Gtaylor@rtqe.net at 15:29, 28 June 2012

2012-06-28T15:29:39Z

Admin at 21:10, 25 June 2012

2012-06-25T21:10:54Z

Admin: Created page with "Click here to open the tutorial patch: 01nEnvelopeFollowing.maxpat ===Introduction=== In this group of tutorials, we'll look at different ways to work with ''dynamics'' ..."

2012-06-22T21:10:19Z

Created page with "Click here to open the tutorial patch: 01nEnvelopeFollowing.maxpat ===Introduction=== In this group of tutorials, we'll look at different ways to work with ''dynamics'' ..."

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Click here to open the tutorial patch: [[01nEnvelopeFollowing.maxpat]]

===Introduction===

In this group of tutorials, we'll look at different ways to work
with ''dynamics'' in audio signals. The ability to use and control
the ''amplitude'' of an audio waveform is important for many applications,
from the macro level (audio compression) to the micro level (distortion).
In this first tutorial, we'll learn to derive control values from amplitude
parameters of an audio signal which we can use to control parameters
elsewhere in the signal chain. This technique is called ''envelope following''.

===The envelope, please...===

We've learned from working with MSP that digital audio is represented as
a stream of discrete samples, each of which represent the ''amplitude'' of
a signal at a given time. However, when we actually think about (and use)
the term amplitude, we aren't necessarily thinking of it on a sample-by-sample
basis. Instead, we discuss the amplitude of a sound based on how ''loud'' it
seems to us as listeners. Alternately, we create objective measurement systems
to attempt to quantify this (subjective) loudness, such as level meters in
audio mixing software. Both of these systems look at the amplitude of an
audio signal not on a sample-by-sample level, but ''averaged'' over time.
This technique of deriving the macro-amplitude of a sound's loudness or
volume (as opposed to the micro-amplitude of the sample values) is
called ''envelope following''.

We use the term envelope in synthesizer design to refer to the overall dynamic
shape of the sound. For example, a sound that fades in smoothly and then fades
out over the same amount of time could be said to have a triangular envelope.
A sound with a short attack, a long sustain, and a short fade might look like
a trapezoid. Similarly, we can take any sound source and, with a little bit
of work, abstract its envelope:

[[Image:Dynamicschapter01a.png|border]]
''"Is that you?": waveform (top) and envelope (bottom)''

To create the envelope above, we looked at the samples within the waveform and
tracked their ''average'' amplitude over time. There are several strategies
to do this in MSP, involving a trio of objects that allow us to smooth audio
signals based on different parameters.

* In the tutorial patcher, familiarize yourself with the area labeled <code>1</code>.
This is a basic sample playback patcher that loads an audio file into a {{maxword|name=buffer~}} object
and plays it in an endless loop using the {{maxword|name=groove~}} object. Start the audio, turn
up the {{maxword|name=gain~}} slider, and make sure you can hear the drum loop as it plays.

===Envelope following in MSP===

* Look at the two objects in yellow attached to the output of the {{maxword|name=groove~}} object.

The first thing that needs to be accomplished when deriving the smoothed amplitude
of a sound is to convert the signal into a ''rectified'' wave; that is, to
ignore the difference between negative and positive sample values and only
concentrate on their distance from <code>0</code>. We can do this by taking the absolute
value of a signal, which can be accomplished using the {{maxword|name=abs~}} object in MSP.

The {{maxword|name=snapshot~}} object allows us to convert one sample of an MSP signal
into a floating-point number output from the object as a Max event. Every time
the object receives a <code>bang</code>, it takes the current audio sample and outputs
it into Max as a number which can be viewed, scaled, or used to trigger Max events.
In the case of our patcher, the absolute value of our {{maxword|name=groove~}} object's
output is being used to drive the height of a {{maxword|name=multislider}} object, with
the {{maxword|name=snapshot~}} providing the intermediary translation from an MSP signal
t. a Max message. As we can see, the bouncing up and down of the {{maxword|name=multislider}}
gives us a vague impression of the amplitude of the drum loop. However, a more
accurate impression of how we ''hear'' the sound can be accomplished by smoothing
those values.

* Look at the patcher logic labeled <code>2</code> in our tutorial. In the {{maxword|name=number}} box
labeled 'Up' type the value <code>1.</code> and hit return. In the {{maxword|name=number}} box
labeled 'Down' type the value <code>100.</code> and hit return. Observe the results as
plotted in the {{maxword|name=multislider}} objects below. Now do the reverse (type <code>1.</code> as
the 'Down' value and <code>100.</code> as the 'Up' value). Notice the difference?

The output of the {{maxword|name=send~}} object (which consists of our drum loop) is
converted to an absolute value, run through an object called {{maxword|name=rampsmooth~}},
and plotted in a {{maxword|name=number}} box and two {{maxword|name=multislider}} objects. The
right-hand {{maxword|name=multislider}} is in one of its scrolling modes, which allows
us to see values as they unfold over time. The {{maxword|name=rampsmooth~}} object
performs a linear smoothing operation on an input signal. Its two arguments
(or numbers given at its middle and right inlets) control the way in which the
smoothing behaves. The middle inlet controls over how many samples the object
will smooth a changing value when it is ''rising''; the right inlet controls
how many samples it takes for a signal to ''fall''. These two values allow
us to independently smooth increasing and decreasing sample values, so that
sharp onsets can be kept in the envelope but abrupt silences generate a tail.

