Basic Principles of Audio Processing

WORK IN PROGRESS - RuthieG

This is the second part of a series of short articles written by a sound engineer with many years' experience. The idea is to explain in plain language how to make a quality sound file.

Post Production 1: Equalisation (EQ) and mains hum removal

Post production tools fall into the categories of

• FX
• EQ
• Dynamics
• Special tools

what are they all about?

FX stands for effects: flange, phaser, fuzz box etc. We don't really need that in LibriVox so let's chuck it out and forget it. Job done. (If you really insist on reading The Pit and The Pendulum like John Laurie, then just chuck loads of reverb on, you'll wing it.)

EQ Stands for equalisation: tone, high middle and low, bass and treble--you know what I mean. Pretty easy, most people are comfortable with that concept and need read no further. But there are one or two tricks that can be done with digital EQ that are really quite effective, more later.

Dynamics is the one that most people don't understand, simply because there has never been a layman's equivalent, but honestly it's no more difficult than working the bass and treble on your old Hi Fi. Dynamics is about manipulating the level of sounds, whereas EQ is about manipulating the tone colour. I can use a compressor with my eyes shut but I have to think about EQ, so dynamics must be easier.

The one special tool I use is Sound Forge noise reduction--there will be a chapter on that later in this series.

Generally I would EQ before processing the dynamics; this is because EQ affects the loudness of frequency bands so you always have to do some dynamics after EQ even if you've already done it first.

So let's deal with EQ first.

When I was a kid I never touched the cheap sub standard tone controls on record players because of a sincere belief I held which years later was echoed almost word for word by a big record producer. He said, ”Tone controls should be absent from record players. After I've spent 12 hours slaving over a million pound mixer you don't have the right to go messing with my mix with a cheap crappy tone control.”

The EQ available in recording software is infinitely better than the old record players because it doesn't require million quid desks, just a bit of nifty mathematics.

EQ is not a black art if you go at it purposefully.

If a sound is low in one band or high in another, EQ can correct it. If it sounds good it probably is--if it ain't broke don't fix it.

Frequency bands

All sound is made up of various frequencies, perfect human hearing hears from 20 cycles per second to 20,000 cycles per second. When you get older your limit drops to 12-14k. Cycles per second in sound is labelled hertz, named after Heinrich Hertz, the Victorian German physicist who pioneered multinational car rental (now would I lie to you?).

If you play a 20 hertz tone through a speaker that can play that low you will feel it more than you will hear it.

• 20 – 100 hertz is where the oomph of a kick drum lies in dance music.

• 100 – 7000 hertz is where we speak and sing, our area of interest.

• 100 – 300 hertz is the bottom end of a voice, the bit that a big man has more of than a petite woman. This is the area you could call 'boom'.

• 300 – 900 hertz is the main body of a voice, the part that is pronounced in operatic singing. This part on its own sounds 'honky'.

• 3000 – 5000 hertz is the fine detail in speech that makes it intelligible. This is the area that disappears on a cheap crappy tone control.

• 6000 – 7000 hertz is the letter S, and the lush swoosh of a splash cymbal, this quite naturally is the 'hissy bit'.

When you give the frequency bands names like boom, honk and hiss it helps you to identify approximate frequencies by ear. I once walked out of terribly mixed Darkness concert muttering that it was all below 400 with nothing above 1k. In layman's terms, it sounded like a party in the house next door but amplified to the threshold of pain.

Coincidentally, electric guitars occupy all the same frequencies as the human voice--perhaps that explains why they've always been so popular. When I used to mix records, my method was to EQ the guitars down a bit from 3000 – 5000 (the intelligibility part) and raise voices in the same band. Consequently the voice came through the wall of guitar sound with just a small adjustment.

EQ controls

Small inexpensive mixers have EQ controls labelled high, mid and low and can be quite useful as long as they've been set at good intervals.

Sound Forge has 3 types of EQs:

• Graphic
• Parametric
• Paragraphic.

