equalizer comes in two basic models, the peak/dip filter and the shelving filter which is a type of variation on the highpass and low pass filters.