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Subtractive
synthesis, something that every
newbie to Industrial tries to learn,
and many don't do so well. Here's
a guide that will help you understand
all those pretty knobs. |
GENERAL
ASSUMPTIONS IN THIS ARTICLE: This lesson
assumes that when you think of tuning,
you think of A-440, which is the note
"A" above middle C. Just understand
that as we use "A" in this
example, we are really talking about
any note you please, but when explaining
the mathematics, the A notes are easiest
to do the math around because whole
numbers are easy. Also, the patch we
discuss was originally written with
a Virus B in mind. Since most synthesizers
only have two oscillators, ignore the
bass triangle wave unless you have a
synth with more than two oscillators,
or if you have two oscillators and a
sub oscillator that is a sine wave,
you can use that too.
Subtractive
synthesis is the most common method
of synthesis today. While other methods
such as frequency modulation and additive
synthesis have some popularity, when
most musicians think synthesizer the
first thing that comes to their head
is a subtractive synth (or one of those
generic ROMplers that flood the market
today, but that's another story for
another time and another genre).
Oscillators:
The
first section we are going to approach
is the oscillator section. The oscillators
are technically the only things that
make sound unless you count self oscillating
filters (more on that later). Everything
else is some form of modification to
this sound. The average synthesizer
has 2 oscillators, while others have
three or more and some have only one
oscillator.
All
sounds, no matter what, consist of "harmonics".
When a sound is made, it consists of
"sine" waves (explained below)
that combine to create the sound. A
natural harmonic is any sine wave in
the sound that is a multiple of the
lowest sine wave (for example, on the
note "A", which has a base
of 440Hz in the A above middle C, possible
natural harmonics are at 880Hz, 1320Hz,
1760Hz, 2200Hz, and so on, adding 440Hz
each time). There are also unnatural
harmonics, which is basically any wave
that does not occur within those bounds,
but is still in the waveform.
The
"saw" wave oscillator creates
a waveform that is characterized by
a vertical leap from the highest point
of the wave or lowest point (the other
way around is common, but it doesn't
make a difference in sound). After this
leap, the wave moves back to the point
where the leap was made in a straight
diagonal line. Honestly it is pointless
to describe the shape of the wave, because
the shape isn't important, the harmonic
content is. What makes the saw wave
special is that it contains every single
natural harmonic that a wave can contain
in gradually decreasing levels. As a
result of this, the wave has a very
"buzzy" sound to it.
The
"square" or "pulse"
oscillator, in it's normal form, contains
only the odd numbered harmonics, resulting
in a bright yet hollow sound similar
to that of a clarinet (except much much
brighter). However, on many synthesizers
the width of the pulse can be adjusted,
which affects the harmonic content of
the wave. As the width of the wave becomes
less and less balanced, the wave starts
to gain a buzz similar to a saw wave,
but not exactly the same.
The
"triangle" oscillator contains
very little harmonic content, but just
enough for there to be a noticeable
"edge" to it's sound.
The
"sine" wave is the simplest
of all sounds. It has no harmonic content,
just the base pitch with nothing else
on it.
Many
synthesizers offer waveforms other than
this (noise, formant sounds, etc.).
Feel free to experiment with those waveforms
to get a better understanding of their
sound. The best way to get a grasp of
an oscillator's sound is to either deactivate
or totally open the filter and turn
any and all modulations such as envelopes
or LFOs off. This will make more sense
later.
While
these explanations don't offer much
of an explanation for the reason a certain
wave is used in a certain context, it
is impossible to explain why certain
waves are used when unless we understand
the filter first.
The
one thing that can be explained is why
certain tunings are used. Normally,
a synthesizer will offer a set of tuning
options that should consist of "coarse"
tuning and "fine" tuning.
Coarse tuning sets the pitch of the
oscillator in relation to the note played
in one note increments. The most common
usage of the coarse tune function is
to change the octave in which the oscillator
plays. This allows you to do something
like create a patch that uses two saw
waves, but has a triangle wave one octave
(12 notes below normal) below the two
saw waves. This will basically be a
loud saw wave (when you use any combination
of oscillators, they add on to each
other. always) that has a "low
note" below it. Hearing it is better
than trying to explain the physics of
it.
