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
Compressors
Limiters
Input filtering
NOM bus
Delays
Most commonly set up functions
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
Dedicated function
Does one type of function
Compression/limiting
Or
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
Input Gain
Filters – Input and/or Output
Equalization
Feedback suppression
Crossovers
Noise reduction
Dynamics
Compressors
Limiters
Gates
Routing
Dynamics
Those functions affecting gain structure and levels
INPUT GAIN
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
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
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
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”
Rate of attenuation on filter - Shown in terms of dB/octave
Octave – doubling of frequency
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
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
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!
Signal Generators
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
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
Often neglected
Primarily used for time alignment
Secondary use for spatial referencing (Haas effect)
Loudspeaker stacks in large venues
Input alignment
Loudspeaker alignment
Localization of sound based on first heard
Set up tip –
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!