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FX Basics
Dynamics Effects
STOMPBOX DESIGN WORKSHOP
Esteban Maestre
CCRMA - Stanford University July 2012
FX Basics: Dynamics Effects
Dynamics effects were the earliest effects to be introduced by guitarists. The simple idea behind dynamics effects is to amplify or attenuate the amplitude of the electrical signal coming out from the pickup or microphone. They first appeared in the 1940s as simple on/off switch boards, evolving to volume pedals in the 1950s. Ex: volume pedal, boost, tremolo, noise gate, dynamic range compressor
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Gain control
0.05 0.055 0.06 0.065
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0.05 0.055 0.06 0.065
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FX Basics: Dynamics Effects
X Input signal Output signal
Gain
Gain > 1
Achieved by means of a simple multiplication.
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Volume Boost FX Basics:
Dynamics Effects
X Input signal Output signal
Gain ON/OFF
Generally used for boosting volume during solos and/or preventing signal loss in long effect chains. Ex: when switching from rhythm guitar to lead guitar, a guitarist may use a clean boost to increase the volume of his or her solo.
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Volume Boost (ii) FX Basics:
Dynamics Effects
0.68 0.7 0.72 0.74
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0.68 0.7 0.72 0.74
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OF
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gain
X
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
00_stomp_dynamics_1.pd
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Tremolo FX Basics:
Dynamics Effects
X
Output signal
Produces a slight, rapid oscillation of the signal amplitude; not to be confused with tremolo bar (pitch oscillation).
Based on the use of a Low Frequency Oscillator (LFO):
~
FREQUENCY (f0) AMPLITUDE
Oscillator
LFO
1/f0
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Tremolo (ii) FX Basics:
Dynamics Effects
X Input signal Output signal
ON/OFF
Typically, two controls are offered: RATE: Sets the frequency of the volume oscillation DEPTH: Sets the amplitude of the volume oscillation
LFO
Amplitude Frequency
+
1
RATE DEPTH
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Tremolo (iii) FX Basics:
Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
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input
0 0.5 1 1.5 24
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seconds
LF
O f
requency
0 0.5 1 1.5 20.1
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secondsLF
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0 0.5 1 1.5 2-1
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outp
ut
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input
X
Inputsignal
Outputsignal
ON/OFF
AmplitudeFrequency
+
1
RATE DEPTH
LFO
01_stomp_dynamics_2.pd
Noise gate FX Basics:
Dynamics Effects
Attenuates signal when its level falls below a given threshold. Both the attenuation and threshold are usually available as user controls (resp. RANGE and LEVEL).
Ex: avoid unwanted noise floor when there is no signal coming from the instrument
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
X
Input Signal
Output Signal
LEVEL
Level Detector
RANGE
< ?
Noise gate (ii) FX Basics:
Dynamics Effects
LEVEL DETECTOR (Envelope Follower):
Often implemented as Root Mean Square (RMS) meter. RMS amplitude provides a measure of effective (short-time averaged) signal intensity.
‘Averaging time’ sets the responsiveness of the meter.
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Input Signal
Output Signal
AVG. TIME
AVG ^2 SQRT
RMS ENVELOPE FOLLOWER
FX Basics: Dynamics Effects
TIME AVERAGE
Acts as a smoothing function:
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Smoothing Function Output
Signal
y[n] x[n]
Input Signal
n
x
n
y
n n
Current sample
Current sample
Average of current and previous input samples
FX Basics: Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
SMOOTHING WITH RECURSIVE EQUATION:
Find coefficients a and b so that equation
y[n] = b0·x[n] + b1·x[n-1] + … + bN·x[n-N]
- a1·y[n-1] - … - aN·y[n-N]
results into a smoothing function.
…digital implementation of a Low Pass (LP) filter.
current and previous input samples
previous output samples
TIME AVERAGE:
y [n] = ( 1/M ) · ( x[n] + x[n-1] + … + x[n-M+1] + x[n-M] )
Obtain M from ‘averaging time’ : M = avgTime · fs
FX Basics: Dynamics Effects
RMS Envelope…
With TIME AVERAGE:
With Smoothing Low-Pass Filter (RECURSIVE):
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
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Averaging using 441 and 882 previous samples respectively (M=441; M=882)
Both filters only using 1 previous sample (N=1) !!
