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This question already has an answer here:

Why Filter Kernel is flipped during convolution process? What are the consequences if the kernel is not flipped?

Note: Filter Kernel means: Impulse response or convolution kernel.

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marked as duplicate by Community Aug 23 '17 at 14:14

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In addition to the linked question, here is a good one from MathWorks:

The time to flip in so miniscule it's inconsequential. If you go through the theory (linear systems theory) you'll understand.

If you want to understand the causality, imagine a filter that's a right triangle, with the small pointy end on the left and the steep edge on the right. Now shift that past a delta function. If you don't flip and just start shifting your filter past it will hit the tall steep edge first and then ramp down. So then your response is opposite to your filter, but they must be the same if it's a delta function.

Basically it's because time goes along the x axis with the small time values on the left and the big (later) time values on the right. So if you start shifting in, you're having the big time values hit your signal first, which is not right (causal). So you have to flip it to make the small time values shift in first.

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