* Look at the area of the tutorial labeled <code>3</code>. As before, enter <code>1.</code>
in the {{maxword|name=number}} box labeled 'Up' and <code>100.</code> in the {{maxword|name=number}} box
labeled 'Down'. Observe the envelope drawn in the {{maxword|name=multislider}}. Now do
the reverse (type <code>1.</code> as the 'Down' value and <code>100.</code> as the 'Up' value).
Type <code>100.</code> for both values.

The {{maxword|name=slide~}} object smooths audio signals through logarithmically
smoothing the input signal. The two values provide it work as denominators
for new rising or falling energy entering the (smoothed) signal. For example,
an 'Up' value of <code>1000</code> means it will take 1000 samples for a
single-sample transition from <code>0</code> to <code>1</code> to occur. The larger
the 'Up' and 'Down' values, the smoother the curve.

* Look at patcher area <code>4</code>. In the {{maxword|name=number}} box labeled 'Delta',
type <code>0.01</code> and hit return. Observe the results. Type an even smaller
number, such as <code>0.0001</code>.

The {{maxword|name=deltaclip~}} object works in a different manner from {{maxword|name=rampsmooth~}}
and {{maxword|name=slide~}}. It sets a threshold for the amount an audio signal can change,
and clips the outgoing signal to that amount. Setting the <code>min</code>
and <code>max</code> values for the delta to <code>-1</code> and <code>1</code> will allow
samples to pass normally provided they don't change by more than that amount.
Smaller values will smooth the audio signal in a ''relative'' way so that
the resulting envelope traces a much narrower set of values than the incoming signal.

If we put a burst of noise through the different envelope following
techniques shown in our tutorial patcher, we can see the difference
between the curves they generate:

[[Image:Dynamicschapter01b.png|border]]
''Different envelope followers on a noise burst.''

Each of these envelope-following techniques have different musical and
sonic applications and have different signatures. A logarithmic envelope
follower controlling the volume of a synthesizer, for example, would
have a very different ''sound'' than a linear one.

===Using envelopes as control signals===

* Look at the patcher logic labeled <code>5</code> in the tutorial. Turn up
the {{maxword|name=gain~}} slider to hear the output of the {{maxword|name=cycle~}} object.
Using the {{maxword|name=umenu}}, select the different envelope followers as a control
input. If you like, change their parameters as well to see what types of
different effects you can achieve.

The three envelope following patcher areas send their output signals
through {{maxword|name=send~}} objects to any {{maxword|name=receive~}} set to the appropriate
name. In the case of patcher logic <code>5</code>, this signal (ranging
from <code>0</code> to <code>1</code> controls the frequency of a {{maxword|name=cycle~}} object,
mapping the control signal to between <code>100</code> and <code>1100</code> Hz. The
difference between the three envelope type should be apparent if they are
set to fairly strong smoothing values.

* Turn down the {{maxword|name=gain~}} slider in patcher area <code>5</code> and look at
the patcher logic labeled <code>6</code>. Turn up the {{maxword|name=gain~}} slider and
select different sources for a control signal from the {{maxword|name=umenu}}.

In this example, we use our envelope control signal to modulate the cutoff
frequency of a {{maxword|name=lores~}} filter, creating a dynamic wah-wah effect on a
mixed and detuned waveform.

* Turn down patcher area <code>6</code> and look at the patcher logic labeled <code>7</code>.
Turn up the {{maxword|name=gain~}} slider and select different envelopes from
the {{maxword|name=umenu}}.

In this example, we are using our envelope to create an 'auto-wah' on the
drum loop itself. This is a very common use of envelope following in music
production, where a control signal derived from an audio signal is used to
control a parameter of an effect applied to that same audio clip further
down the chain. Using this logic, we could use our envelope follower to
apply control signals to any signal-processing procedure we want in MSP.

===Summary===

When talking about the ''amplitude'' of an audio signal, we distinguish
between the sample-by-sample amplitude and the overall loudness of a sound.
This second shape is called the ''envelope'' of a sound, and the signal
processing involved in deriving it is called ''envelope following''.
Envelopes of audio signals can be created by looking at the absolute (rectified)
waveform (created with the {{maxword|name=abs~}}) object and smoothing it using MSP objects
such as {{maxword|name=rampsmooth~}}, {{maxword|name=slide~}}, or {{maxword|name=deltaclip~}}. These smoothed
control signals can then be scaled and use to control parameters on an synthesis
or signal processing operation you like. The MSP {{maxword|name=snapshot~}} object is useful
for converting MSP audio signals into floating-point Max numbers which can be
viewed using objects such as {{maxword|name=multislider}}.

===See Also===

{{maxword|name=abs~}} - Absolute value of a signal

{{maxword|name=snapshot~}} - Convert signal values to numbers

{{maxword|name=rampsmooth~}} - Smooth an incoming signal

{{maxword|name=slide~}} - Filter a signal logarithmically

{{maxword|name=deltaclip~}} - Limit changes in signal amplitude

[[Category:Teaching Material]]

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−	Click here to open the tutorial patch: [[01nEnvelopeFollowing.maxpat]]	+	Click here to open the tutorial patch: [[Media:01nEnvelopeFollowing.maxpat]]

	===Introduction===		===Introduction===

MSP Dynamics Tutorial 1: Envelope Following - Revision history

Gtaylor@rtqe.net at 15:29, 28 June 2012

Admin at 21:10, 25 June 2012

Admin: Created page with "Click here to open the tutorial patch: 01nEnvelopeFollowing.maxpat ===Introduction=== In this group of tutorials, we'll look at different ways to work with ''dynamics'' ..."