Computer Music Synthesis
Chapter 7
Based on “Excerpt from Designing Sound”
by Andy Farnell
Slides by Denny Lin
Copyright © 2011 by Denny Lin
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Pure Data essentials
• The following is a collection of patches
that implement essential concepts used for
mixing, reading and writing files, and
sequencing
– 7.1 Channel strip
– 7.2 Audio file tools
– 7.3 Events and sequencing
– 7.4 Effects
Copyright © 2011 by Denny Lin
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7.1 Channel strip
• These patches consist of various mixer controls:
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Signal switch
Simple level control
Log law fader
MIDI fader
Mute button
Panning
• Linear, square root, cosine, and MIDI panner
– Cross fader
– De-multiplexer
Copyright © 2011 by Denny Lin
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Signal switch and direct level
control
• Both of these patches
control audio levels
by multiplying the
audio signal input
• These controls are
prone to clicks and
zipper noise, which is
caused by sudden
level value changes
on an audio block
boundary
Copyright © 2011 by Denny Lin
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Log Law Fader
• Sliders are linear
(“lin”) by default. Can
be changed to
logarithmic (“log”) in
the slider’s properties
• Half of the slider’s
output range is
compressed into the
final ten percent of
the movement range
Copyright © 2011 by Denny Lin
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MIDI data scaling
• MIDI controller messages
are generally linear
signals in the range of 0
to 127
• Can be normalized by
dividing by 127, or
multiplying by 0.0078745
• Normalized output may
be:
– scaled to a log curve
– multiplied by 100, and
converted to decibel scale
via the dbtorms object
(see fig. 7.6)
Copyright © 2011 by Denny Lin
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MIDI Fader
• The ctlin object connects
the MIDI fader patch to
an external MIDI device
• Only volume messages
from channel 1 can
control the spigot
• MIDI message is
normalized (between 0
and 1), and transformed
to an audio signal
• 2Hz low-pass filter used
to smooth out fader
Copyright © 2011 by Denny Lin
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Mute Button and Smooth Fades
• Slider value (0 to 100) is
in decibel scale
• Fader value is stored in
cold inlet of * object
• Left inlet of * object gets
inverted value of toggle
switch
• Message data is
transformed to audio
signal and low-pass
filtered to smooth the
fader
Copyright © 2011 by Denny Lin
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Panning
• The purpose of pan control is to place a
monophonic sound source within a
listening panorama
• Different from balance, which places a
stereo sound source
fig. 7.10: Linear, square root, sine/cosine panning laws
Copyright © 2011 by Denny Lin
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Linear Panner
• Consists of two volume
controls, one for each channel
(signal source is monophonic)
• Fader smoothing achieved by
converting messages to audio
signal, and low-pass filtering
• Cold inlet control directly alters
left channel output
• Right channel is controlled by
the complement of the left
channel control value
• Problem: When a signal
amplitude level of 1 is sent, the
perceived loudness at each
channel is halved
Copyright © 2011 by Denny Lin
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Square Root Panner
• Similar to linear panner
• Takes the square root of
control signals to produce
panning that follows an equal
power law
• Has a 3dB amplitude increase
at the center position
• Problem: The square root
curve is perpendicular to the xaxis when x is close to 0 and 1.
The sound in one of the
channels suddenly disappears
at the slider’s extreme ends
Copyright © 2011 by Denny Lin
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Cosine Panner
• The curve makes a 45
degree angle approach at
the edges
• Cosine and sine are
easier to compute than
square root
• Tends to mimic
placement of sound
source on a circle
• Problem: The response
around center position
not as desirable as
square root panner
Copyright © 2011 by Denny Lin
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MIDI Panner
• Combines the MIDI
fader (see fig. 7.5)
with a cosine panner
• Pan information is
sent to MIDI controller
number 10 (64 is
center position).
