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Author Topic: DC Blocking, HPFs and their position in your chain  (Read 5522 times)

Hermetech Mastering

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DC Blocking, HPFs and their position in your chain
« on: March 31, 2011, 09:10:43 pm »

Just trying to work through a few things in my mind, and would be happy for anyone else to jump in too!

The way I understand it, a DC blocking filter is the same as a HPF with the cutoff point set very low. The reason for using a DC blocking filter is to remove DC offset from the audio, which can happen sometimes, and blocking it frees up some more headroom for the rest of the audio. So far, so good.

However, from the research I've done, and the various posts I have read on the subject of DC blocking in a mastering context, this DC offset thing is rarely, if ever, a problem, and it seems that  many people rarely use this feature.

Going back to the fact that this is the same as a HPF, and that these do often seem to be used in a mastering context, I was wondering how many people use them, and where in the chain they get used.

I understand the use of a HPF with a lowish cut-off frequency can be used as a type of EQ, for example, if the bass is too much, or to free up more headroom by reducing frequencies which most people's systems (and even ears) would be unable to perceive, or even to be able to make things LOUDER (if that's your thing).

I use the Elephant limiter which has a very full featured "DC Blocking Filter", with adjustable frequency and about ten different slope types. I find myself using this more as a "Bass Reduction EQ" at the end of the chain, than I do as a DC blocking filter. I notice the Dangerous Music Bax EQ also has this functionality, and that many people put this at the end of their chain.

Any one else use a DC blocking filter as a bass reduction EQ, or am I out on a limb here? I've noticed substantial differences in the clarity and bass weight of tracks by careful adjustment of this setting.

My chain is usually: digital source file, digital surgical EQ, D/A, outboard compression, outboard program EQ, A/D, SRC (if needed), Limiter (including the DC Filter) and Dither.

Something I have been wondering about recently is, wouldn't it be better to reduce any extreme low frequency information before the file hits the D/A into the outboard (around the same time I do any surgical EQ on the file, if needed), thereby achieving greater headroom throughout the entire chain? And if so, what digital tools or EQ do you use for this task?

Like I said, I'm just wondering out loud really, and will do some practical experiments with this in the coming days!

Thanks in advance for any input.

Gregg

Gold

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Re: DC Blocking, HPFs and their position in your chain
« Reply #1 on: March 31, 2011, 11:15:16 pm »

I think the difference between a DC blocking HPF and a "musical" HPF is a big fat zero. DC blocking is just the application. Filters can be defined by the slope xxdB/octave and the type. Bessel, Butterworth, Chebychev and some that are in between. The types have to do with optimizing certain parameters at the expense of others. For instance maximum amplitude flatness versus minimum phase shift. I sometimes drink red wine with fish and white wine with lamb.
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KAyo

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Re: DC Blocking, HPFs and their position in your chain
« Reply #2 on: April 01, 2011, 01:09:19 am »

Many a times, I’ve pondered the same, while using the TC MD3.
With the DC ON engaged it sounds more like a CD should, but then.. it could be just placebo etc.. It does feel, the mix starts to sound leaner and less lumpy, thus, the apparent loudness.

None of the TCMD3 presets have it engaged either, that throws another, why? in the perception of its usage. But, as you’ve read; most mixes have the slightest negligible offset in them, and applying the DC function isn’t paramount or a must.

Is that the consensus around. Would be an insight, to know.

Ciao’
KAyo
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Hermetech Mastering

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Re: DC Blocking, HPFs and their position in your chain
« Reply #3 on: April 01, 2011, 01:16:32 am »

@ Gold: Yep, that's what I had figured Mr. Gold!

I'm not sure what type/algorithm the filters in Elephant are, but the cut-off is tweakable in tenths of a Hz from 1.0 to 100.0, and the roll off is selectable from 6, 12, 18, 24, 30 or 36 dB per octave. Apart from the 6dB slope, all the others are also selectable from "Soft" or "Steep", which has always confused me, as I had always assumed you were selecting that with your roll off choice. Perhaps different filter equations are implemented here? I tend to stick to the 6dB most of all, as it seems to have less of an effect on the audible content than the steeper slopes, and usually sounds better to my ears.

Aside from the technical specifications of the filters, I am really interested in how you use them and where in the chain. Do you think it's worth implementing one before analogue outboard in order to preserve more headroom? And if so, what tools do you use for this?

How about custard with HP sauce?

@KAyo:

Glad I'm not the only one wondering about this! Anyone else chime in?

Viitalahde

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Re: DC Blocking, HPFs and their position in your chain
« Reply #4 on: April 01, 2011, 01:49:42 am »

6dB/oct filtering can be good, as well as 12dB/oct. When I use HPF's, I tend to use low order filters.

But most often I don't use a strict filter at all. 6dB/oct can be seducting for reducting lows in general, and while the rolloff starts well, there's just something in deeper cutoff I don't like. With the new room & monitoring here, this is even more obvious.

Low shelves get the most use for reducing low end here. My shelves start from 21Hz, 6dB/oct. They kind of do the same type of thing a 6dB/oct filter does, but they don't obviously stop it all. Sounds much better to me.
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Hermetech Mastering

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Re: DC Blocking, HPFs and their position in your chain
« Reply #5 on: April 01, 2011, 01:56:01 am »

Thanks Jaakko, that certainly gives me another option to think about! Would the Sonoris MEQ be a good digital tool for this kind of extreme low end work? Been meaning to give it a try for a while.

phonon

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Re: DC Blocking, HPFs and their position in your chain
« Reply #6 on: April 01, 2011, 08:44:48 am »

おはよう, from Ohio, Gregg-san!

