Transcript Slide 1

Setting Up DSP Processors
• Gordon Moore CTS
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[email protected]
1-800-821-1121
DSP
 Digital Signal Processors
 Can be any device that modifies a digital signal (video,
audio – anything)
 In this class – we will be talking about Audio
 NOT a “How-to” for specific manufacturers
 Use their training
 Apologies to any Manufacturers whose screens are not
featured – no slight intended.
 The CONCEPTS are the core of this course - no brands
endorsed or rejected.
What to expect
 Signal pathway organization
 Setting the array of “modules” or functions available
Why?
 Most DSP processors are barely used to their best capability.
 Units returned for service usually have just the simplest functions
enabled while many enhancements are not touched.
Most commonly unused or forgotten
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Compressors
Limiters
Input filtering
NOM bus
Delays
Most commonly set up functions
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Input Gain (But often badly set)
Routing (inputs to outputs)
Equalization for outputs
Controls interfaces
DSP organization
 Flexible Architecture
 Fixed Architecture
 Dedicated function (one capability only)
 Multi-function with fixed pathway
Hybrid Architecture – some fixed, some flexibility in routing
Flexible Architecture
All functions can be configured in a “drag-n-drop”
environment. Audio functions can be placed
almost anywhere along the signal chain in any
order.
Characterized by a drag and drop GUI (Grpahical
User Interface) and/or “fuel gauge”.
Sample
Advantages
Complete flexibility – you can do some amazing things within a single
box and develop very complex signal paths. Excellent choice for
systems where complexity and/or multiple applications may come
into play.
Airports
Large scale paging systems
Complex communications systems
Very little you cannot accomplish.
Disadvantages
 Flexible Architecture may be more costly on a per channel basis
 Requires more DSP power – memory register stacks must be
allocated for any eventuality – code space cannot be optimized.
Fixed Architecture
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Dedicated function
 Does one type of function
 Compression/limiting
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 Equalization
 Or
 Signal Routing
Advantage
Fixed architecture is simple to set up and operate – may not
even require a computer
Less cost for the box – may not be as cost effective as a
combined DSP capability
Sets up much like analog counterpart
Excellent choice for existing system upgrade
Disadvantage
 Very limited in scope of function
 May not be very scalable
Fixed Architecture
Multi-function
Has multiple functions in a fixed pathway
Generally fairly cost effective
Limited in terms of routing and or set up choices.
Advantage – predictable known good pathway
No gauge
Disadvantage – few if any routing choices
Hybrid Architecture
 Combination of routing choices plus fixed multiple function signal
pathway
 Advantages –
 Allows flexibility as far a signal routing goes – what inputs show up at what
outputs
 Optimizes DSP processing power – memory stacks and registers can be
more tightly packed
 Disadvantages
 May not fulfill all needs in a system
Hybrid Architecture
Enough Boring stuff – Let’s set up some functions
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Input Gain
Filters – Input and/or Output
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Equalization
Feedback suppression
Crossovers
Noise reduction
Dynamics
 Compressors
 Limiters
 Gates
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Routing
Dynamics
 Those functions affecting gain structure and levels
INPUT GAIN
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Most important setting – GET THIS RIGHT!
Always set up – but not necessarily well set up
Microphones
 Handheld Vocals = 35dB minimum
 Handheld Presentation = 45dB
 Gooseneck desk = 45dB
 Boundary mic = 55dB
 Any further away = 60db+
 Ceiling – as hot as you can get it
INPUT GAIN
 Multimedia
 Unbalanced? Consumer = +10
 Balanced? Professional = 0 to -8
Signal to noise ratio
Poor S/N
Ratio
Line
Level
Mic
Level
Noise Floor
Mic Preamp Mixer
Signal processors
Amps
Signal to noise ratio
Line
Level
Good
S/N
Ratio
Mic
Level
Noise Floor
Mic Preamp Mixer
Signal processors
Amps
Signal to noise ratio
Line
Level
Mic
Level
Noise Floor
Mic Preamp Mixer
Signal processors
Amps
THRESHOLDS
 The level at which the desired function becomes active
 Generally speaking a lower threshold level means it will activate
earlier.