• Patch only responds
to pan information on
channel one
Copyright © 2011 by Denny Lin
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Cross Fader
• Used to smoothly transfer from
one sound source to another
by mixing them to a common
output
• Two inlets are used for signal
sources
• Third inlet controls the ratio of
the mixture
• Can be used to control
proportion of reverb effect, or
to cross fade between two
tunes
• Constant power fading
versions using sine and square
root functions are sometimes
better
Copyright © 2011 by Denny Lin
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De-multiplexer
• Signal source selector to
pick one out of several
inputs to be routed to the
outlet
• A number at the inlet
selects and bangs the
corresponding Boolean
message list to the
unpack object
• The signal~ and 80Hz
low-pass filter objects are
used to remove clicks
Copyright © 2011 by Denny Lin
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7.2 Audio file tools
• The following example patches can be
used to record and playback .WAV or
.AIFF files:
– Monophonic sampler
– File recorder
– Loop player
Copyright © 2011 by Denny Lin
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Monophonic Sampler
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Abstraction used to record and
play a few seconds of audio
Sounds are recorded into a table
that stores 88200 samples (2
seconds, given 44100 samples
per second)
First inlet gets signal
Second inlet controls the gain
(clicks are removed with signal
and low-pass filter)
A bang on the third inlet records
the signal into the table
A bang on the fourth inlet plays
back the recording to the outlet
Sampler can be used by
connecting an adc~ input object, a
gain slider, bang buttons to control
record and playback, and a dac~
output object
Copyright © 2011 by Denny Lin
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File Recorder
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Fixed length file recorder
abstraction
Takes the recording length as first
argument (sent to $1), and
directory housing the file as
second argument (sent to $2)
Uses the writesf~ object which
takes the signal, and three
commands: start recording, stop
recording (after delay of $1 ms),
and name of file to write
A counter tracks the number of
times the file recorder was run,
and appends the number to the
filename via the %d
A bang message is sent to the
done outlet when the recording
has ended. Recording is stopped
by the delay object
Copyright © 2011 by Denny Lin
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Loop Player
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Use this abstraction to play a .WAV or
.AIFF file endlessly
The openpanel object provides a file
dialogue to select a file to play
The pack object takes the filename
and the $0-a symbol as $1 and $2
items in the read message
The soundfiler reads the sound file into
the $0-a table. When complete, the
number of bytes read is used to trigger
a bang message to the tabplay~
object
When done playing, the right outlet of
tabplay~ sends a bang that is routed
via spigot to trigger the tabplay~
object to repeat
A zero received on the right inlet stops
the spigot object from re-triggering the
tabplay~; a one received on the right
inlet triggers a bang to play the sound
file
Copyright © 2011 by Denny Lin
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7.3 Events and sequencing
• The following example patches create time,
event, and sequence triggers:
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Time base
Select sequencer
Partitioning time
Dividing time
Event synchronized Low Frequency Oscillator (LFO)
List sequencer
Text file control
Copyright © 2011 by Denny Lin
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Time base
• This time base abstraction
accepts beats per minute
• The control inlet activates the
metro object, and resets the
internal counter whenever the
time base is stopped
• The second inlet takes beats
per minute
• The third inlet takes the
number of beats per measure
(2, 3, 4, 6, etc)
• The fourth inlet takes the swing
percentage, multiplied by the
number of beats per measure,
sent to the delay object before
the counter increments
fig 7.19: A more useful musical
time base abstraction with BPM
and swing
Copyright © 2011 by Denny Lin
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Select Sequencer
• This patch demonstrates that
repetitive patterns can be
generated with a time base,
mod and select objects
• Uses the time base abstraction
• The mod object wraps the time
base counter output to 8 beats,
so events within the 0 to 7
range can be triggered
• A separate event at time=1024
can be triggered with the
select object using the same
time base
• Instead of bang boxes, note
and frequency related objects
can be triggered to generate
repeating sequences
Copyright © 2011 by Denny Lin
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Partitioning Time
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Specific events at musical measure
