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Ten MinuteMaster No116

LimitingIn the ongoing quest for ever-increasing

loudness, is a limiter always thesimplest and most effective solution?Mark Cousins takes it to the limit...

W hen you

consider all the

plug-ins

developed over

the years, it’s

the humble limiter that has arguably

had the most fundamental impact

on the way we listen to,appreciate and produce music.

In short, the limiter has played

a lead role in the so-called

‘Loudness War’, where every last

decibel of a track’s dynamic range

is squeezed to within an inch of

its life, resulting in a waveform

that appears to ‘flatline’ at 0dBFS

and an output level that demands

your attention – whether you like it

or not.

But beneath the deceptively

simple controls, what actually

makes a limiter tick? Can limiting

really achieve the perceived boost

in output level that manufacturers

claim, and, ultimately, is their

impact on audio quality an

acceptable one?

Know your limitsThe limiter is based on the same

technology as a compressor, with

the signal being attenuated inresponse to movements above a

given threshold. However, the

fundamental distinction between

compression and limiting is

defined by the device’s ratio.

In the case of a compressor,

the intention is to preserve some

of the original signal’s integrity. For

example, if the input strays 8dB

over the threshold, the amount of

gain reduction is only -4dB – in

other words, a 2:1 ratio (1dB

reduction for every 2dB rise above

the threshold). A limiter, on the

other hand, has a ratio of infinity:1,

so any movement above the

A conventional brickwall limiter operates with a fixed threshold. Varying the amount of input gain, therefore, achieves differing amounts

of limiting. Increasing the gain too far, though, may begin to generate audible distortion in the signal.

Our perception of loudness is based on average signal levels. While the upper track has a

higher peak level (0dBFS), its lower average level means that it’s perceived as quieter.

threshold results in a corresponding

reduction in gain. In the case of the

previous example, an 8dB move

above the threshold would result in

8dB of gain reduction.

Shaping up

Given its somewhat brutal nature,the limiter was initially seen more

as a preventative measure rather

than a tool for deliberately shaping

a mix in a creative way. For

example, FM radio stations were

originally equipped with limiters to

prevent the programme output

overloading the FM transmitter.

Ideally, the limiter’s gain-

reduction circuitry would rarely be

called into action, but should

excessive peaks occur, the

station’s volume would be

attenuated by the limiter rather

than risk distorting the FM

transmitter. In music production,

the compressor was seen as the

preferred tool for gain control, with

limiters (such as those found on

classic compressors like the Neve

33609 and Urei 1176) used more

to control stray transients or for

extreme special effects, but

certainly not as a solution forprocessing the entire mix.

The sea change in attitude to

limiting came in the mid-90s, when

different approaches to mixing and

mastering began to be explored.

Fundamental to this change was

the concept of loudness – the

difference between the absolute

level (the loudest metered point)

and the audience’s perception of

how ‘loud’ a mix sounds.

Intriguingly, our ears have a

clear bias towards averaged signal

levels (also known as RMS) rather

than short peaks. A track with a

few scattered peaks at 0dBFS will

sound subjectively quieter than a

track that peaks at -2dB but has

an average RMS level of -6dB.

The problem in the early 90s

was that the newly digitized

recording process was very good

at preserving absolute levels, but

less effective at masking stray

transient peaks than distortion or

tape saturation – in other words,

digital recordings and theirassociated CD masters sounded

distinctly quieter than their

analogue counterparts. By running

the mix through a limiter, though,

peaks could be ironed out,

52 November 2007  MusicTechMAGAZINE   www.musictechmag.co.uk 

Input gain -0dB

Peak limited

Input gain +3dB Input gain +8.25dB

Threshold at 0dBFS

Master without limiting

 Master with limiting

Peak level (0dBFS)

Average level (-12dB)

Peak level (-4dBFS)

Average level (-8dB)

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Ten Minute MasterNo116 Limiting

achieving a more satisfactory

overall loudness. However, this

kind of power was addictive,

resulting in engineers pushing the

limiter increasingly hard to

produce output levels that were

louder than the competition.

Hitting the wallTo achieve these extreme average

levels, a new breed of limiter was

developed – the digital brickwall

limiter. Riding the levels so close

to 0dBFS means that the limiter

needs a fast-acting response

time plus super-precise amounts

of gain reduction. Even the

quickest movements above the

threshold need to be matched by

an instantaneous, corresponding

amount of gain control.