Graphic EQ

Graphic EQ is the most familiar one to most people: the bands are labelled with freqencies and you can raise or lower the sliders to lift or cut in that band. Anyone can play a graphic EQ by ear. Just try a slider; if it doesn't do what you want put it back and try another. Totally unscary EQ.

Parametric EQ

Parametric EQ is what you find on pro mixers. You still have the high mid and low controls, but for each band you have three controls

• Freq
• Q
• level

The Freq control is labelled in hertz and simply allows you to center that band on a particular frequency, so you can be quite specific about where you want to lift or cut.

Q is a great mystery, a black art known only to those adepts who have been baptised by Phil Spector. Not! Q translates as bandwidth. You can make a very narrow notch on the frequency spectrum by turning up the Q or a wider gentle slope by turning it down. If you find an EQ with the bandwidth labelled as Q then high Q means narrow band and low Q means wide band, it's really that simple. If you're not that confident about adjusting the bandwidth then just set it about half way and use the freq and level controls--you won't go far wrong.

The level control should start off centred and is capable of adding or removing up to 15dB at the centre frequency, tapering off each side according to the Q setting. A good trick I learned is to turn up the level to +15dB and swing the frequency control across the band, as soon as you hit the problem frequency it will stick out like a sore thumb. You can then return the level to zero and continue down to make a gentle cut.

I go to the trouble of explaining Parametric EQ because it is the magic bullet that can kill a mains hum and you'd never know it was there. You couldn't do it with a graphic EQ. More on that later.

The greatest problem you can get on recordings, the most annoying, and the easiest to remove is a constant drone such as mains hum.

A constant drone noise may not be a mains hum but all you have to do is identify the frequencies and then you can filter them out.

Highlight a bit of the file that is supposed to be silent and pull up the Spectrum Analyser. The only things here that will show up on the Analyser are random noise and the hum or buzz.

The Sound Forge spectrum analyser presents itself as a graph and it can be a useful tool if you want to identify a problem sound. There just have to be plugins for all the other programmes too--seek and you will find.

The base of the graph is Frequency and the side bar is Level in dB. The line across the graph shows the level of various frequencies. The line is generally pretty straight, slightly canted to the right and tails off at the top frequencies. If there is an obvious hump or spike in the line you have a problem that is easy to find. Zoom in on the spike. You can identify the center frequency in the hump or spike and remember it while you dig up the parametric EQ.

The nature of human hearing is that if you have to raise EQ it's better done with a wide Q but if you have to cut out a problem frequency with EQ it's better done with a narrow Q.

Paragraphic EQ

Paragraphic EQ is my favourite tool for this job, it's a four band parametric EQ with a little graph which shows you what you are about to do to a sound.

There are presets: one of the presets is 60hz mains hum removal. All the presets do is set the controls.

Mains supply in America is alternating current at 60 Hertz, in Europe it's 50 hertz. If the mains induces interference in your recorded signal, then it's at mains frequency and sometimes at the first harmonic of 100 or 120 hertz so if I have a sound with a mains hum the spectrum analyser tells me which side of the Atlantic it was recorded on.

If you've got the Sound Forge paragraphic EQ showing, you can pull down the preset for mains hum removal and you'll see what I describe.

Other parametric EQs will do the job just as well though.

Set the bandwidth to narrowest (high Q). On Sound Forge, this will narrow down to one note, set the centre frequency to 60hz U.S. or 50Hz U.K. and pull down the level control to -25. On the Sound Forge EQ, all of the four bands can have the same settings so you can stack four filters to give 100 decibels of hum removal. Your little graph will have a 100 decibel hole one note wide where the mains hum should be. Now when you consider that there's only 96 decibels to the bottom of a CD that's pretty good filtering. If there is still a droning hum it might be that there is a first harmonic of the mains at 100 or 120. In that case I usually use two filters for each frequency: 50 dB is as good as dead.

If you then spectrum analyse the same bit of file you'll now see a narrow chasm in the graph with a tiny mains hum spike in the bottom of it at about -80dB. The hole is so narrow and so far below the human voice that this filtering will simply clean your sound.

If you've still got hiss or random noise in the silent parts there are a couple of tools you can use to clean them out too. More on that in the Noise Removal section.