# What are the limitations of FIR filters?

I know that for type II FIR filter it adds a zero at $$z=-1$$ so it cannot design a high-pass filter.

My question:

what type of limitations the other types have (type I, III, IV)?

• Wrong, an FIR filter can be a low-pass, high-pass, band-pass or bandstop filter... – Ben Nov 7 '19 at 0:54
• Sorry I edited it – Anwer Ak Nov 7 '19 at 1:00
• Those limitations are for linear phase FIR filters... – Fat32 Nov 7 '19 at 10:54

Note that this is not about general limitations of FIR filters, but about the special case of linear-phase FIR filters. If you understand why a type-II linear-phase FIR filter has a zero at $$z=-1$$, then the limitations of the other types should be obvious too.

It's always about zeros at either $$z=1$$ (DC) or $$z=-1$$ (Nyquist). Given the transfer function

$$H(z)=\sum_nh[n]z^{-n}\tag{1}$$

it is easy to see that

$$H(1)=\sum_nh[n]\tag{2}$$

and

$$H(-1)=\sum_n(-1)^nh[n]\tag{3}$$

Now look at examples of the four linear-phase FIR filter types. An example of an impulse response of a type-I filter is

h = [1 -1 2 3 2 -1 1]

Using $$(2)$$ and $$(3)$$ can you say anything about zeros at $$z=1$$ or $$z=-1$$?

An example of a type-II filter is

h = [1 -1 2 2 -1 1]

Clearly, applying Eq. $$(3)$$ results in a zero at $$z=-1$$, regardless of the actual values of the impulse response.

The following two filters are type-III and type-IV filters, respectively:

h = [1 -1 2 0 -2 1 -1]

h = [1 -1 2 -2 1 -1]

Now apply Eqs $$(2)$$ and $$(3)$$ and see what you get.