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!igital recording

systems running at

24-bit resolution are

designed to capture

truly high-quality

recordings, ensuring

the best possible signal-to-noise

ratio and dynamic range. That

said, signal noise and internal

distortion are still common

frustrations, often leading

anxious home recordists to goon expensive – and sometimes

unnecessary – spending sprees.

In the majority of cases, though,

it’s not the equipment that’s at

fault; rather, it’s the gain structure

of our recordings and mixes that

needs to be reviewed.

Structural integrityCorrect gain structure within

either an analogue or digital

system will ensure that the end

result utilises all of the availabledynamic range, has a minimal

noise floor and doesn’t

introduce unintentional

distortion to the signal. This

may appear to be a very generic

statement that can cover all

‘good recording practices’, but it

is very specific when referring to

gain structure as any distortion

added to the signal path at one

stage cannot be removed at any

subsequent stage.

Every stage in the signal path

should be set so that each

processor (be it hardware or

software) receives the correct

input level and therefore operates

optimally within its own dynamic

range. A good example is that of

a reverb effect being used on an

auxiliary buss channel.

For this example, let’s

assume that we want to add a

small amount of reverb to thesource signal. This can be

achieved in two ways. Firstly,

we can send a small signal from

the channel strip (via the aux

send) to the reverb unit and set

the outputs of the reverb to

unity gain or higher to bring the

signal to the correct level within

the mix. The problem with this

approach is that the signal level

for the reverb is very low in

terms of the input range of the

device, and consequently closeto the noise floor, giving a poor

signal-to-noise ratio. As the

output of the device is very

quiet, it is likely that its level will

have to be boosted beyond

unity gain, raising the noise

floor of the reverb along with

the signal.

The second approach is to

have the aux send from the

channel strip set at (or near)

unity gain, ensuring that the

processor receives a sufficiently

high input signal (watch out for

clipping). The mix level of the

effect is controlled by the return

level. This means that the

reverb processor is operating at

a level within its dynamic rangeand the noise floor is much

further from the signal.

The weakest link The concept behind good gain

structure states that each stage

should be working capably

within its operating range, but

unwanted signal distortion can

still occur at any point in the

signal path. Microphone

preamps, for example, are

particularly prone to internaldistortions, especially those

found on lower-end mixing

consoles. It is feasible for the

microphones themselves to

distort (check the SPL levels for

1% THD on the mic’s spec

sheets), but it is far more likely

to be the mic preamp that is

distorting. In this case, careful

metering can help only as a

guide to the problem, as PFL

(Pre Fade Listen) metering is

generally post the EQ sectionand the channel insert point,

narrowing the problem down

only to these areas. Once this

distortion is within the signal it

cannot be resolved by reducing

the overall fader volume on the

channel – the gain on the mic

preamp has to be set correctly

in the first place.

From real to reelThe most critical time for

ensuring optimal gain structureis during the recording process

itself. When you’re working with

analogue equipment there is

always a certain amount of

headroom available before

distortion becomes apparent –

Eliminating extraneous noise is a primary task in the studio, but we may be inadvertently

generating it ourselves. Grant Bridgeman explains the importance of good gain structure...

Gain structure

"#$ &'$("#&)*"#+ No99

38 March 2007 MusicTech MAGAZINE

Microphone Preamp EQ Volume fader

Irrespective of what each device in your signal chain actually does, it is important to

ensure that the output level of each device is suitable for the following device.

Once a signal has clipped, boosting the signal at a later stage will merely result in a

louder clipped signal. Preventing clipping from occurring in the first place is key.

A clipped signal thathas been boosted. The

unwanted distortion isboosted proportionally.

The same signal afterdigital clipping. The

loss of informationcauses distortion.

Original audio signal

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to the extent that most true

analogue mixers have an

additional amount of headroom

(around 6dB) beyond the final

+12dB VU mark on the meter

before distortion takes place

within the output stage of the

desk itself.

When moving from theanalogue to the digital world (as

is necessary when recording

takes place) you no longer have

the luxury of this headroom;

there is a definite point of no

return, where all the bits

become one and beyond which

the signal is corrupted.

To accommodate this, digital

recorders and soundcards are

configured with a false internal

‘headroom’, so they have to be

driven quite hard from the mixerto actually create digital

distortion. The result is that it

can often be quite difficult to get

a decent signal level into a

soundcard as an analogue desk

may have to be outputting

+12dB more than you wouldexpect before reaching 0dBFS

on the digital meter. The irony is

that to reach 0dBFS, the mixer

could now be running towards

its own internal distortion

threshold – the reasoning being

that minor analogue distortions

are far less of a problem than

any digital distortion.

One workaround is to adjust

the sensitivity settings on theinputs of the soundcard. Instead

of matching the output of the

mixing desk to the input of the

card, it is possible to configure

the card so that it is set up with

-10dBV sensitivity. Feeding this

with the +4dBu outputs from the

desk means that 0dBFS can be

achieved at the digital inputs, at

around +4dB VU on the mixer.

Feel the squeeze

Making the most of the signal-to-noise ratio of a digital input is not

necessarily as straightforward.

With a 16-bit system, for

example, leaving 6dB of

headroom reduces the signal-to-

noise ratio to 90dB. One option

is to use some gentle analoguecompression on the way into the

soundcard to take some of the

edge off the transient peaks andincrease the level of the quieter

signals using make-up gain. It is

while recording quieter signals

that the converters suffer due to

a lack of resolution and

problems with quantisation noise

become apparent. The

advantage to compressing the

signal before the A/D process is

that it is making far better use of

the total number of bits, giving a

better signal integrity once in the

digital domain. The disadvantageis that this compression cannot

be removed from the recorded

signal, and that the make-up

gain will also raise the noise floor

within the analogue signal,

potentially highlighting the

frailties of the preamp stage.

Digital dynamicsOnce within the digital domain,

incorrect gain structure can have

a far more catastrophic effect

than the equivalent problem on

an analogue desk. Overdriving

any virtual channel beyond

0dBFS causes damage to the

audio signal. The advantage to

virtual mixers is the metering,

which means it is very easy to

trace and resolve problems withgain structure. Every channel –

be it auxiliary, buss, group or

main output – has a peak meter

that provides additional visual

information about the audio

signals themselves.

When working with a busymix, the Peak Hold functionality

can be a real benefit as it makes

it possible to search through the

channels to see which have

overstepped over the mark and

resolve the problem.   MTM

MusicTechMAGAZINE March 2007 39

 The most critical time for ensuring optimalgain structure is during recording itself.

 Setting the gain structure for a liveperformance is addressed in thefollowing two sites, the first moretheory-based than the second:www.mmproductions.co.uk/gain1.html

www.prosoundweb.com/install/tech_corner/elwl.php

 Many mixer manufacturers providesome helpful pdf files about setting upgood gain structure using theirmixers. This is Mackie’s link:

www.mackie.com/pdf/CMRefGuide/Tips_Ch4.pdf

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