Logic Pro 9 Channel EQ hi-pass means gain boost?

Doug Zangar

I'm working with someone today and he has a vocal track that is set to 0.0 db and I tell him to add a hi-pass filter to cut out some background crap below 80hz. So bam, do it and all of a sudden the track is clipping by .03 db. Bypass the EQ, no clipping. Turn off the hi-pass, no clipping.

So I try it on tonight on one of my projects - set the channel level to to peak at 0.0 db, set hi-pass filter at around 80hz, and BAM - clipping at +2.4 db.

Has this gain boost always been going on and I'm the last one to notice? Basic logic (pardon the pun) would state if you reduce frequencies, output level should drop.... Or so I always thought.
Yes, it can happen.
I have read somewhere(I can't remember where exactly) that the filter can produce a notch that can actually attenuate yor signal. Nothing wrong. Have you tried different octaves and q-values?
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Hey Doug,

I am assuming you are using the Channel EQ? It would be interesting to try the exact same settings with the Linear Phase EQ and see if you get the same results. Maybe the unintended gain is a result of some subtle phase changes introduced by the Channel EQ??? Just a thought....
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It has to do with the shape of the filter. You are creating a resonance. Try altering the q-factor, that will lower the amount of resonance.
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@ Eli

With Linear Phase EQ and same setting, the output gain is raised by 0.1 db, a 2.4 db decrease from the Channel EQ. So that's a possible answer, except for the latency it introduces.

@ Falk

1) this a frequency specific issue (removing unwanted lows). I don't think an octave shift would be effective for the end result.....

2) speaking of effective, changing the Q factor changes the effectiveness of the frequency removal, making it ineffective. It does however, lessen the gain boost.
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Hi Doug!

I'm sorry, I'm not being very clear. When I speak about an octave, I speak about the Gain/slope parameter. It determines how steep the cut is in dB's per octave. This is not about pitch-shifting... :)

From the manual:

  1. Frequency fields: Adjust the frequency of each band.
  2. Gain/Slope fields: Set the amount of gain for each band. For bands 1 and 8, this changes the slope of the filter.
  3. Q fields: Adjust the Q factor or resonance for each band-the range of frequencies around the center frequency that are affected.

So for technical reasons I'm sorry to admit I don't know enough about, when using the low-cut, the q-factor will in fact raise the level around the freq your cutting at. It's nothing to be afraid of, just lower your output if necessary.
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Regarding gain/slope, that too does make a difference - I was using 48db/octave. But again, lessening that makes it less effective.

The link is great! Explains what happens and also explains why the Linear Phase EQ (with phase correction) had virtually no gain increase. I need to explore its latency issues some more.

And yes, one can just turn down the volume. The only reason it came up is the client had everything mixed and wanted to learn the mastering process within the Logic project. I was doing my usual clean up of getting rid of unneeded frequencies and did the hi-pass filter on his edited vox track and was surprised to see the gain increase.

I rarely mix individual channels that hot, and I usual do hi-pass filters at the start of a project, so I'm mixing with those in place, not after the fact.

Good education on phase issues with the Channel EQ.
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