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Gain structure, Getting it right

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1 Gain structure, Getting it right
Gordon Moore CTS

2 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

3 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)

4 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.

5 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.

6 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

7 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

8 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…………………..

9 Signal to noise ratio Poor S/N Ratio Line Level Mic Level Noise Floor
Preamp Mixer Signal processors Amps

10 Signal to noise ratio Line Level Good S/N Ratio Mic Level Noise Floor
Preamp Mixer Signal processors Amps

11 Signal to noise ratio Line Level Mic Level Noise Floor Mic Preamp
Mixer Signal processors Amps

12 Two methods to be discussed
Unity gain staging System optimization

13 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

14 Oscilloscopes, Audio analyzers or other stuff requiring $$$$$$$$$$$$
Good news – Neither method requires really expensive equipment. You DO NOT need Oscilloscopes, Audio analyzers or other stuff requiring $$$$$$$$$$$$

15 IT DEPENDS Why two methods?
The correct answer in all questions audio is IT DEPENDS

16 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

17 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.

18 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

19 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

20 Unity Gain Method Requires signal generator – tone and pink noise
Meter to measure output


22 Another option
Or Google “Tone generator free”

23 Insert Known signal into input at TOP of chain
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)

24 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”.

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

26 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.

27 Demonstration -

28 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.

29 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.

30 System optimization Two ways to do this – one requires expensive gear, the other doesn’t Lets go cheap first You need

31 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.

32 Piezo Speaker




36 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.

37 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.

38 Running up gain until clipping

39 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 ………….

40 Demonstration

41 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.

42 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

43 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.

44 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

45 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

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

47 Benefit 1 – Far side v near side in conferencing
– relative levels between sources will be even Far side v near side in conferencing Dial tones v incoming phone level Multi-media v voices

48 Benefit 2 Processing functions will work at their best
Compressors, limiters, automixers, equalizers, etc

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

50 Summary Maximum modulation with good headroom will give best signal to noise ratio. Set ALL stages (don’t forget any inputs)

51 Questions?

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