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Old May 6th, 2006, 02:23 PM   #1
Fred Retread
 
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WOW! Audio cleaned up by removing 60Hz harmonics

Most of us have wound up with some recorded 60 Hz hum at one time or another and know how to eliminate it. But the last stage event I taped was plagued with that other noise --that higher pitched hum/tone/whine. You'd know it if you heard it, it's fairly common. It was coming through the PA system. Awful audio.

In the past I've tried to use SoundSoap on this, with some sucess, but I had to accept some distortion of the desired audio. SoundSoap is great for broadband noise, but it turns out that my culprit was not broad band noise.

It's a combination of odd harmonics of 60 Hz. Specifically, the 1st, 3rd, 5th, 15th and 21st harmonics. The 5th was the loudest, followed by the 3rd, the 15th, the 21st and the 1st. Yes, 60 Hz itself was the least of them. Chalk it up to electrical resonances at those frequencies.

I identified these by taking a sample and doing a spectral analysis in Cool Edit. Then I removed them with the paramentric equalizer in Vegas using a width of 0.3 octaves for each frequency. It took several iterations since the equalizer can only do four bands at a time, and also because it simply took two applications to get rid of the worst two frequencies.

The result is amazingly clean audio with virtually no audible distortion. If this is old hat to anyone you'll have to excuse my excitemet, because it was a revelation to me.
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Old May 6th, 2006, 02:41 PM   #2
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Any electrical equipment that handles audio can fall prey to the dreaded 60hz and its harmonic compatriots, so this is always a good place to start any audio cleanup...in the digital age...if it doesn't clean up any of the audio problems...UNDO so you can continue to work with pristine copies of the audio.
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Old May 6th, 2006, 05:41 PM   #3
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I have only run across a problem with 60Hz on
one occasion where it really stood out. This
occurred when I was working around a lot
of fluorescent lights in a dropped ceiling.
The ballasts in the lights, if they are older,
can really start to buzz. And some ballasts are
just designed to be quieter than others. For
example, ones used in a library are quieter than
household ones, which are quieter than ones
used in a factory.

Anyways, I was wondering what advantages
you find with Cool Edit as compared to Vegas
for your audio work? Currently have Vegas
and was wondering if Cool Edit would
help me out in any way, other than for
stuff like spectrum analysis?
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Old May 6th, 2006, 06:48 PM   #4
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Keep in mind that 60hz AC is usually transformed into 120hz pulsing DC before filtration (commonly referred to as full wave rectification.) It only takes an open, or way out of tolerance power supply filter cap to let some of that 120 hz ripple make its way into the system. If the wave gets clipped anywhere, it will generate harmonics in a pattern much as Fred described. That's because a square wave is the fundamental frequency plus a large amount of odd harmonics. Oure ears are much more sensitive to odd harmonic distortion than even harmonic distortion. The basic guitar fuzz pedal is nothing more than a clipping diode circuit to distort the incoming waveform. Overdrive is the distortion produced at the output when the input signal exceeds the amplifiers ability to faithfully reproduce the input signal.

So what I was really trying to convey is that as Fred discovered, audio hum is best removed by eq'ing out 120 hz and the odd multiples.

Good work, Fred. I know how exhilarating it is to clean up messy audio that you thought was never going to work. Like many of life's adventures, getting there is half the fun!

-gb-
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Old May 6th, 2006, 07:44 PM   #5
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Hey good job on the hum removal. I use Protools with a Waves De-noise plug-in that has a EQ pre-set like you describe.

I sometimes find 60 hz hum helpful. Recently I was transfering some very old transcription disks that were recorded at a varible speed. They started out around 78 rpm and went slower towards the end of the disk. I was able to transfer them to a varible speed analog recorder and then while watching a PAZ frequency analyzer manually adjusted the speed by keeping the spike at 60 hz . In this case the hum made the job easier.
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Old May 6th, 2006, 08:20 PM   #6
Fred Retread
 
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Quote:
Originally Posted by Dave Largent
...I was wondering what advantages
you find with Cool Edit as compared to Vegas
for your audio work?...
Two things arise from Cool Edit (now known as Adobe Audition) being dedicated to audio: The interface is graphically intuitive with easier access to the tool set, and a lot of its functions are more powerful. For instance, the paragraphic equalizer in Vegas would only apply four notches at a time and would only take them down to -25 dB. In Cool Edit I can make as many FFT notches as I want, control the width precisely, take them down to virtually - infinity, and save the preset.

Being able to look at the signal at any zoom level you choose is a big plus too.
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Old May 6th, 2006, 08:28 PM   #7
Fred Retread
 
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Thanks everyone. I knew the genesis of harmonics, I was just surprised that the higher ones were so prominent, and that they could be removed so cleanly.
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Old May 7th, 2006, 04:02 PM   #8
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Quote:
Originally Posted by Greg Boston
Keep in mind that 60hz AC is usually transformed into 120hz pulsing DC before filtration (commonly referred to as full wave rectification.) I
True but more to the point today is the nearly all pervasive use of "switching" power supplies which do not draw sinusoidal current from the mains but rather pulses of current at a 120 Hz rate. This results in large harmonic currents in the transformers, distribution wiring and so on with the result that harmonics of 60 Hz are everywhere and worse (especially the higher order ones) than they were in the old days of linear supplies.
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Old May 7th, 2006, 07:01 PM   #9
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Quote:
Originally Posted by Fred Retread
Thanks everyone. I knew the genesis of harmonics, I was just surprised that the higher ones were so prominent, and that they could be removed so cleanly.
With my fluorescent light buzzing problem, I found that the
majority of the buzzing was in the higher harmonics of
60Hz, like around 240Hz.
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Old May 7th, 2006, 07:46 PM   #10
Fred Retread
 
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I found significant noise at 900, 1020 and 1260.
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