Fine
tuning is like coarse tuning, but extremely
mild. Instead of changing the pitch
in semitones (basically a note up or
down the entire keyboard), the pitch
is changed in fractions of a semitone
(depending on the synthesizer. Most
true analogs will go an infinite number
of fractions up or down a note, some
digitals only go 64 fractions each way,
some actually divide it into "cents",
which is 1/100th of a note, and some
"fine tuning" knobs, like
on the Korg MS2000, change pitch slightly
at first and gradually increase until
the change becomes almost as drastic
as a semitone per value). The best way
to explain how this would work is to
look at our two saw waves and a low
triangle wave again. If we were to take
the fine tune knob of one of the saw
waves and turn it up some, a very slight
type of "noise" (dissonance
to be technical about it), will start
to come in, which leads to a more natural
and organic sound. It's kinda like hearing
two people sing the same note. Neither
one gets it perfect, and as a result
you can tell it's two people singing
it.
The
best way to get a grasp of an
oscillator's sound is to either
deactivate or totally open the
filter and turn any and all modulations
such as envelopes or LFOs off.
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Filters:
Now
that you know what's making the sound,
let's talk about changing a sound (which
is really what everything but the oscillators
do). How filters work is that they are
assigned a certain frequency (which
can be modulated by various sources,
but more on that later) and, depending
on the type of filter, remove all frequencies
above (lowpass), below (highpass), not
close to (bandpass), or close to (band
reject). A lowpass filter makes a tone
seem "darker", a highpass
filter makes a tone seem "brighter",
and bandpass and band reject simply
remove a section or two of harmonics
which can have some interesting results.
Once again, not all synths have all
of these filters. Band reject isn't
very common, and some synthesizer only
have a lowpass filter, which leads to
what I call "lowpass hell"
(writing a song that has so many sounds
made by lowpass filters that it's all
low frequencies and sounds like beating
on a large dead animal). Filters almost
always have two main settings, cutoff
and resonance.
Cutoff
rate is usually stated in "poles",
or more understandably, decibels per
octave (6 x the number of poles). For
example, with a 2 pole lowpass filter
(12db/oct), every harmonic located an
octave above the filter is 12 decibels
quieter. This means that if on the note
A at 440Hz you set the cutoff at 2200Hz,
the harmonics would decline at a rate
such that the 4400Hz frequency is 12db
quieter, the 8800Hz frequency is 24db
quieter, and so on and so on. With a
highpass filter, this means that 1100Hz
is 12db quieter, 550Hz is 24db quieter,
and so on. On bandpass, 1100Hz and 4400Hz
are 12db quieter, 550Hz and 8800Hz are
24 db quieter, and so forth. On a band
reject filter, this means that 1100Hz
and 4400Hz are 12db louder, and so forth.
As
the cutoff rate increases, how quickly
the filter reduces the volume before,
after, or around the cutoff point increases.
On a lowpass this means that the sound
will become "darker" as the
cutoff rate is increased because the
higher harmonics are less and less audible.
When
you increase a filter's resonance, you
basically increase how loud harmonics
near the cutoff point are. Using only
a little resonance, you can make a sound
seem a tad brighter in it's harmonics
around the cutoff. There are three possible
scenarios when you crank the resonance
extremely high, depending on your synth.
Most modern synthesizers will compensate
for the gain created by the filter's
resonance and you will hear only the
harmonics closest to the cutoff. On
many synthesizers that have analog filters
(instead of digital emulations), the
filter cutoff will be very loud, but
you can still hear the stuff under it,
however you'll probably be tempted to
turn the volume down some if your synthesizer
is like this because this can get loud.
The final possibility happens on some
analog synthesizers (like the Minimoog),
and this is "self oscillation",
where the filter will actually start
to feed back and CONSTANTLY create the
harmonic at the input whether or not
there is sound coming from the oscillators.