 Recommended starting threshold for most line level (post
preamp) functions = 0dBu
AGC and/or Levellers
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Automatic Gain Control
RAISES gain if signal too low
Compresses if signal too high
BE VERY CAREFUL with these
 Can run a room into feedback if used on amplified inputs
 Primary application – to capture weak signals for recording or
transmission
 Start with threshold set at 0dB – keep gain centered at line level
Ambient Level Control
Uses a reference microphone to measure room noise level the
automatically adjusts system for noisier environment
Reference microphone may be dedicated microphone – only purpose is
reference, or may be designated microphone – used in system but
designated to be the reference signal
Read the manual
COMPRESSORS
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Control dynamics - loudest to softest
Useful for keeping level under control
Meek versus motivational speaker
Rarely set up
Compressor settings - Ratio
 Ratio – The amount of actual level increase above
threshold that will yield ONE decibel in actual gain
change after the compressor.
 Example – 3:1 ratio
 For every 3DB the gain increases above threshold, the final
level will change only one dB
 SO, if level jumps 9DB, the final level will jump only 3dB
 FM is broadcast at typical 10:1 ratio
Compressor settings – Threshold, Attack, Release
 Threshold - Level at which compressor begins to
engage and affect level.
 Attack – time in milliseconds the compressor begins to
make changes after level exceeds threshold
 Release – time in milliseconds the compressor lets go
after level settles below threshold.
 Makeup or post compressor gain
 Compensation in level to make up for compressor reduction in signal.
Suggested setting for compressors
 Speech systems (conference rooms, boardrooms, etc)
Ratio = 3:1
Attack = 10-20ms
Release = 200-500ms
Threshold = 0
If initial input gain was set to result in 0dB level, then it would take a
60dB increase at the mic to hit the +20dB limit of input (clipping)
MUSIC or Multimedia – try increasing ratio to 6:1
BE CAREFUL – too much = bad
LIMITERS
 Basically, a compressor with an infinite ratio
 Absolute ceiling to maximum level
 Protects downstream gear by preventing severe clipping and
overdriving amps and speakers
 Many amplifiers have built in limiters to protect themselves.
 ALWAYS set limiter threshold above threshold of compressor
 Otherwise, compressor will never engage
 Good for spikes like dropped microphones, cymbals, plosives
(P,D,T)
Suggested Limiter settings
 Threshold –
15db higher than compressor = +15dB if 0dB for compressor
 Attack – faster than compressor = 2ms or quicker
 Release – 200ms or less
Some dynamics filters can be frequency
specific
 Compressor with low pass filter good for controlling proximity effect
while allowing high frequencies to pass unaffected.
EXPANDERS
 Increases gain if signal very low such as weak talker
 BE VERY CAREFUL HERE – Expanders, in an
amplified environment, can push system into ear
bleeding feedback.
 Primarily intended for recording and or transmission.
GATES
 Gates activate a channel, allowing it to pass, once the
level passes above the threshold.
 Found in some automixers
 Useful for noise control (noisy multimedia source, for
example)
 Originated in music
Attack , Release, Threshold
 Attack – try relatively fast settings, 1ms to begin
 Release – start at 50ms
 Threshold – depends on place in chain
 If after input gain – 0dB is a good starting place
 Lower if not getting a reliable start
Noise Gate
Spectral
 Affects the frequency response of the system
EQUALIZATION (Inputs)
 Equalization is one of the most commonly used functions
 Input EQ is generally for tonality control – adjusting the tonal
content so each input sounds similar.
EQUALIZATION (Outputs)
 Generally used for speaker compensation
 Adjusting for “quirks” or characteristics in the loudspeaker response.
 You cannot EQ a “room”
Equalization patterns
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Pass – Low, High or Band
Shelving – Low or High
Parametric - Notch
Graphic
“Q”
 No, not James Bond’s gadget guy
 The ratio of filter width to depth at 3dB roll off points
Low Q – Wide band width
High Q – Narrow Band width
Filter Slope or “Order”
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Rate of attenuation on filter - Shown in terms of dB/octave
 Octave – doubling of frequency
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First order = 6dB per octave
Second Order = 12dB per Octave
Third Order = 18dB/Octave
Fourth Order – 24dB/Octave
Each order equals another 6db of roll off.
 That means 4 times factor in power level
 If 6 db down from 100watts = 25 watts.
