Why don't we use steep highpass filtering?

[Mark Hammer]

Someone on another forum was kvetching about hum from their single-coil-equipped guitar.  It prompted me to google what the lowest normal frequency was on a regular old 6-string guitar (I'm sidestepping baritone guitars and 7 and 8-string instruments which will provide more low frequency content).  Apparently, it's 82hz.

So why are we going through all these gymnastics, and careful level setting of gates and such, when a simple 4-pole (or greater, if you feel like it) highpass filter, rolling off at 70hz, for example, would improve our situation?

I suppose there is likely SOME sort of sonic "price to be paid", or else the market would already be flooded with such units.  But still, I have to ask, why don't we do this?  It could certainly make the job of noise gates placed further along the signal chain easier.

ADDENDUM: Back in the day of vinyl, a "rumble" filter, intended to remove subsonic components via steep highpass filtering, was considered a very useful addition to a home sound system.  Why not make use of something like Rod Elliot's infrasonic filter project #99 ( https://sound-au.com/project99.htm ) and set the cap values to something that kept the music but pushed down the hum?

[idy]

More steep means more distortion, ringing. 60hz is less than an octave below 82. So maybe 4 poles..but that will still affect 82, a musical tritone below +or -. Also, the power hum has harmonics.




but practically, hum blockers do work, sort of. EHX has a "Hum Debugger." Others too.

What about using a "balance" line? Make a hum antenna, invert the phase, add as much as you need to your signal?

[anotherjim]

If only it was just 60Hz (or 50Hz for the rest of us). The harmonics also come to the party, particularly the 2nd.

[Steben]

Many circuits have 1 pole in between highpass filters with 70 to 100 Hz 3dB points. 

[PRR]

Why not make use of something like Rod Elliot's infrasonic filter project #99...

Because that is two opamps for the filter and will need another to buffer into guitar chain. Three opamps (and power supply) is "a lot" for something so boring and utilitarian.

And harmonics. In casual everyday use it can be hard to know 60 from 120; your ear finds the fundamental even if it is absent.

[Mark Hammer]

Good replies.  So here is a related question: if an EMI source provides not only 60hz hum, but harmonics of 60hz as well, would modest reduction of 60hz, specifically, reduce the "burden" placed on any noise reduction further down the line?  In other words, 120hz may well be more audible than 60hz for many of us, but in terms of setting any threshold-based noise-reduction devices, is that 120hz (and/or any other multiples of 60hz) noticeably lower actual amplitude than its fundamental?

[PRR]

You actually want to dip JUST the 60Hz. It may be better to notch the narrow band than to fit a hyper-steep slope between 82Hz and 60Hz.

> hum antenna, invert the phase

You can't get the antenna right AT the pickup. Always a mm or two one way or another. So unless the guitar and all the electricty in the room is locked-down, you will lose the null with any motion. You also null part of the string vibration....

It's basically a "humbucker", with the same problems. Incomplete null, variable as the player moves, and loss of signal (starting with treble).

[Rob Strand]

We already discussed this a few years back,

https://www.diystompboxes.com/smfforum/index.php?topic=120242.20

I did an experiment at the end of the thread (reply #30) which proved that removing hum alone isn't enough. The harmonics are more annoying (and harder to remove because they are in the audio band).

[andy-h-h]

I used to do concert sound back in the day - with a large system the first thing most people do when setting up a mix is to turn on all of the high pass filters, excluding kick, floor tom, bass guitar and keys.  Why?  Reduces rumble and tightens up the low frequencies for everything else.  Sometimes subs are run off an auxiliary send / matrix out so you only send certain things to the subs. 

What has this got to do with this topic?  Most high pass filters that I recall were 80hz and they would have steep poles. 

You might not want to touch the low end of your signal, but the person out front probably will, and they won't ask for your opinion on it. 

[phasetrans]

I used to do concert sound back in the day - with a large system the first thing most people do when setting up a mix is to turn on all of the high pass filters, excluding kick, floor tom, bass guitar and keys.  Why?  Reduces rumble and tightens up the low frequencies for everything else.  Sometimes subs are run off an auxiliary send / matrix out so you only send certain things to the subs. 

What has this got to do with this topic?  Most high pass filters that I recall were 80hz and they would have steep poles. 

You might not want to touch the low end of your signal, but the person out front probably will, and they won't ask for your opinion on it. 

This is 100% true. I might add that these days all the consoles have adjustable high pass frequency. Both 2nd order and 4th order filters are seen. Between cleaning up the rumble and offsetting proximity effect from the predominantly cardioid mics, they are useful and often necessary tool. Very common in the studio, too.

Putting on my mix engineer hat (20+ years experience and day job for a pro audio manufacturer), balancing low end is a big part of a pleasing final product. Control of the low end also heavily influences how much time based (e.g. attack and release) control you have over sources, as the amount of LF content influences the speed of release times without excessive IMD.

For typical electric guitar close miced with cardioid microphone, a 2nd high pass between 70-180hz AND a second order lowpass between 9kHz - 15kHz, instrument dependent, is extremely common for my mixes and of other mixers I know.

[amptramp]

If you want to reduce the fundamental and odd harmonics, a comb filter may be what you want.  It is just a delay going into a summing junction with the dry signal input.  It doesn't do much for even-order harmonics but maybe you can use another comb filter at 120 Hz.  Notch filters seem to be the way to go here.  You can get steep sides without much effect on the rest of the signal.  You could even do this for bass guitar where the output goes down to 41 Hz.

[PRR]

> fundamental and odd harmonics

What we want is "all" the harmonics (1, 2, 3, 4, 5, 6, 7..) of 100/120Hz, plus that pesky 50/60Hz.

(In an ideal case, power supply ripple lacks the 50/60 fundamental, but stage buzz always has some.)

[andy-h-h]

I used to do concert sound back in the day - with a large system the first thing most people do when setting up a mix is to turn on all of the high pass filters, excluding kick, floor tom, bass guitar and keys.  Why?  Reduces rumble and tightens up the low frequencies for everything else.  Sometimes subs are run off an auxiliary send / matrix out so you only send certain things to the subs. 

What has this got to do with this topic?  Most high pass filters that I recall were 80hz and they would have steep poles. 

You might not want to touch the low end of your signal, but the person out front probably will, and they won't ask for your opinion on it. 

This is 100% true. I might add that these days all the consoles have adjustable high pass frequency. Both 2nd order and 4th order filters are seen. Between cleaning up the rumble and offsetting proximity effect from the predominantly cardioid mics, they are useful and often necessary tool. Very common in the studio, too.

Putting on my mix engineer hat (20+ years experience and day job for a pro audio manufacturer), balancing low end is a big part of a pleasing final product. Control of the low end also heavily influences how much time based (e.g. attack and release) control you have over sources, as the amount of LF content influences the speed of release times without excessive IMD.

For typical electric guitar close miced with cardioid microphone, a 2nd high pass between 70-180hz AND a second order lowpass between 9kHz - 15kHz, instrument dependent, is extremely common for my mixes and of other mixers I know.

True - digital certainly changed things.   Nothing like having an RTA running behind your EQ curves on every channel.  Luxury...