However, not many synthesizers do this
and you probably are not on your first
synthesizer if you have one where this
happens.
Another
thing that's worth mentioning is key
tracking. Most modern synthesizers offer
variable key tracking. When the key
tracking is set at 100% or a 1:1 ratio,
that means that the cutoff rate will
rise or decrease at the same rate the
notes change. This means that the patch
will sound exactly the same no matter
what note you play. The only difference
will be the pitch. On acoustic instruments
this really does not happen because
as the string, pipe, head, or whatever
grows shorter, it becomes harder and
harder for it to make the higher harmonics.
Wind instruments don't experience this
nearly as drastically as percussive
and string instruments do, so don't
automatically assume that you should
have total or no keyboard tracking.
I'll go into more detail on this one
later.
An
example of how to use a lowpass filter
is to once again look at our patch that
consists of the two saw waves and the
triangle wave below them. Currently
this has a very bright edge to it, but
we will be taking off that edge with
a 24db/oct lowpass filter. So since
the general sound we're going for here
is probably most likely to be used as
a bass sound, we want the filter cutoff
pretty low (but not too low, or as stated
earlier it will sound like you're beating
a large dead animal). We'll go ahead
and set the cutoff at somewhere around
1300Hz. It is not important that you
get the filter cutoff on a harmonic
or octave. Filters are a gradual decline
so it doesn't make TOO much of a difference
if you aren't totally exact. There is
a difference, but you don't have to
be Adolf Filter about it.Also, if your
synthesizer doesn't offer exact values
for the cutoffs, just set it to something
that sounds pretty low but doesn't make
your patch totally bassy.
If
you're hell bent to find close to the
frequency I said turn off every oscillator
except a saw wave, turn the filter cutoff
all the way down, turn the resonance
all the way up, and start slowly turning
the cutoff knob up. You will start to
hear a frequency grow really loud, and
then you will hear another frequency
fading in as the previously loud frequency
begins to fade out, and the same thing
will happen after passing the second
harmonic. Once the third harmonic has
reached it's peak, you're there. Also,
turn the resonance back down to a rational
amount after doing this and turn your
other oscillators back on. This is useful
if you're on an analog and for some
reason or another want the cutoff dead
on a harmonic.
OK,
so at 1300Hz our 220Hz triangle wave
is untouched until the 6th harmonic,
and our 440Hz saw waves start to experience
cutoff near thier third harmonic. Since
this is a 24db/oct filter, that means
that the cutoff will be pretty drastic,
so it won't be a very bright sound (keep
in mind that you don't want to use a
24db lowpass at a low frequency on every
sound, because if you do that your track
will be a bunch of muddy lowpass shit).
So now we have a relatively dark bassy
sound that has a tad of dissonance from
one of the saw wave oscillators being
slightly detuned from the others. Just
for fun, lets add a tad of resonance,
only about 10-15%. Normally resonance
in a bass makes you sound like a techno
weenie, but the reason we are adding
resonance here is to make something
that comes later a little more noticable.
Try
to avoid abusing resonance, but if by
the end of the article you're starting
to think of weird things to do with
it feel free to experiement with higher
resonance levels. Just don't release
it unless you think you've done something
pretty unusual with it.
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Filters
are a gradual decline so it doesn't
make TOO much of a difference
if you aren't totally exact. There
is a difference, but you don't
have to be Adolf Filter about
it.
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Amps
and Modulations:
The
amp section is simple. It turns the
sound into something audible. Amazing..
However, this is a very good turning
point for going from the basic sound
crafting to manipulation of the static
sound, known as modulation, because
when you combine the amp with envelopes,
LFOs, and other things, it can quickly
become the single most drastic factor
on what you sound turns out to be.
