There is a very important technical difference between the 2 and there seems to be a bit of confusion in one of the posts about this. The phase characteristics of a HP filter are VERY different than a shelf EQ. In most cases, A HP filter will impose a much more dramatic phase shift than a Shelf EQ.
The most common filter people use in analog gear or emulate in plugins is the "butterworth" style filter, so all of this will most likely apply for the Original Post'er. It is also the type of filter being emulated in the GML (
MDW) EQ plug in. I will use that as the example.
Let's say you want to roll off low end below 100hz and end up with 50hz being about 12dB down. You select the 12dB/octave HP filter and set the frequency at 100hz. You will have -3dB at 100hz and, predictably, -12dB at 50hz. The phase shift at 100hz is about 90 degrees and the phase shift at 50hz is about 135degrees!!! The phase shift will continue to travel closer and closer to 180 degrees as the frequency gets lower.
Now we change the EQ band to a shelf and select 12dB of cut at 100hz. There is an obvious difference in the slope that is created (it falls slower and levels out). We are still about -12dB at 50hz, but the amount of phase shift at 50hz is very different. It is way less (somewhere in the 20 to 30 degree range).
This difference is a huge part of when I decide to use a shelf EQ or a HP filter. If I need to remove some low end from the snare drum mic but it is really IN phase with the over heads, I will use a shelf EQ. If the snare mic sound is not adding up well with the over heads and needs to have some low end removed I will use a HP filter to try a and kill 2 birds with one EQ move (so to speak). Another good example is if you mic the front and back of a guitar cab. With a HP filter, you can completely change how those 2 signals blend together and create sound that absolutely would not happen with a shelf EQ. Using HP fitters to manipulate phasing between multiple mics on a sound source is a very powerful tool.
Here is the important thing to know when using a HP filter for phase manipulation. The steeper the filter gets, the more dramatic the phase shift. We can use the
MDW EQ plugin as an example again. with the Cut Off Frequency set at 100hz, here are the degrees of phase shift at 100hz for the various slopes:
6db/Oct - 45 degrees
12dB/oct - 90 degrees
18dB/oct - 135 degrees
24dB/oct - 180 degrees
So as you can see if you use a very steep filter (24dB/oct) the phase difference is huge. You are literally reversing the phase at the selected frequency.
EV