Gain structure, Getting it right

Gain structure, Getting it right

• Gordon Moore CTS• [email protected]• 1-800-821-1121

Gain

The change in level imposed on an electrical signal

Generally measured in decibels 3 dB is a doubling of power You need 6 dB to make up for moving twice the

distance from a source – a quadrupling of power

Gain

10 dB changes the level by a magnitude of ten (10X or 1/10th )

20dB changes the level by hundred (100X or 1/100th)

60 db raises the level a thousand (1,000 or 1/1,000th)

Why is Gain needed?

Microphones generate VERY low voltage levels – measured in millivolts.

This very low signal needs to be amplified to around a volt to be strong enough for long cable runs.

Why is Gain needed?

Very low levels are susceptible to interference from electro-magnetic and radio frequency sources. Amplification makes these signals much more resistant to these sources.

Processing works much better and quieter at higher voltage levels.

Where is the biggest amplifier?

The microphone preamp has the greatest effect on your relative signal levels and S/N ratios. It is, in general, the most amplification you will apply in most systems.

Get the preamplifier right and everything else falls into line

Gain is KING

Proper gain staging makes or breaks performance at all levels. It can:

Affect signal to noise ratio in the entire system Affect the proper operation of signal processors

DSP Conferencing systems Audio processors

Affect wireless microphone performance

What is gain staging

The optimization of the signal level so all components in the signal chain are operating within their optimal performance range without detriment to signal integrity.

Or

Setting the levels right…………………..

Signal to noise ratio

Mic Preamp Mixer Signal processors Amps

LineLevel

Mic Level

Noise Floor

Poor S/NRatio



LineLevel

Mic Level

Noise Floor

GoodS/NRatio



LineLevel

Mic Level

Noise Floor

Two methods to be discussed Unity gain staging System optimization

Common attributes Both, when properly engaged, will provide improved signal to noise

ratio. Both require some minor specialized equipment

Signal generator Tone and/or pink noise

Signal analyzer Simple as Multimeter or even cheap piezo tweeter

Misc cables with all sorts of connector combinations – great soldering practice

Good news

– Neither method requires really expensive equipment. You DO NOT need

Oscilloscopes, Audio analyzers or other stuff requiring $$$$$$$$$$$$

Why two methods?

The correct answer in all questions audio is

IT DEPENDS

Unity Gain Method

Involves setting all line level outputs to one common level Most often 0dBu (.775Volts) or +4dBu (1.94V) Use – permanent installed sound systems such as courtrooms,

conference centers, boardrooms, paging etc – primarily voice reinforcement systems

Unity Gain Method Advantage – maintenance is simple –

pull out device A, replace with device B, set output level to 0dBu, voice test, Go home, drink cold beverage

Unity gain – 0dBu in to 0dBu out.

Unity Gain Method

Why 0dBu or +4dBu?

Most systems need “headroom” – generally 20dB will handle anything

Most professional gear can handle up to +20dBu (77.5V) signals (professional theatre or touring gear up to +28dBu)

Capping “normal” level at 0 allows up to 20dB of extra loud signal before distortion or clipping

Clipping

Driving an amplifier circuit to the point where signal levels cannot go any higher.

OVERDRIVEN signals create distortion and harmonics Can overheat components and destroy speakers. Guitar amplifiers/speakers that must handle distorted

guitars require much heftier components than most home systems can handle.

Example

Unity Gain Method

Requires signal generator – tone and pink noise Meter to measure output

http://www.nti-audio.com/Home/Products/DiscontinuedProducts/tabid/97/Default.aspx

Another option

http://www.world-voices.com/software/nchtone.html

Or Google “Tone generator free”

Unity Gain Method Insert Known signal into input at TOP of chain

Top = first audio insertion point – microphone input on mixer

Known signal can be any level – but you must know what that level is.

Set input trim to set 0dBu level (if input signal is -20dBu, set input trim at +20 to get 0)

Unity Gain Method

Set output level to measure 0dBu (or .775V with multimeter set on AC)

Go to next device – set input and output gain so level at output is also 0dBu

Set amplifiers last – if all else is set right and amps are properly chosen, typical amp level will be “low”.

Unity Gain Method

Until you get to amplifiers, there is NO need to have amplifiers ON.

When setting amplifier, use typical test signal and set levels in room using SPL meter (or even by ear, not documentable however)

Equalize then reset equalizer output to 0dBu.

Unity Gain Method

Basic system gain is correctly set. Any changes to any signal processors, (compressors, limiters, etc)

will require a reset of that piece of gear back to unity gain.

Demonstration -

System optimization

Setting every component so that clipping occurs at exactly the same time in all devices. If “clip” indicator shows nothing on main console, you know everything else in your system is not going to “clip” either. All components can be driven to their maximum without danger.

System Optimization

WHEN to use? Live sound systems and theatrical installations where the sound operators may need to drive the system to “Max”. This optimizes the signal to noise ratio and permits full signal range. Requires complete re-calibration if any components are replaced.

System optimization

Two ways to do this – one requires expensive gear, the other doesn’t

Lets go cheap first

You need

CHEAP STUFF

1 – very cheap piezo tweeter – generally available for less than $10.

2 – one signal generator that can generate an A on the scale 440Hz (or something near that level)

If you have a laptop, you can get free on-line

If not, music stores have generators for less than $50.

Piezo Speaker

System optimization

Step one – connect speaker to output of first device (generally the mixer)

Step two – set output level at 0, faders at 0, subs at 0. Step three – insert tone into input of mixer. Increase

gain at input until speaker makes squealing sound (you are clipping) – back input gain down until silent.

System optimization

At this point, leave all controls at mixer alone – do not change Repeat test at each input and output of all line level devices

downstream. When you are finished, all devices will clip at the same time.

Running up gain until clipping

System optimization

Expensive method – replace free laptop software with fancy signal generator and speaker with O-Scope.

Results will be exactly the same. If you lose the speaker or it gets stolen – oh

well. If you lose the O-scope ………….

Demonstration

Setting mics

Based on application and varies according to mic. Set input trim to give 0dBu nominal level

Can shouting distort the audio (clipping!)? If Yes, turn it down a bit until answer is no.

Setting Multi-Media

Use known reference – as in KNOWN to YOU Ideally a measured sample – so you can document the setup

and result Does not have to be boring test signals

Setting Multi-Media

I use Diana Krall, “Live in Paris” Reasons –

Wide dynamic range (soft to loud) Wide frequency range (bass to cymbals) Available in DVD and CD format

I also use boring test CD as well. ZZZZZZZZZZZZZZZZZZ Pink noise drives out unwanted visitors.

Setting wireless mics

Transmitter gain affects FM modulation The modulation of the signal affect S/N ratio

Poor modulation makes apparent range lower Getting transmitter gain “hot” improves FM performance

Setting Wireless Microphones Again – avoid distortion but don’t set too low

Depends on situation – does scene require screaming? Or simply talking?

Then set output of receiver according to input in the mixer you are using. Line level preferred – better resistance in RF and EMI

So, why is this important again? If gain is consistently set at optimal levels you get three benefits

Benefit 1 –

– relative levels between sources will be even

Far side v near side in conferencing Dial tones v incoming phone level Multi-media v voices

Benefit 2 Processing functions will work at their best

Compressors, limiters, automixers, equalizers, etc

Benefit 3 – Signal to noise ratio will be at best possible result

Summary

Maximum modulation with good headroom will give best signal to noise ratio.

Set ALL stages (don’t forget any inputs)

Questions?

Documents

Gain structure, Getting it right