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

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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

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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

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

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frequency (Hz)

FREQUENCY DOMAIN

Fourier Transform

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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)

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frequency (Hz)

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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]

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frequency (Hz)

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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

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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