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How to determine HPF filter frequencies?
Old 22nd February 2013
  #1
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Scott Whigham's Avatar
 

How to determine HPF filter frequencies?

I bought a compressor that came with a hand-built sidechain HPF. The guy I bought it from bought it from another guy and now no one knows the frequencies...

It has four options, which I assume relate to capacitor values:
  • 0.1
  • 0.15
  • 0.22
  • 0.47
Is there anywhere that tells you what the actual Hz values are? I can listen, of course, but that only tells me "It's about 150hz". If possible, I'd prefer to know if there's a way to determine the value based on the cap value.

Thanks!

Old 23rd February 2013
  #2
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Run pink noise through it, then into DAW of choice.

Run RTA plugin of choice and start pushing buttons and look at the frequencies.

Should be the easiest way to tell
Old 25th February 2013
  #3
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Thanks for the help. I'm sorry to be dense but I'm struggling a bit here. I've run it like this:

pink noise -> compressor w/ sidechain filter in -> frequency analyzer

Is that what you're talking about? I'm just either not doing it right, or using the wrong analyzer, or something. I'm just not really finding the specific ranges that it's really hitting.

On a side note, I then tried just running a tone of various frequencies through it and I could definitely see the comp moving based on the settings of the HPF.
Old 25th February 2013
  #4
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Run the pink noise through the HPF and not the compressor. Pink noise through the HPF and into your DAW. Then strap an RTA plug on the DAW channel and you will get a visual cue of the slope. Good luck.
Old 25th February 2013
  #5
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There are some formulas here for calculating hpf cut-offs based on capacitance and resistance of a circuit. Little over my head.

High Pass Filter - Passive RC Filter Tutorial
Old 25th February 2013
  #6
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It's going to be hard to simply look at cap vaules to determine this, as other design features will help determine the points (whether it's active or passive, load impedance on input, etc.).

My advice? A ballpark guess of the filter points should suffice. I would guess the front panel is saying .1 khz and etc. So 100 hz, 150hz, 220hz, and 470 hz. Those would all be useful in sidechain compression.

Use a signal generator (can be a plugin) and start with a low frequency, say 80hz. Compress it. Engage the sidechain filter at the lowest setting, does the compression change? Sweep the signal up until it compresses again, then change the filter point and so forth. That should get you in the ballpark. Of course, you will also have no idea what the slope of the filter is either, so it will be hard to nail down exactly.
Old 25th February 2013
  #7
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gannonsamuel's Avatar
 

Quote:
Originally Posted by j2dafo View Post

My advice? A ballpark guess of the filter points should suffice. I would guess the front panel is saying .1 khz and etc. So 100 hz, 150hz, 220hz, and 470 hz.
that's what i was gonna say!!


Or i would say try not to think of it in so much as "what frequency is this setting" and more "What does this setting do to my sound?" when you have an idea of how each one sounds on a variety of sources, knowing what frequency it is should become reasonably irrelevant.
Old 25th February 2013
  #8
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Quote:
Originally Posted by Aisle 6 View Post
Run the pink noise through the HPF and not the compressor. Pink noise through the HPF and into your DAW. Then strap an RTA plug on the DAW channel and you will get a visual cue of the slope. Good luck.
The HPF has no input(s); it just has two TRS cables coming out that go sidechain to the comp.

Thanks, everyone. j2dafo is right, for me - a ballpark guess is absolutely fine for my needs.

I'm still struggling with the interpretation of what I'm seeing using the signal generator. Using 80hz, it looks like this:
  • Bypass - -13
  • 0.1 - GR of -7
  • 0.15 - GR of -8
  • 0.22 - GR of -10
  • 0.47 - GR of -12
When I change to 150hz, it looks like this:
  • Bypass - -13
  • 0.1 - GR of -9
  • 0.15 - GR of -10
  • 0.22 - GR of -11
  • 0.47 - GR of -12 but maybe -13

When I change to 220hz, it looks like this:
  • Bypass - -13
  • 0.1 - GR of -10
  • 0.15 - GR of -11
  • 0.22 - GR of -12
  • 0.47 - GR of -13 (no change)
Does this seem right? I'm just struggling here to understand this...
Old 25th February 2013
  #9
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A few things: What is the comp it is paired with? What did the previous owner usually use it on?

Also, the Filter has an input and an output. The compressor's sidechain circuit will have a send and return, thus the 2 trs jacks coming from the filter. You should be able to use those to interface separate from the comp.


And, I'm confused by the readings you provided. Are you giving output levels or gain reduction readings?
Old 28th February 2013
  #10
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Scott Whigham's Avatar
 

Quote:
Originally Posted by j2dafo View Post
A few things: What is the comp it is paired with? What did the previous owner usually use it on?

Also, the Filter has an input and an output. The compressor's sidechain circuit will have a send and return, thus the 2 trs jacks coming from the filter. You should be able to use those to interface separate from the comp.

And, I'm confused by the readings you provided. Are you giving output levels or gain reduction readings?
Thanks for the help.

This is a Pendulum OCL2. Previous owner is clueless unfortunately...