For example, although ‘soft’

analogue limiters have a fast

attack (in the region of a few

nanoseconds) it isn’t uncommon

for fragments of loud transients to

slip through the net. In contrast,

a digital brickwall limiter has a

look-ahead buffer that stores a

few milliseconds of the audio

input ahead of the gain reduction

circuitry. This enables it to

respond quickly and precisely to

movements over the threshold.

On the attack Given the importance of a fast

response time when it comes tobrickwall limiting, you’ll rarely find

any controls to vary the attack

time. However, varying the release

is vital to achieving a result that is

sympathetic to the programme

material. Ideally, the release

should be optimised to be fast

enough to restore the signal level

between peaks, yet not too fast

so as to produce signal distortion

when catching low-frequency

sounds. In some cases, a

programme-dependant Auto

Release control will automatically

configure the release setting, with

the limiter setting the time in

response to the input it is

presented with.

Finally, a limiter’s threshold

value is often fixed (usually just

below 0dBFS), with increasing

amounts of gain reduction – and,

therefore, loudness – defined

solely by how hard you drive the

input level.

 As one of the last processors in

the mastering signal path, some

limiters also incorporate dithering

controls. Technically, dithering isn’t

part of the limiting process, but it’s

useful if you intend to render your

16-bit masters direct from the

limiter’s output, taking advantage

of the additional perceived

resolution dithering offers. (For

more information, see the Ten

Minute Master on dithering in

Issue 39, June 2006.)

 Alternatively, you could use a

dithering plug-in after the limiter, or

use the dithering options in your

DAW’s ‘bounce to disk’ feature.

Side effectsWhat is difficult to assess are the

cumulative negative effects that

heavy peak limiting can bring.

Undoubtedly, the squaring-off of

transients produces small levels of

distortion, although the ear is

relatively forgiving when this

occurs over a short duration.

Pushed even harder, the

aggressive, quick movements in

and out of gain reduction can also

create their own form of distortion,

which is particularly evident in the

low frequencies. Ultimately, what

you’ll find is that different limiters

deal with these issues with varying

degrees of success, which is why

well-respected third-party limiters

tend to see far more professional

use than the majority of those

included with DAWs.

One such plug-in is Waves’

L3-16, which splits the signal into

16 frequency bands before

individually limiting each band and

summing them to form the final

output. Until now, conventional

wisdom has suggested that a

single broadband limiter provides

more than enough gain reduction

for the required amount of volume.

But as musicians yearn for

ever-hotter masters, it has become

apparent that the current

technology is being pushed well

beyond accepted levels of gain

reduction. By focusing on specific

frequency bands rather than the

mix as a whole, the limiter can

cope better with peak energy

without the entirety of the mix

being aggressively attenuated.

The end of the War? Although it’s hard to imagine

contemporary music production

and mastering completely

foregoing the limiter, it is

interesting to see at least some

individuals finally growing weary of

its effect. While heavy limiting and

mp3 file compression might have

made music louder and more

convenient than ever before, we

may well begin to see a return of

older values as the progressive

solution for making your music

stand out from the crowd. MTM

 Tech Terms■ 0dBFS 

Indicates the maximum peak levelavailable in a digital system – in otherwords, a digital byte that has all itscomponent bits set to 1.

■ RMS  An RMS reading is an averaged signallevel rather than the peak reading,providing a figure that has closer parityto our ear’s perception of loudness.

■ Tape saturation Tape saturation is the process ofdeliberately overloading a signal beingrecorded to tape to impart smalllevels of distortion and peak limitingas the signal is recorded.

FURTHER INFO■ For a detailed examination of

the principles and approaches toloudness in mastering, visit:www.musictechmag.co.uk/mtm/ features/mastering-the-quest-for-loudness

■ For expert insight into theLoudness War, visit:www.soundmirror.com/articles.html

■ For more information oncompression in general, see:www.musictechmag.co.uk/ mtm/features/the-ultimate-

guide-to-compression-part-1-compression-essentials

■ For more information on pro-level brickwall limiters, visit:www.waves.comwww.uaudio.com

Waves’ L3-16 splits the input into multiple bands, enabling a greater amount of limiting to be

applied. Gain reduction is applied to problematic frequencies rather than the mix as a whole.

 The humble limiter has had a fundamentalimpact on the way we produce music.

The principal difference between compression and limiting is defined by ratio. A

compressor will use a soft ratio, whereas a limiter uses a hard, infinity:1 ratio.

MusicTechMAGAZINE  November 2007 53www.musictechmag.co.uk 

Compression – 2:1 ratio Limiting – infinity:1 ratio