Introduction
Introduction
So, what is this analogue (or analog) synthesis that we have been hearing so much about?
Most modern synthesizers use some form of digital synthesis method, where sounds are created
as a series of numbers & processed, before finally being turned into an analogue signal.
However, with analogue synthesis, the sound starts as an analogue signal, and is then processed
by more analogue signal, before being issued from the Out socket.
The first synthesizers were analogue. They were developed from various bits of electronic test
equipment used in
laboratories (signal generators, filters, etc) which were used to produce early electronic
music.
The key idea was to put all the 'modules' into one
large box (or a series of boxes), provide a means of connecting the modules together & provide
a standard mechanism for controlling the modules. The result was the voltage controlled
synthesizer. Originally synthesizers consisted of a large number of modules, which could be
connected together in any way desired - these often took up entire walls! (E.g. the modular
Moog synths used by Keith Emmerson, Walter/Wendy Carlos, etc.) Later, the key modules were put
together in the one box, with a 'standard' connection hard wired in and a keyboard attached.
These synths were less flexible than the big modular jobbies, but were a lot more portable and
easier to use (the Minimoog is the classic example here).
The Components Of An Analogue Synth
In essence, an analogue synthesizer consists of 3 main components - a voltage controlled
oscillator (VCO), a voltage controlled filter (VCF) and
a voltage controlled amplifier (VCA).
The VCO
produces a basic tone.
The pitch is usually (though does not have to be) controlled with a
keyboard. This tone is then fed into the VCF, which
'colours' the sound - changing the harmonic content of the sound (it's like a very powerful tone
control). Finally the sound is fed into the VCA. This is controlled by an
envelope generator to
control the volume or loudness 'shape' of the sound - ie how quickly it starts up when a key is pressed,
how loud it stays while the key is held down, and how quickly it dies away when the key is
released. There is also usually a Low Frequency Oscillator (LFO) which can
apply a changing control voltage to modulate the other components.
The Keyboard
The keyboard is the device that is normally used to play the synthesizer. This does not have
to be the case - there are some synths controlled by a guitar interface, wind synthesizers, etc.
When a key is depressed, the keyboard sends out a trigger signal. This is basically an on/off signal -
when it is on, it means a key is being pressed, when it is off, no key is
pressed. This can be used by other modules (in particular the envelope generator)
to start doing something when a key is pressed (and maybe to start something else when the key is released).
NB the trigger is not necessarily a positive voltage - it is on Roland, Sequential and some other synths. But on Moog & Korg synths the trigger
output is a positive voltage when no key is pressed, and falls to zero volts when a key is pressed.
Note that unlike most MIDI keyboards now, original analogue keyboards did not detect how hard a key
was pressed.
In addition to sending the trigger voltage, the keyboard also sends a control voltage when a key is
pressed. This voltage is normally sent to the oscillator.
The voltage depends on the key being pressed - each key has a unique voltage. This means
the oscillator knows what pitch to sound for each key pressed. This is an analogue, not digital,
relationship - if the voltage the keyboard sends is slightly too high, then the pitch sounded will
be slightly too high. The circuitry in keyboards (and oscillators) must therefore be very precise and stable
to maintain good tuning in the synthesizer.
NB as well as two trigger standards, there are 2 standards for oscillator control voltages (CV) - 1 volt per Octave (used by Roland, Moog, Sequential, etc synths) and linear volt/hz (used by Korg and probably some others).
It is thus not normally possible to control, say, a Roland synth from the keyboard trigger/CV output of a Korg synth without some sort of CV (and trigger) conversion device. It is also important if you
buy a MIDI to CV convertor to get one that supports the CV and trigger standard used on
your synth. Aren't standards great?
The Voltage Controlled Oscillator (VCO)
The module that actually produces sound is called the oscillator. This produces
a constant tone. The pitch (or note) of this tone is determined by the value of a control voltage
sent from the synth's keyboard - usually the higher the key played, the higher
the voltage sent & the higher the pitch. Because the pitch (aka frequency) of the oscillator's
output is
determined directly by a voltage, we can vary this voltage to produce various effects.
For example, by 'wobbling' the control voltage up and down a little bit at a relatively low
frequency (abt 5 times a second, or 5 'hertz') we can produce vibrato - the same effect as when you
'wobble' your finger on a guitar fret. This effect is produced using the LFO module.
Similarly, we can mix the keyboard control voltage with a variable voltage from a pitch bend wheel
- the more you push the wheel up, the higher the voltage, so the pitch of the oscillator is 'bent' up.
When you return the wheel to the centre, the pitch returns to normal. You can bend the note down by
adding a negative voltage to the keyboard control voltage.
There are many other 'treatments' you can apply to the oscillator control voltage - e.g. by making the
voltage start high & fall quickly (by feeding the envelope generator output into it),
you get the 'peeeooow' syndrum type effect (and other more usable
effects!).
There is also often a portamento or glide control on many synths.
This allows you to 'slur' note changes. With the portamento turned up, when you press a new key the frequency will not
change instantly to the new note. Instead it will 'glide' from the original to the new note. The higher
the portamento control, the longer the glide takes.