First Order – 6dB per octave (High pass)
Same filter – Second order
Eighth Order- 48 dB/octave
Parametric
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Fully configurable
Boost or Cut adjustable
Center Frequency selectable
Q selectable
Parametric
Band Pass
Low Pass
High Pass
Shelving – Boost or cut, High or Low
Filters - Graphic EQ
CROSSOVERS
 Used for bi-amplified and tri-amplified systems
 Low frequency content sent to bass amplifiers
 Mid and high range sent to appropriate amplifiers
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Separate amplifiers involved
Large scale concert systems
High order - 4th to 8th order filters
Bass – 250Hz or lower
For tri-amplified – start at 4K for high pass
FEEDBACK SUPPRESSION
 Should be set up LAST after Equalization
 Smooth response FIRST, then take care of
Feedback nodes
 A time domain issue coupled with Frequency
domain
 Notch Filters – very tight
 But too many can badly affect content
Set up tip – One mic at a time
 1. Before final EQ – run feedback “eliminator” first
 2. Make note of first three feedback freqs
 3. Construct three very tight notch filters at INPUT on those
frequencies
 4. Reset the feedback filters
 5. engage feedback filters again after equalization and system
is at operational levels.
NOISE REDUCTION
 Popular new algorithms that “sample” the noise floor
 Noise floor – acoustical and electronic noise that is NOT wanted in
system
 Air Conditioning
 Fan noise
 Laptops near boundary mics
 computers
 Projectors
 Electronic noise – noisy sound cards in computers etc.
Setting Noise Reduction Filters
Canceller Depth – depends on the amount of noise
Quiet conference room with little to no noise may not need this.
Computer and projector fan noise – try starting at 9dB.
Heavy room noise – large attendance training room or bad air
conditioning rumble, try 12dB
 Remember, these filters remove spectral content – none are perfect
and they will all affect your room response.
 DON”T GET CARRIED AWAY!
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Signal Generators
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White noise – equal energy per frequency
Pink Noise – equal energy per octave
Tones
Primary use – test and measurement
 Use pink noise to set up your levels in the room
 If your gain structure is correct – this will allow amplifiers settings tobe accurately set up
 Use test tones for gain staging and/or speaker alignment
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Secondary Use
 Noise masking – covering conversation or background ambient noise
 Alarms
ROUTING
 Matrix –
 Rarely overlooked
 Determines which inputs go to which outputs
 (technically speaking “gozindas to gozoudas”)
 Some traps here
 Watch for
 Feedback loops
 NOM bus assignment – VERY IMPORTANT
Selecting NOM bus and action
 NOM= Number of Open Microphones
 Determines interactions of microphones in automixing
 Failing to select correct NOM interaction can affect echo
cancellation, and gain before feedback
 Choices may include
 Chairman Over-ride
 Auto mix (or Normal)
 Background (or Ducking)
Feedback loops
 Common error in setup – requires careful
documentation and double checking – especially
when dealing with mix minus conferencing
systems.
Sending an input BACK to itself.
Delays
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Often neglected
Primarily used for time alignment
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Secondary use for spatial referencing (Haas effect)
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Loudspeaker stacks in large venues
Input alignment
Loudspeaker alignment
Localization of sound based on first heard
Set up tip –
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Try setting for 10ms or 10ft or 3.3m FARTHER than distance would indicate.
Example – Speaker is 68 ft from stage – instead of 60.5ms, try 70.5.
Some architectural Do’s and Don’ts
 In Flexible Architecture – General rules – always exceptions
 Do NOT put compressors before Equalizers/Filters
 The EQ can take out energy that would falsely trigger compressor – so put
compressor AFTER filtering
 Do NOT put limiters before compressors  Do NOT put limiter thresholds lower than other devices – especially
compressors
 Be careful about thresholds for automixers – (gated designs)
 Too low and room noise will open mics
 Too high and you may lose the first characters
 Shared gain mixers will not have thresholds
Hybrid Architecture Do’s and Don’t
 Don’t create a feedback loop – be careful where you send inputs
 Don’t go overboard on settings
 “Too much of a good thing is wonderful” does NOT apply
here.
Troubleshooting tips
 Ringing or severe echo in audio
 Internal feedback loop
 Signal generators or incoming conferencing too loud
 Poor input gain structure – HOT IT UP!
 HISSssssssssss or noisssssssse in system
 Bad gain structure – inputs too low – Amplifiers too high
 University reasoning – wrong solution to the right problem.
 Amps set to full output
 Inputs set low
 Correct solution – lock up the amplifier controls.
Questions?
 Fill in your reviews please!