What is a Linear Phase EQ?

Back in the days, when Analog gears was the only option, linear phase wasn't possible, but today with modern technology we can use if needed a Linear Phase EQ, but what is it?

For an easier understanding let's imagine to EQ a sine wave

As you can see it start from zero then continue the full cicle.
If you use an eq in linear phase mode, the shape will change accordingly with the curve you are using, but the cycle will be the same. Will still start from zero and end to zero.

If you do the exact same eq but with a normal eq, you will have the same shape you had before, but with a different phase.

To have an idea on what a different phase is, just thake a look at the following picture

You can see here, the same sine we had before, and another one that seems to "start earlier". This is what a normal eq does, they just do not change the frequency response but also shift the phase.

So you may think linear phase eq are a better choice. But this is not true... it depends.

Linear Phase EQ are able to mantain the same phase, but with side effect, called "pre-ringing".
On certain sounds, it's not a problem while in others it's a big issue.

But what is pre-ringing?
It's a kind of echo that occour before the audio.
In the following image you can see on the right the original waveform, while on the left the waveform after a linear phase processing.

We don't have just a different waveform because of the EQ, but we also introduce this echo right before the sound.

Generally speaking my suggestion is to use normal eq and reach for linear phase processing only when you do some parallel processing that may introduce audible phase cancellation.