Not Wanted:
AC Hum!

AC Hum – another source of unwanted noise – is one of the more easier types of nasties to eliminate.

Before I go further, I want to make it clear that I'm going to be talking about this in a post production sense only. Although you should always try to eliminate hum at the source, only software solutions are going to be discussed here.

So there!

All types of hum, whether 60 cycle hum or ground loop hum, are 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 simply inserted at the required frequency and presto – it's gone!

Well, there's also the issue of harmonics. These are tones that are mathematically related to the fundamental, or base, tone.

For example with AC hum – or 60 cycle hum – in addition to tones at 60 Hz, the tones at the multiples of that base frequency (60 Hz) would be 120, 180, 240, 300 Hz, and so on but with continually decreasing volume levels.

So to remove those you would use an, appropriately named, harmonic filter!

Material that was recorded in Europe may have hum problems at 50 Hz. (When I get files with ac hum at this frequency, it gives 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 is: unrelated to the base frequency. This is referred to as buzz.

In a situation like that, I 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.

If AC hum comes off an analog source such as an open reel tape, and the tone is near 60 (or 50) Hz, there is a clue there for getting the correct playback pitch!

So how do I do this?

Waves makes a software program called X-Hum.

But I use a plug-in designed by the Roger Nichols Digital company called Uniquelizer.

I find Uniquelizer extremely flexible because in addition to having presets for both 50 and 60 cycle hum, and of course the required notch and harmonic filters necessary for acting as hum eliminators, the frequencies are finely adjustable by a tenth of a degree, as are the bandwidths.

To locate the undesirable frequencies, I'll loop the section where the hum clearly stands out on its own, and then plugging in Voxengo's Redunoise solely for its spectrum analyzer's accuracy, I'm able to locate which tones need to be eliminated.

Using the carefully selected notch filters I remove the tones; and then I remove Redunoise – unless there's some hiss to be removed!

Often, when there is a really heavy hum presence in the audio, and it's successfully removed, the resulting clean-up will sometimes give the impression of lacking bottom end or even warmth, depending on what type of hum needed to be removed.

I once did a restoration for a client involving very heavy hum removal from an old radio interview file. 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 impression 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!

Nevertheless, what impression the original file gives, is an important consideration before finalizing your work as a hum eliminator.

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