Topic: Strange Results from Using High Pass Filter in Post

[gratefulphish]

I am working on the Lotus show from Langerado.  When I first opened it in SF8, I went in and removed or reduced a few glitches and clips.  I then had to use a high pass filter to reduce some of the overwhelming bass.  I just used the straight low pass filter, with no other EQ.  When it was done processing, all of a sudden I had a whole lot of new much hotter peaks than I had started with, and some clips or at least peaks against 0db that were not there before.  Is this normal?  Why is it increasing the signal, when all it should be doing is reducing some of the low end?  Am I doing something wrong?  Thoughts?

[Gordon]

my thought are don't use any low pass filter

[gratefulphish]

I normally never do either, but these recordings are so overwhelminly bassy, that it drowns everything else out, both because of sheer volume, and because in order for our recorders to figure out when we are going to clip, the loud bass causes the rest of the instruments to be just lost behind the wall.  A simple run through the high pass filter solved it, ansd took an otherwise unlistenable recording, and turned it into something that sounded really good.

[newblue]

Those 170s can be a little hefty on the low end.   A HPF is sometimes necessary with that setup, esp. at show that are sloppy with the bass.

[guysonic]

Software has different features, what software are you using?

There are MANY types of filters in CEP and Adobe Audition, and each allows ADDING GAIN that is usually not a good idea so check all options.   Best filter I use with those programs is simple 1 pole butterworth type High Pass (a SCIENTIFIC FILTER) set at anywhere from 50 to 200 cycles depending on how much low end needs be removed.

See thread: http://taperssection.com/index.php?PHPSESSID=dc5cb19c3eae03a2a98a47d2a8b6b290&topic=81253.0;all

[gratefulphish]

I am doing the EQing in SF8 and just using the high pass filter, actually set at 300.  I experimented with it lower, but that is where it needed to be.  What I still don't understand is why it increased the levels of other sounds.  When I have EQ'd before, usually the whole wav file is reduced in decibel size afterward, because it was the bass that was really expanding the waveform.  Then I would normalize, and things would sound really good.  This is the only time that when I ran the filter, the other signals actually increased.

[Dede2002]

I am doing the EQing in SF8 and just using the high pass filter, actually set at 300.  I experimented with it lower, but that is where it needed to be.  What I still don't understand is why it increased the levels of other sounds.  When I have EQ'd before, usually the whole wav file is reduced in decibel size afterward, because it was the bass that was really expanding the waveform.  Then I would normalize, and things would sound really good.  This is the only time that when I ran the filter, the other signals actually increased.

Hi there,

300hz? That's good to know. I'm trying to get rid of bass in one my last recordings. I've just tried 180 hz and the damn thunderous bass is still there.I'll give the 300hz a try.

[John Kary]

I am doing the EQing in SF8 and just using the high pass filter, actually set at 300.  I experimented with it lower, but that is where it needed to be.

The EQ is adding gain somewhere.  No HPF should ever increase the volume of the recording.  Re-check the plugin itself for a gain control and set it to 0db.

Also, instead of cutting the entire low-end from your recording, just do a shelf from 300hz so you reduce the low-end by an amount that sounds good, instead of cutting it all out from 300hz down.

[Dede2002]

I am doing the EQing in SF8 and just using the high pass filter, actually set at 300.  I experimented with it lower, but that is where it needed to be.

The EQ is adding gain somewhere.  No HPF should ever increase the volume of the recording.  Re-check the plugin itself for a gain control and set it to 0db.

Also, instead of cutting the entire low-end from your recording, just do a shelf from 300hz so you reduce the low-end by an amount that sounds good, instead of cutting it all out from 300hz down.

Excuse me, but how is he going to do that? Pardon my lack of knowledge. Can you elaborate a little bit more about the shelf thing?

[Brian Skalinder]

Excuse me, but how is he going to do that? Pardon my lack of knowledge. Can you elaborate a little bit more about the shelf thing?

Cutting everything below 300 Hz would look something like the 1st pic attached.

A sloped rolloff looks something like the 2nd pic attached.  (Not sure of the proper term, but when I visualize a shelf, I think of the first image;  slope, I visualize the 2nd image.)  At any rate, the idea is to reduce volume gradually across a frequency range, rather than all at once.

Most in-line high pass filters I've seen (usually on preamps) reduce levels by 6 dB or 12 dB per octave.  Every halving / doubling of a frequency = one octave down / up respectively.  So if we start at 300 Hz and apply a HPF of -6 dB / octave, our slope is 0 dB at 300 Hz, -6 dB down at 150 Hz, another -6 dB down at 75 Hz (total of -12 dB relative to our starting point at 300 Hz), and still another -6 dB down at 37.5 Hz (total of -18 dB relative to 300 Hz).  A -12 dB / octave slope would be -12 dB per each octave, or 0 dB at 300 Hz, -12 dB (total) at 150 Hz, -24 dB (total) at 75 Hz, -36 dB (total) at 37.5 Hz.  I typically find a -12 dB / octave slope too severe and usually start with a -6 dB / octave slope as my starting point.

[gratefulphish]

Actually, I was using the flat high pass filter, which has adjustable db rolloff, which I took down 12 dbs after trying it at a number of different settings on a couple of clips that I was using for tests.  I tried the slope, but I really needed a fairly sharp dropdown at about 300hz, so I just used the normal shelf filter.

OTH, I think that I may have discovered my issue.  I think that I may have moved a gain slider on the filter window, that added 3dbs to the post product.  That would explain how the levels increased.