FX Basics: Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
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TIME DOMAIN
1/1200 1/500
0 0.5 1 1.5 2 2.5
x 104
0
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magnitude
frequency (Hz)
FREQUENCY DOMAIN
Fourier Transform
500 1200
fs/2 (Nyquist)
FX Basics: Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
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x(t) = 1.0·sin(2·π·500·t) + 0.4·sin(2·π·5000·t)
0 0.5 1 1.5 2 2.5
x 104
0
0.5
1
1.5
magnitude
frequency (Hz)
500 5000
FX Basics: Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
f
Magnitude
Slower Components
fs/2 (Nyquist)
Quicker Components
Low Frequencies
High Frequencies
FX Basics: Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
f
Gain
Low Frequencies
High Frequencies
1 One can design a Low-Pass filter so that components above a certain ‘characteristic’ frequency (fc) get attenuated…
LP
FX Basics: Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
y[n] = 0.0344·x[n] + 0.0344·x[n-1]
+ 0.9312·y[n-1]
0 0.5 1 1.5 2 2.5
x 104
0
0.2
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magnitude
frequency (Hz)
500 5000
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How to ‘design’ the coefficients? (e.g. how many coefficients? which values?)
Basics of DIGITAL FILTERS (to come…)
Noise gate (iii) FX Basics:
Dynamics Effects
RMS Envelope Follower
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
x[n] y[n] LP filter ^2 SQRT
RMS ENVELOPE FOLLOWER
0.3 0.31 0.32 0.33 0.34 0.35
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0.1
0.2
0.3
0.4
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seconds
Rapid oscillation (quicker components) have been attenuated
Noise gate (iv) FX Basics:
Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Input
TH
Output
TH
Gain 1
RANGE ON ON
ON
Chattering
Abrupt ON-OFF / OFF-ON transitions
Example of basic operation
Noise gate (v) FX Basics:
Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Input
THON-OFF
THOFF-ON
Avoids chattering
Gain
Output
1
RANGE
Release
Attack
Smoother transitions
02_stomp_dynamics_3.pd
Noise gates often include HYSTERESIS and ATTACK/RELEASE times
Dynamic Range Compressor
FX Basics: Dynamics Effects
Attenuates the signal when its level its higher than a certain threshold. Both the amount of attenuation and the threshold are the most typical user controls (resp. COMPRESSION/RATIO and LEVEL).
Ex: reduce intensity differences, soften the amplitude of very loud attacks
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
X
LEVEL
Level Detector
COMPRESSION
>?
Input Level
Output Level
LEVEL
COMPRESSION
Bypass
Hard Limiter
Dynamic Range Compressor (ii)
FX Basics: Dynamics Effects
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
X
LEVEL
Level Detector
COMPRESSION
FEED-FORWARD basic structure
Gain Computer
X
LEVEL
Level Detector
COMPRESSION
FEED-BACK basic structure Gain
Computer
Dynamic Range Compressor (iii)
FX Basics: Dynamics Effects
Example of basic operation
Stompbox Design Workshop July 2012- CCRMA, Stanford University
Input Level
Output Level
LEVEL
OFF: Gain = 1
ON: Gain < 1
1:1
2:1
4:1
Inf:1
Gain = 1
Gain < 1
ON OFF OFF OFF ON
Input
Output
Gain
Dynamic Range Compressor (iv)
FX Basics: Dynamics Effects
Further available controls, depending on application:
• ATTACK / RELEASE TIMES
• HARD vs SOFT KNEE
• MAKE-UP GAIN
Stompbox Design Workshop July 2012 - CCRMA, Stanford University
Input Level
Output Level
LEVEL
SOFT KNEE
MAKE-UP GAIN
HARD KNEE
Make-up Gain
Reduced Gain
ON OFF OFF OFF ON
Input
Output
Gain
03_stomp_dynamics_4.pd