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60Hz roll-off results in overall volume gain - what's going on?

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60Hz roll-off results in overall volume gain - what's going on?

Postby Ken P » Mon Dec 28, 2009 11:04 pm

I am currently working with some unusual material for a compilation; sound scapes made using experimental techniques. Some of the artists are deliberately using sonic artifacts which would normally indicate a problem, but in this case are intentional, such as digital clipping.

Some of the tracks build, slowly to a sustained raging burst of noise with levels peaking at 0dB. Part of my mastering strategy on this project is to firstly reduce the volume of the files, to create some headroom, and then to roll-off frequencies below 60Hz to eliminate unnecessary low freqeuncy energy that might, for example, dominate playback where a sub woofer is present.

I'm using sound forge and I've foudn that simply rolling off low frequencies with the standard EQ processing function is resulting in the overall volume being increased, by as much as 4dB. This is contrary to what I would expect - normally, when you filter out frequency content the overall volume level decreases.

Anyone have any ideas what's going on here?
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby Hugh Robjohns » Tue Dec 29, 2009 1:09 am

Filtering the extreme LF can only reduce the total signal energy levels, so if you're saying the level goes up by about 4dB (which I presume is assessed on a meter) I presume you have a compressor somewhere downstream of the filter.

By removing the extreme LF, the compressor will see a lower level in the side chain and apply less compression. Therefore the output will be higher.

Does that make sense?

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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby Waltz Mastering » Tue Dec 29, 2009 3:13 am

Ken P wrote:
I'm using sound forge and I've foudn that simply rolling off low frequencies with the standard EQ processing function is resulting in the overall volume being increased, by as much as 4dB. This is contrary to what I would expect - normally, when you filter out frequency content the overall volume level decreases.

Anyone have any ideas what's going on here?


This is typical for a minimal phase eq design, especially if you are using a steep cut off. The amplitude and phase response does not work like it would in a linear phase eq.

With minimum phase eq, phase shift varies with frequency and can "advance" or "retard" as the amplitude and frequency is changed.

With linear phase eq, this won't happen so much but you have other things to deal with like pre ringing or pre echo.

You can try decreasing the steepness of your roll off and or using a bell with a wide Q to cut the sub low's.

Quote:Traditional analog EQs are termed "minimum-phase," meaning they introduce a small time-shift that varies with the frequency of the signal. As greater boosts and cuts are applied, some frequencies are time-shifted more than others, essentially pulling apart the precise alignment of overtones that define a musical instrument's character. There is a point of diminishing returns. If the EQ is pushed to far, its benefits are overshadowed by damage to the wave structure caused by phase-shift.
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby Ken P » Tue Dec 29, 2009 7:45 am

Waltz - thanks that makes sense, the roll-off is a bit steep.

Hugh - I'll send you an example file if you like so you can see the bizarreness in action. There's no compressor or limiter affecting the output - just roll-off the LF and watch the gain go up.

I think this is partly caused by the extreme material I am working with ...

Thanks guys.
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby abba_x » Tue Dec 29, 2009 9:09 am

You might find that the EQ (filter) you are using is slightly resonant i.e. it boosts frequencies around the cut-off point. What you end up doing is cutting away the very low frequencies (of which there are very little) and boosting the lows (of which there is quite a lot) resulting in an overall gain.

Your EQ may have a graphical display of it's response. Does it go above 0dB at 60Hz? If so then this is what you are experiencing. Changing the 'Q' value (if available) may help.
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby Hugh Robjohns » Tue Dec 29, 2009 9:58 am

Sorry Ken -- bad assumption on my part.

Yes, as others have said, it could be down to the shape of the filter response you're using -- some do indeed peak slightly before attenuating -- or the phase response of the filter.

What kind of EQ are you using?

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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby Ken P » Tue Dec 29, 2009 11:57 am

Thanks abba_x - I think the most likely cause is indeed some resonance around the cut-off point, perhaps just a little, but enough to push it over.

Hugh it's just the standard built-in Sound Forge EQ.

Intuitively, and I have no real empirical basis for this, but what I feel is going on here, is some spurious maths at work in Sound Forge resulting in gain increase, that is caused by the extreme material I am working with ... when I get chance I'll post some files and meter readings so you can see what's going on.

In the mean-time, thanks for all your help!
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby dmills » Sat Jan 23, 2010 7:14 pm

This actually sounds like "Tilt", a common problem in badly thought out FM transmission chains where the low frequency roll off inherent in a PLL based exciter can result in over mod.

What happens is that the clipping can in effect produce large infrasonic components that must be there to bias the rest of the signal to stay within range, remove these and the peak level does increase (but the total energy does of course decrease).

It is exaggerated in your case by the fact that some of your sets of samples are probably NOT actually valid sample values (as in you could never produce them with an analogue signal staying strictly within the full scale range of the converters).

Every analogue signal that stays within the range of the converters and that its in below the nyquest limit results in a reproducible set of samples, but NOT every set of samples represents a valid signal!

I wouldn't worry about it, knock 10db off the top, strip the infrasonic crap out, and master what is left as appropriate.

Regards, Dan.
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby The Robot » Mon Jan 25, 2010 9:26 pm

I do believe it is related to phase shift. And it's not just in soundforge or the material you're working with.

This topic came up somewhere else, so I immediately loaded pro tools, printed some white noise at -6db then stuck the phasescope as the last insert on the channel.

Hit play and observe the scope, nice even lines across the image. A solid -6 on the level meter.

Introduce an EQ before the scope (EQ-3 7 band in this case), engage the filter and BAM! Watch as your phase goes to pot and your level increases erratically.

Increasing the filter Freq upto 1-2k gives a level increase of nearly 6db, not a solid increase either, jumps around a lot; and the phasescope is all over the show.

Print your white noise any louder than -6 and its clip city!

And to confirm what WaltzMastering said above, the steeper your rolloff, the more extreme the symptoms.
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby tomafd » Mon Jan 25, 2010 10:03 pm

Maybe this is why I've taken to not using any eq on individual tracks, as far as possible (sometimes it has to happen) these days, and just sticking a nice, hardware, passive eq over the mix buss for a bit of top end 'air' sometimes - and getting, much, much nicer results ....
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby Shingles » Mon Jan 25, 2010 10:17 pm

Hugh Robjohns wrote:Filtering the extreme LF can only reduce the total signal energy levels...

But it can - and often does - increase the peak signal level.

Low pass filter a square wave and see what happens to the peak amplitude.
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Re: 60Hz roll-off results in overall volume gain - what's going on?

Postby Hugh Robjohns » Mon Jan 25, 2010 11:24 pm

Shingles wrote:Low pass filter a square wave and see what happens to the peak amplitude.


Good idea.. for those without the capability of doing this, here's a demo I knocked up quickly in Adobe Audition.

Started with a square wave:

Image

Applied a third-order (18dB/oct) high-pass filter:

Image

And this is the result:

Image

As Shingles says, note the difference in peak amplitude between the raw square wave and the filtered result. There's a decibel scale on the right hand side, and it's about a 5dB increase!

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