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HPF and roll off - doing it wrong

 
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Foog
DC


Joined: 27 Oct 2013
Posts: 608
Location: Upper Canuckistan

PostPosted: Wed Jan 18, 2017 6:16 pm    Post subject: HPF and roll off - doing it wrong Reply with quote

I just discovered that I had been setting my HPF wrong. With the best intention of "subtle changes are better than more radical changes", I had my 80 Hz HPF cutoff curve set at -12db. It looked like this:



But "cutoff" is a misnomer here. A gentler curve starts dialing things back at a higher frequency than a steeper curve. Just look at the different "starting point" of the -24db curve:



I always thought my ears were just playing tricks on me when I heard a significant difference in before and after HPF sound. Turns out even my wooden ears were right. I was starting to roll back the sound all the way up around 200 Hz. Yikes!

Here is a link to the article (Pro Tools specific) where I found this info.

http://www.protoolsproduction.com/filtering/

Thought I'd share this on the outside chance that someone else has been making the same damned fool mistake. I also wonder what the curve is for hardware (on-mic or on mixer) and other HPF formats. If you don't know what your cutoff curve looks like, you may want to find out!
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Andrew Fogarasi


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ConnieTerwilliger
Triple G


Joined: 07 Dec 2004
Posts: 3381
Location: San Diego - serving the world

PostPosted: Wed Jan 18, 2017 7:23 pm    Post subject: Reply with quote

The bags of plonkers are nothing when compared to the extreme legibility of the flanged pottery glossary.

You know what I mean?

(Um, this seems like a gear post?)
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Foog
DC


Joined: 27 Oct 2013
Posts: 608
Location: Upper Canuckistan

PostPosted: Wed Jan 18, 2017 7:40 pm    Post subject: Reply with quote

I almost posted it in the gear subforum, but for some fantastic reason that I can't quite recall, I put it here instead. It really was, err, must have been a brilliant rea... that is to say... *sigh*

Apologies for posting the thread in the wrong spot.
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Bish
3.5 kHz


Joined: 22 Nov 2009
Posts: 3738
Location: Lost in the cultural wasteland of Long Island

PostPosted: Wed Jan 18, 2017 7:56 pm    Post subject: Reply with quote

(with apologies to Connie)

Back in the seventies, apart from working on conditioning data lines over the POTS network and futzing with individual capacitative and inductive components to create just the right frequency response curve, I used to build synths and stuff. This is back in the days when Moog was the gold standard, and if a filter wasn't going to work at 24dB/octave, it was nothing but a pale pastiche of "the real thing". Then, Doug Curtis made chips that made the classic Moog sound possible without the laborious work of building the (voltage-controlled) oscillators, amplifiers, and filters by hand on breadboards. I remember building my first Curtis-based synth... pure joy!

If you need to get rid of something (mains hum or any unwanted and useless energy below 100/75Hz), then attacking it with a 24dB/octave filter is a sharp tool. The roll off is fast, and you can select frequencies with far more accuracy. Subtle changes are great up in the meat of the frequency range, but the surgical removal of anything below a certain frequency needs something decidedly unsubtle.
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Lance Blair
M&M


Joined: 03 Jun 2007
Posts: 2279
Location: Atlanta

PostPosted: Thu Jan 19, 2017 6:19 am    Post subject: Reply with quote

Agree with Bish. The best HPF leaves things alone around 80 Hz and then sharply diminishes frequencies below 70 Hz.

My Mackie VLZ 4 has a HPF at 100 Hz at -18 per octave. But if you dial the 80 Hz eq knob up 3 to 5 DB, then the 70-110 range is preserved and the sharp drop happens below 70.
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todd ellis
A Zillion


Joined: 02 Jan 2007
Posts: 10491
Location: little egypt

PostPosted: Thu Jan 19, 2017 7:13 am    Post subject: Reply with quote

ya know something, i really learned a lot from this thread. thanks to all. and connie - you can't find a good flanged pottery glossary these days.
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