I agree with Mr. Gold.   If using a DC-blocker sounds better than not using it, use it! (:   (But how often do they actually make the music sound better?)

Of course, with all those center frequencies on the Elephant, how do you know which one to use? \:

I'd recommend trying first the 18 dB/oct slope HPF'ing for gentle "garbage disposal" filtering.   

Also, I'd put the limiter before the SRC so that the dynamics are also done at 2x Fs.   Otherwise, just go ahead and capture at 1x...


さよなら,

Andrew
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Re: DC Blocking, HPFs and their position in your chain
« Reply #7 on: April 01, 2011, 09:03:45 am »

I've never seen DC offset so great that it would effect the headroom available for the analog chain in any appreciable way (assuming one runs their analog chain with ample headroom as I do) and I'm fairly certain no modern ADC will pass / generate DC, so I don't think DC offset is an issue unless one does only digital processing (or uses ancient ADCs).

But then, I'm a technical moron, so please correct me if I'm wrong here...
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Brad Blackwood
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phonon

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Re: DC Blocking, HPFs and their position in your chain
« Reply #8 on: April 01, 2011, 09:45:35 am »

You're right, Brad, of course...  and, technically, "more on" than most.   ;D

The Lavry AD122 has a brutal DC-blocking filter as part of the design.  It's only supposed to be defeated for factory calibrations and it's not linear phase (Dan says that was not needed, or wanted, and would have been too expensive to implement ---  Matthew Gray begs to differ, I believe, at least about the center f and slope, whence his ADC wars)...

And the analog gear with trafos should have DC blocking qualities,  anyway.

However, for crafting the sound, Gregg-san and others might like how with DC-context filtering you can warp the phase and balance of energy of the whole mix, sometimes, without actually putting your grubby hands directly in the audio zone (20+ cps). 




Andrew
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Hermetech Mastering

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Re: DC Blocking, HPFs and their position in your chain
« Reply #9 on: April 01, 2011, 09:48:17 am »

@phonon: Thanks! I do experiment a lot with the DC Filter with every track. I don't currently have the best monitoring in the world by a long shot, but I do hear "more effect" or "less effect" or "better sound" or "worse sound" when playing with the various options Elephant has to offer. It's very subtle though, but sometimes I can seemingly get the mix to "open up" a little without affecting any of the bass that I can actually hear. Will probably be a lot easier to work out once I get the ATCs!

And re: putting the SRC after the limiting, in an ideal world that's exactly what I'd do, but the SRC I use (Voxengo R8Brain Pro Full) often creates peaks 6dB or higher than the source file, sometimes going over 0dBfs, and so I have found it easier/less time consuming to put it before the limiting stage.

@Brad: yeah, I mean basically I never use the DC Blocking Filter for blocking DC, if that makes any sense, as the transformers in the analogue chain pretty much knock out any of it, and that offset introduced by the final limiting tends to be extremely tiny (as you said) and not worth worrying about. I'm more thinking of using a HPF before the analogue chain, not to block DC, but to "carve out" any inaudible sub bass frequencies in order to increase headroom in the following stages. The reason I want to do this is that I think it might sound better/cleaner that way, as, for example, the compressors then won't be reacting to a load of inaudible frequencies. Guess I need to try it out on a few mixes I know well and compare the results... And I'm pretty much a technical moron too. :)

John Roberts {JR}

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Re: DC Blocking, HPFs and their position in your chain
« Reply #10 on: April 01, 2011, 11:11:03 am »

As has been correctly stated, DC blocking is not (should not be) a head room issue.  There is no DC music or sound, so all DC is unwanted noise or perhaps artifacts of conversion or processing that should be removed.

In analog path design, there is a need for DC blocking in places like microphone phantom power supplies, and the like. DC blocking, especially in older designs will also be used in several places to reduce switch clicks and scratchy pots, what will generally be interpreted by users as inferior path quality (even if it is better than with a cheap blocking cap).

On a per input basis it is useful to filter out LF content that is below the source frequency range of the instrument being tracked, unless you want to capture the musician's grunts and room air conditioner noise. Piano's and weighted keyboards can have some LF content from the action that is below the lowest strings. 

The shape (slope and alignment)  of these LF filters only matter if they are close to the passband and alter the sound in their transition region.

For DC blocking the general design philosophy is to make the poles as low as practical so they don't impinge on the audio passband. Several very low poles in series will accumulate and creep up higher in effective pole frequency, so these are typically set quite low.  In the analog world it was generally good practice to use one high quality HPF to define the lower response skirt, and keep all the DC blocking well below and out of the sound. The LF filters in series will all interact if close in tuning. 

JR
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SafeandSoundMastering

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Re: DC Blocking, HPFs and their position in your chain
« Reply #11 on: April 01, 2011, 02:18:16 pm »

I never use it for EQ i.e. cutting sub but purely for removing DC. If a track has more than 1pct of so I use a DC filter. Of course always A/B ing to ensure it's transparent.
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Barry Gardner
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bkuijt

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Re: DC Blocking, HPFs and their position in your chain
« Reply #12 on: April 01, 2011, 03:09:02 pm »

Low shelves work all the way down so a HPF makes sense in a Bax EQ.
In terms of place in the chain I mostly apply it at the start or at least before any compression.
 
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SafeandSoundMastering

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Re: DC Blocking, HPFs and their position in your chain
« Reply #13 on: April 02, 2011, 04:26:08 am »

I tend to use it just before limit and dither.
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Barry Gardner
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Re: DC Blocking, HPFs and their position in your chain
« Reply #14 on: April 02, 2011, 08:39:41 pm »

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