AC Hum – another source of unwanted noise – is one of the more easier types of 'nasties' to eliminate.
Especially in a post production setting like mastering.
And to be clear: I'm going to be
talking about this in a post-production sense only. Of course, you should
always try to eliminate hum at the source. But sometimes software solutions like the ones
I'll discuss here are needed.
No matter the source or type, all hum is simply a constant, steady tone fighting for your unwanted attention with the music you're trying to listen to.
To remove them, hum eliminators, better known as
notch filters, are plugged in at the required frequency and presto –
There's the issue of harmonics. These are tones that are mathematically related to the fundamental tone.
Have a look at my image below to see my point:
In addition to tones at 60 Hz, the tones at the multiples of that base frequency (60 Hz) would be heard. So in the image above, this shows 120 and 180Hz being eliminated. But it can continue upward with 240, 300 Hz, etc. It depends on the sound source itself.
So to remove these I use a harmonic filter. Waves provides options to link these frequencies and scale them according to the sound file.
Material that was recorded in Europe may have hum problems at 50 Hz. (This also provides a clue as to where the music was originally recorded as well!)
Often hum problems come at other frequencies, and with in-harmonic tones, that are unrelated to the base frequency. This is referred to as buzz.
In a situation like that, I might use notch filters at the needed frequencies, with as narrow a bandwidth as possible and as few as possible. Too many will alter the audio files in odd ways.
A simpler solution may come from Sonnox. Their dehumming plugin also includes buzz removal and a tracking feature that comes in very handy.
Older sources like vinyl and tape will often have traces of AC hum in them. So by analyzing the frequency (be it either 50 or 60 Hz), I have a clue for getting the
correct playback pitch for off-speed sources! Pretty neat! Only then do I zap the hum.
As you can see on this page Waves, Sonnox and Steinberg are some of the makers noise reduction software to eliminate hum.
Waves has X-Hum. WaveLab includes Sonnox' De-Buzzer and Steinberg's PostFilter.
Some time ago, Roger Nichols Digital had an EQ named Uniquelizer. It was extremely flexible. It had starting presets for both 50 and 60 cycle hum. It included the required notch and harmonic filters. The frequencies and bandwidths were finely adjustable by a tenth of a decimal. And it was linear phase. Sorely missed here at Mister's Mastering House.
I do this by looping a section where the hum clearly stands out on its own. Then using an accurate spectrum analyzer, I locate which tones need to be eliminated.
Using carefully selected notch filters I simply lower the level of the tones!
Often, when I successfully remove a really heavy hum presence in the audio, the resulting clean-up will sometimes give the impression of lacking bottom end or warmth.
I once did a restoration for a client involving very heavy hum removal from an old radio interview. And while everything ended up nice and clear, it seemed to lack the body of the original. So in addition to providing a straight, cleaned up file, I also made two attempts to restore the perception of the original, by EQ'ing in some bottom end warmth.
However in the end, the client enthusiastically preferred the first, un-eq'ed restoration!
What impression the original file gives, I have to consider it thoroughly when I remove AC hum.
Or any unwanted noise.
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