Envelopes:
We
will start with envelopes. The most
common type of envelope is ADSR, which
means that it allows you to set how
fast something goes from unchanged to
it's largest change (A: Attack), the
change starting to wear off (D: Decay),
what level it will remain at a level
that is still different from the starting
point (S: Sustain), and how long it
takes for the sound to go from wherever
it's at back to the starting point after
you release the key (R: Release). The
most common use of envelopes is to change
the amp's output level. So lets take
our patch once again, and set an envelope
to have an instant attack (if you synthesizer
is digital or even VA, nudge this up
slightly. Digital synths usually don't
take well to instant attacks and will
have a "pop" near the beginning
of the sound on the fastest attack,
so nudge this up a value or two), a
moderate decay (around 1/4 of a second
is good), a sustain of about 1/2, and
a pretty fast release (not instant,
because that sounds extremely unnatural.
Not even palm muting a guitar cuts off
the sound as fast as an envelope with
an instant release). Now you have a
pretty punchy bass. Now lets assign
this envelope to change the filter cutoff
as well. Change the ammount of the effect
according to taste, but due to the resonance
that we added you will hear an initial
punch and then the brightness of the
sound dropping (listening to the resonance
you can hear it better). You can also
assign an envelope to pitch (there should
be a way to route it to the pitch of
all of the oscillators. so just to prove
a point take a second envelope (or if
you only have one, just use it for now
and set it back), and go ahead and set
the attack rate to the fastest possible,
have the decay just slightly above instant,
no sustain level, and a release time
of zero. Assign this envelope to the
pitch and go ahead and make it have
as much of a positive (meaning that
the envelope raises the pitch instead
of lowering it, which can be done too)
effect as possible. When you hit a key
with an envelope assigned like this,
you will hear the pitch of the patch
start somewhere extremely high and quickly
shoot down to the proper pitch.
LFOs:
LFOs
are totally useless for this patch as
it currently works, so for the purposes
of establishing how LFOs work set the
attack time to something pretty slow
(perhaps 1/4th to 1/2 a second) and
play higher on the keyboard. LFO waves
are generally pretty similar to oscillator
waves, but they are slower and therefore
the harmonic content of an LFO doesn't
matter at all, the waveform does. Lets
go ahead and take a triangle waveform
and have it mildly effect pitch, and
set it to something around 6Hz (6 cycles
a second in case you don't know) Now
when you hit the key, the pitch of the
sound will move up and down and you'll
have vibrato. If you don't hear anything
different, turn up the amount the LFO
effects the pitch until you do. If you
were to do this to the filter cutoff
(crank the resonance up to 30-40%, have
the LFO have a moderate effect on the
cutoff, and slow the LFO down some),
you'll have a slow "wah" effect.
If you don't turn the resonance up,
it will sound sort of like tremolo,
which will be described now. If you
set the LFO speed back up to 6Hz or
so and assign it to the amp, the volume
will move up and down, creating tremolo.
A
Comment on Effects and Other Goodies:
Since
all synthesizers have different features
(effects, different oscillator waveforms,
various filter types and so on and so
on), I cannot give any sort of valid
commentary on them. However, with minimal
research on the internet you can probably
find out all you really ever wanted
to know about them.
There
you have it, the basics of subtractive
synthesis. While this document explains
why synthesizers work and to some extent
how, you'll have to read your synthesizer's
manual to know the exact process for
how to make a change (most manuals will
give you all the "how" you
need to know). As for what you make,
that can only be done by your own experimentation.
Everything about the basics of subtractive
synthesis has been explained here, but
in order to truly understand it you
have to work with it. Explore this whole
"slow attack" concept some
more. Assign a square wave LFO to change
the pitch 2 octaves, use waves other
than triangle and saw, see what happens
when you mix a square wave and a two
sine waves tuned to an octave and three
octaves above, experiment with different
filter types and cutoffs (contrary to
the ideas of many, 24db/oct lowpass
is not the only filter in the world).
Also, not all synthesizers follow this
exact signal path. Synthesizers like
the Ensoniq ESQ-1 have volume envelopes
available for all three oscillators
before the filter and have a final volume
envelope at the amp stage, and some
synths like the Access Virus have multiple
filters. The concepts explained here
will still work on these synths. Now
that you know why your shiny new synth
works and your manual should tell you
how to use it, go see what you can make.
-Mister Bill
