Effects

1
Effects
2
Effects in Music

• All music that is recorded or amplified relies on
  effects to enhance certain characteristics of the
  sound.
• Guitarists typically have an array of
  foot-activated effect units in front of them.
• These effects are the result of filter
  combinations.

3
Combining Filters

• There are two ways to combine filters
• Parallel
• Cascade

4
Combining Filters
Parallel the signal is split and put through
one or more filters.
BPF
Output signal
Input signal
BPF
RESULT the respective frequency responses are
added in the output.
Note A bandpass filter could be constructed by
using a highpass filter and a lowpass filter in
parallel.
5
Combining Filters
Cascade the signal is sent through a succession
of filters.
Input signal
BPF
BPF
Output signal
RESULT the respective frequency responses are
multiplied in the output.
6
Delays

• The most basic type of effect is simple delay.
• Delay gt 50 ms audible echoes
• Delay lt 10 ms coloration, filtering
• Between, enhancement, increase in volume

7
Simple Delay
The output is combined with a delay.
Output
Input
X
0.25 sec.
Notes every 0.4 seconds
0.8
8
Multitap Delay
This is an analogy to plumbing
Picture water flowing through a pipe
We can take water from different points along the
pipe by inserting taps along its length.
9
Multitap Delay
The output is combined with a succession of
delays.
Output
Notes every 0.75 seconds
Input
0.85
0.15 sec.
0.7
0.45 sec.
0.85
0.6 sec.
0.6
0.9 sec.
10
Feedback Delay
The output of the delay is combined with the
input.
Output
Input
X
Notes every 1 second
0.25 sec.
2 seconds
11
Delays

• Shorter delays lead to two filter types
• Comb filter
• Allpass filter

12
Comb Filter
Current sample combined with a delayed sample.
Produces a series of spectral peaks and nulls,
resembling a comb.
The position of the peaks and nulls correspond to
the sampling rate (SR) divided by the delay (D).
13
Feedforward Comb Filter (aka inverted comb
filter)
From 0 Hz to the Nyquist frequency
Produces peaks at nSR/D Hz

Produces nulls at (2n - 1)SR/2D Hz
Dry
Example
SR 44100 Hz D 21 samples SR/D 2100 Hz
Delay 15 ms
The longer the delay, the greater the number of
peaks and nulls
14
Feedback Comb Filter
From 0 Hz to the Nyquist frequency
Produces peaks at nSR/D Hz

Produces nulls at (2n - 1)SR/2D Hz
The shape of the spectrum is inverted, producing
a ringing at SR/D Hz the ringing depends on the
coefficient b.
Dry
Example
SR 44100 Hz D 21 samples SR/D 2100 Hz
Delay of 15 ms
The longer the delay, the greater the number of
peaks and nulls
15
Comb Filtering
Comb filtering can appear unintentionally,
leading to undesirable coloration, if reflections
are accidentally mixed with a signal.
This is an averaged spectrum of the audio sample.
The white line shows the portion without comb
filtering, and the red line shows the portion
with the comb filtering.
16
Allpass Filter
Combination of feedforward and feedback combs.
Steady state passes all frequencies equally, but
alters phases.
This description might lead to the
misunderstanding that an allpass filter has no
audible effect.
The non-uniform phase response of an allpass
filter means that its transient response will
alter the phases of the sounds attack spectrum.
Since the attack is the definitive portion of a
sound, an allpass filter can audibly color its
input signal there is a ringing at the
frequency 1/(delay time), where the delay time is
D/SR seconds.
In contrast to the delays discussed earlier,
allpass filters produce a delay that is
frequency-dependent.
Frequency-dependent delay is sometimes called
dispersion.
Dry
Allpass filtered
17
Flange
The signal is combined with a feedback comb
filter having a short delay time 1 to 10 ms.
The delay time oscillates.
Result is a comb filter with oscillating teeth,
expanding and compressing.
Characterized by a pronounced whooshing.
18
Chorus
Also combines input with an oscillating delay,
like a flanger, but
It is typically not a feedback delay.
The delay time is longer, typically 20-30 ms.
Due to the longer delay time, the result is not
comb filtering, but rather a simulation of human
singers, who can never sound at exactly the same
time, and thus have a group sound.
Chorus units often use a multitap delay, with
control over the time and gain of each delay.
19
Phase Shift
Another delay effect with modulating delay times,
producing holes in the spectrum.
A generalized example of flanging.
Phase shifters use a series (cascade) of allpass
filters.
The result is gentler than flanging, without the
whoosh.
Dry signal
Phase shifted
20
Reverberation
Simulates the natural propagation of sound in a
closed space, in which sound waves reflect off
the spaces surfaces.
Three stages
Direct sound from source to listener gives the
impression of sources location.
1)
Early reflections first reflections to reach
listener from surfaces give the impression of
room size.
2)
Diffuse reverberation later and more frequent
second (and higher)-order reflections, give the
impression of the rooms sound.
3)
Listener
(REMEMBER everything is a filter. Even a room!)
21
Reverberation
Acousticians measure a rooms impulse response by
creating a short sound burst (hand clap, flick of
a lighter, click from a toy) and measure the
intensity and timing of the reflections.
22
Reverberation
As reverberation is an essential component of
natural sounds, it is considered an essential
effect in recorded and amplified sounds.
Analog recordings had three main methods of
simulating reverberation. There was not a great
deal of control over the reverb parameters, and
each had its own distinctive, artificial color.
However, many digital reverberators now have
settings to imitate these vintage effects. (They
simulate the early simulations!)
23
Analog Reverberation
Reverb chamber
Audio is played into a highly reflective room
the signal from this room is mixed with the
original signal.
24
Analog Reverberation
Spring reverb
Mechanical fluctuations are transduced into
electrical fluctuations
Electrical fluctuations are transduced into
mechanical fluctuations
Mechanical fluctuations are reflected back and
forth along the spring
25
Analog Reverberation
Plate reverb
Transducer converts signal into mechanical
vibrations that spread onto a metal plate
Transducers on the plate convert the vibrations
into a signal that is mixed with the original
26
Digital Reverberation
First computerized reverberation models were
introduced in the early 1960s by Manfred
Schroeder of Bell Labs.
27
Digital Reverberation
Parallel comb filters and cascaded allpass
filters are still considered fundamental building
blocks of reverberation algorithms.
Companies invest considerable resources into
reverberation and other effects products the
precise algorithms they use tend to be highly
confidential.