The sidechain on my Pendulum OCL2 has two TRS plugins. The HPF has two TRS plugs. So I take both and plug them in. According to the Pendulum's manual, the Tip is the send and the Ring is the return. When you Link, both inserts are active.

Sorry for the confusion - the readings I gave are the gain reduction (GR).
Old 28th February 2013
  #11
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In that case, if you have a female TRS-2 TS cables, you can use the filter independently.

If you are at 80hz and the gain reduction GOES UP as the filter is stepped up, that doesn't seem like a HPF.

I would run a drum mix through them with a lot of kick and floor tom and squash it. CREATE ARTIFACTS, suck out the low end. Then start stepping up the filter, if it's a HPF, the low end should clean up as you engage the filter.
Old 28th February 2013
  #12
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to me it seems that when you increase the resistance value, more low frequency coming through, hence the more GR you get.
Maybe it's the other way around; 0.1 is the highest (e.g. 220 Hz) and 0.47 is the lowest (e.g 80 Hz)
Old 28th February 2013
  #13
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I'm about to give up haha. I'm just totally "not getting it". I can hear changes being made when I use a tone generator. I think that, sometimes, I can hear changes being made when I run drums really squashed through it (like you suggested). But it's so subtle....

I ran a tone generator through it and am monitoring the effects w/ Blue Cat's Freq Analyst:

Screenshot #1 - HPF is bypassed
Screenshot #2 - HPF engaged and turned on the 0.1 setting
Screenshot #3 - HPF engaged and turned on the other extreme: the 0.47 setting

If I read this correctly, this shows that the 0.1 setting filters out the 100hz before the comp but the 0.47 doesn't really filter it out at all.
Attached Thumbnails
How to determine HPF filter frequencies?-100hz_filterbypassed.jpg   How to determine HPF filter frequencies?-100hz_filterat_0.1.jpg   How to determine HPF filter frequencies?-100hz_filterat_0.47.jpg  
Old 28th February 2013
  #14
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why don't you just send an impulse through and get the impulse response...
Old 28th February 2013
  #15
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I think it's time to give up.... trying to figure it out anyway. Just use the comp and probably sometimes you'll find the filter useful, and sometimes you won't.

OR, you could get a simple stereo EQ, even a Behringer Ultragraph, and use that as the sidechain. You'll know exactly what that's doing. (The audio is not affected by anything patched into the sidechain, it just narrows the focus of the detector circuit of the comp).
Old 28th February 2013
  #16
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Scott Whigham's Avatar
 

Quote:
Originally Posted by stinkyfingers View Post
why don't you just send an impulse through and get the impulse response...
I don't quite understand how to do that - do you know of any tutorials anywhere?

Quote:
Originally Posted by j2dafo View Post
I think it's time to give up.... trying to figure it out anyway. Just use the comp and probably sometimes you'll find the filter useful, and sometimes you won't.

OR, you could get a simple stereo EQ, even a Behringer Ultragraph, and use that as the sidechain. You'll know exactly what that's doing. (The audio is not affected by anything patched into the sidechain, it just narrows the focus of the detector circuit of the comp).
Thanks for the help - I'm with you on just saying, "Screw it - use your ears". I like the idea of the UltraGraph - hadn't thought of that.
Old 28th February 2013
  #17
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here is an impulse. it is a perfectly flat line.
run a loopback like you were doing with the tones, but with this file.
that will give you the frequency response.
(44.1 kHz / 24 bit)
Attached Files
File Type: zip sample.zip (1.2 KB, 10 views)
Old 3rd March 2013
  #18
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did you figure this out yet?
Old 3rd March 2013
  #19
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I'm thinking that this is a situation where the numbers really don't matter that much. Just use the compressor as it was designed and if you find that it's triggering to much off the low end information of the source then turn that knob until it isn't!

Honestly, as much as we like to assign data to switches and knobs the art of audio engineering is really quite simple - turn a knob one way and if the source sounds better than keep turning until it starts to sound bad again and then back it off a hair. If the source sounds worse when you turn the knob, then try turning the knob in the opposite direction and then refer to the above. Easy as pie!

Or, if you really want to label it and know the resistance values, give Greg at Pendulum a call - I'm sure he'll be able to give you a good idea as to the frequency that each one affects.
Old 3rd March 2013
  #20
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Quote:
Originally Posted by AllBread View Post
I'm thinking that this is a situation where the numbers really don't matter that much. Just use the compressor as it was designed and if you find that it's triggering to much off the low end information of the source then turn that knob until it isn't!

Honestly, as much as we like to assign data to switches and knobs ...
this is not simply the smart-aleck answer, it is the Real World answer. Even after you have identified the values, you are still going to turn the knob until it sounds good.

One of my former students said in his audio school, they were relentlessly Quizzed with pink noise cuts and boosts until they could pretty much tell by ear what frequency was affected.

I bet with a little bit of that type of work you could confirm the "100, 150, 220" hypothesis just by listening. Those values certainly make sense.

In any case, turning the sound into a number is not as important as simply 'remembering' the sound.
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