numbers can be triggered. This patch
ignores events from time 0 to 127
using a chain of moses objects
Given time at 208, the first moses
object routes the time value to the
second moses object
Time value is shifted down, treating
this section as if this were the
beginning of the piece
mod 64 creates 2 measures of 64
beats each, in the range 128 - 256
Event at time 208 is beat 80, which
would be the second measure of the
partitioned timeframe
Each of these 64 beat measures can
be further subdivided into 4 measures
that each contain 16 beats. Each bang
box marks the downbeat of the first of
4 measures
Copyright © 2011 by Denny Lin
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Dividing Time
• Output from the timebase can be divided to
tell which measure the
music is at
• The time base number is
divided by the beats per
measure value (2 or 4)
• Uses the int object to get
whole measure numbers
• Uses the change object
to remove repeating
measure numbers
Copyright © 2011 by Denny Lin
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Event Synchronized LFO
• If the time base is to be
used as a control signal,
dividing the time base
using change and int
objects could result in
several consecutive equal
values
• Output from the Low
Frequency Oscillators
(LFOs) becomes choppy
• Using higher mod and /
values fixes the problem
fig. 7.23: Synchronous message LFOs
Copyright © 2011 by Denny Lin
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List Sequencer
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This list sequencer does not rely on a
time base
Uses relative time from a message list,
so the list {61 0 60 500 59 500} means
play note 61 at time 0, then note 60 at
time 0 + 500, and note 59 at time 0 +
500 + 500
The list split 2 object processes two
items at a time; the remainder of the
list is sent to the cold inlet of the list
append object
First item (note number) is sent to the
cold inlet of float object
Second item (time value) is sent to a
delay object that bangs the note
number at the end of the delay time,
sending output to the synthesizer
The list append object is banged,
sending the remainder of the list to the
list split 2 object, which processes the
next two items. Continues until list is
empty
fig. 7.24: An asynchronous list sequencer
Copyright © 2011 by Denny Lin
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Synthesizer Part
• The patch on the right
half of fig. 7.24 is a
monophonic
synthesizer that
accepts MIDI note
numbers
• The phasor~ output
is modified with a
400ms curved
envelope
fig. 7.24b: Monophonic Synthesizer
Copyright © 2011 by Denny Lin
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Text file control
• Text files can be used to store MIDI data. File contains note number
and note duration data
• The notein object gets MIDI note numbers, and the stripnote
removes 0 velocity notes. Note data is sent to the pack object’s hot
inlet (first item)
• The timer object measures the time the note was pressed, and
sends its output to the pack object’s cold inlet (second item)
• When the user clicks on start-record, the timer object is reset, the
list accumulator is loaded with a bang, and the textfile writer is
cleared using a clear message
• After the user has finished playing, the write button is banged so that
the textfile object writes the performance data from the list append
and list prepend set data into the text file using the textfile object.
The name of the text file is specified by the write message object
• The load-playback button rewinds the textfile to the beginning, and
sends it to the monophonic synthesizer
Copyright © 2011 by Denny Lin
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Fig. 7.25: MIDI Sequencer that uses
text files to store and play data
Copyright © 2011 by Denny Lin
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7.4 Effects
• Two simple effects are introduced:
– Stereo chorus and flanger effect
– Simple reverberation
• Use these effects sparingly
• External mixers or plugins may already
have better implementations of these
effects
Copyright © 2011 by Denny Lin
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Stereo Chorus/Flanger
• Produces multitude of
sound sources by
doubling up several
copies of sound using
delays
• LFOs are always 1Hz
apart, and vary between
1Hz and 5Hz
• Fig. 7.27 uses the stereo
chorus sub-patch and
passes the feedback,
rate, and depth
fig. 7.26: A chorus type effect
fig. 7.27: Testing the chorus
Copyright © 2011 by Denny Lin
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Simple Reverberation
• Designed to mimic
reverberating sounds on 4
surfaces
• Minimum delay should be at
least a quarter of the reverb
time
• To avoid phase cancellation
and reinforcement between
echoes:
– Delay times in the delread~
objects should be prime
numbers
– Amplitude for each buffer
output is scaled at different
levels
• rev1~, rev2~, rev3~ are
reverb abstractions
fig. 7.28: A re-circulating Schroeder reverb effect
Copyright © 2011 by Denny Lin
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