It's not a matter of which one is correct. The matter is that there are two type of expression for IIR filters. And DSPLinks takes (A), Wikipedia also takes (A) but UCCS takes (B). See the following figure.

What sort of symptom occurs if you confuse (A) and (B)?? Let's change the polarity of b1 and b2 like following.

The frequency response changed like this. It's still a LPF but not what you want.

That's it for this tutorial. Many beginners make mistakes at this point. To make things worse, the response becomes "a bit weird" even if you make the mistake. If the response of IIR filter is not what you want, please remember this tutorial and change the polarity of b1 and b2. Please note that DSPLinks takes (A), not (B).

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