While implementing/studying brickwall low pass filters for digital audio, I decided to run a unit impulse through FabFilter's Pro-Q 3 brickwall low pass filter to see what the resulting impulse response would look like.
When in "Linear Phase" mode (bottom half of image), the resulting IR sure enough looks exactly like what I expected—a sinc function. However, when in "Zero Latency" mode (upper half of image) the resulting IR looks kind of similar to the right half of a sinc function, except the decay is much smoother, the frequency is lower and it starts from a zero crossing instead of a peak (and the initial rise also looks more s-curve shaped).
Convolving a signal with both these IRs gives basically exactly the same result (or frequency response, at least). Which piqued my interest, because the mysterious "Zero Latency" IR seems to give the same results with (presumably) half the number of points. I'd very much like to know exactly what it is, and how one can calculate (generate) it.
I have very little experience with digital signal processing, so any insights would be much appreciated! (also sorry if everything I just said makes no sense)