# Translations:Wavelets - book/7/en

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Recall our discussion in Chapter 4, in which we mentioned the three types of wavelets: *causal*, *noncausal*, and *purely noncausal* wavelets. Now we will learn more about them. Also recall that in Chapter 5, we saw that a *digital filter* is represented by a sequence of numbers called its *impulse response* or its *weighting coefficients*. A digital filter is said to be causal if its present output (at time *n*) depends only on present and past inputs (that is, depends only on inputs at times *n*, *n* – 1, *n* – 2, … , and so on. Another term for a causal filter is a *realizable* filter. In equation **2** of Chapter 5, we saw that the most general causal filter with a finite number of delay elements has the form