Oscillators can normally be set to generate one of a number of waveforms. The waveform is the 'shape'
of the output produced by the oscillator. It determines how the oscillator will sound - in particular
how 'bright' it sounds. The brightness is actually determined by the number & type of harmonics present
in the waveform (harmonics are multiples of the original - or fundamental - frequency of the note,
which are present at a lower volume). There are a number of standard waveforms:
This is a very 'bright' 'fizzy' sounding waveform
(it actually contains all the odd and even harmonics of the
original frequency). It is very good for trebly sounds & dramatic filter sweeps (because it is so
bright, the filter has something to work on).
This is more of a 'reedy' slightly
'hollow' type of sound.
It is better than a sawtooth for deep bass sounds,
as there are fewer harmonics present to obscure the 'real' frequency.
On many synths, you can vary the pulse width of the wave - this is the ratio of the
on part to the off part.
By making the on part very short relative to
the off part, you can produce a very 'thin'
sound, very reed like. Often you can modulate the pulse width using an Triangle Wave
This is very close to a sine wave
(which contains only the fundamental frequency and no harmonics). It is a very 'pure'
sounding waveform, producing a flute like sound.
The filter will have little effect on this waveform, as there are no
harmonics to be acted on.
This is isn't a 'tone' at all - it is a
random mixture of all frequencies. It sounds like the sort of hiss you get on a radio when
you're not tuned into a station. There are two types of noise - white noise
which is truly random and pink noise, in which the lower
frequencies get more emphasis (this produces a 'heavier' hiss). Noise is good for generating
percussive sounds like snares & hand-claps, for simulating wind or sea sounds (when put through a
filter modulated by an LFO) and for producing
breath effects in flute & other wind instrument sounds (obviously you then need another oscillator
to produce the actual flute etc tone, unless your synth has a separate noise generator!).
Some synths only have one oscillator. However many synths have two (some have more). You can
generally get a much 'fatter' sound with two or more oscillators. You can set them an octave
apart to thicken the sound, and you can detune them very slightly to produce a chorus
effect. (Single oscillator synths often use pulse width modulation on the square wave to produce
a similar, though less fat, result.)
Another major benefit of 2 oscillators is that you can 'synch'
one to the other. If you synchronise
oscillator B with oscillator A, then every time oscillator A starts a new wave cycle, it forces
oscillator B to do so too, no matter where it was with its own cycle. If the 2 oscillators are
detuned slightly, this can create very complex waveforms:
Notice in this simple example how oscillator B has to restart its sawtooth wave each time
oscillator A restarts - even though B is already part way through its next wave! So you get a sort of
double sawtooth waveform. If you then pitch
bend oscillator B but NOT oscillator A, you can great some great screaming lead sounds as B fights
against the synch!
The Pitch Bend Wheel
The pitch bend wheel is simply a convenient method of applying a modulating control voltage
to the oscillator(s). By pushing the wheel away from you, you can
bend the pitch (ie frequency) of the oscillator up. Similarly, you can bend the pitch
down by pulling the wheel towards you. The wheel is normally spring loaded so it will return to
the centre position (ie no bend applied) if you let go of it. It is generally used in synth solos
(along with much facial grimacing) to give 'expression' to your solo. It does a similar job to
the tremolo arm (AKA wang bar) on electric guitars.
Note that on some synths (eg Roland) the 'wheel' is actually a spring loaded lever that moves
left/right.
Some synths (eg Korg MS20) do not have a pitch bend wheel as such - there is a control wheel
but you will have to patch it yourself to the oscillators to do pitch bend. (This is just a
case of connecting the control voltage out from the wheel to the modulation control in on the
VCO)
The Voltage Controlled Filter (VCF)
The filter is the module that shapes the tonal qualities (the bass & treble, if you like) of the sound. It works by removing, or at least greatly reducing, certain frequencies from the sound. It is effectively a powerful tone control. There are three types of filter - Low Pass, High Pass and Band Pass.
This is the most common (and normally most useful) type of filter. It removes high frequencies
from the sound. The effect is progressive - ie the higher the frequency, the more it is removed
(or attenuated). The 'strength' of a filter is measured in db/octave - normally 12db/oct
(mainly Japanese synths such as Korg, Roland) or 24db/Oct (usually US synths - Moog, SCI). 12db/oct
tends to sound a little 'brasher' than 24 (though this is not necessarily a bad thing - it depends
what sort of sound you are after). Moog synths in particular were famous for the smoothness
of their filters.
It may seem pointless to create a nice bright sound with an oscillator, only
to remove all the brightness with a filter! The point is that not all the sounds we want to create
are trebly - a deep bass sound needs a lot of treble removing to give it that thundering earth
shaking effect. Different sounds will require different levels of brightness. Having a variable
filter acting on a very bright sound allows us to determine the 'colour' of the sound we want very
precisely.
Also, we want movement in our sounds - we can vary the cut-off
frequency (ie the point at which the filter starts to remove frequencies) in real time during the
course of the note. This gives
life to the sound, and allows us to replicate the behaviour of real sounds.
For example, a piano note starts out very bright as the strings are first struck, then decays
to a much 'purer' sound as it dies away. We can simulate this by having the filter wide open as
the
note starts, then 'close it down' (ie sweep the cut-off frequency down) gradually as the note
fades.
Simlarly, punchy synth bass sounds use a dramatic quickly closing filter sweep (by applying an
envelope generator to the filter) to give a peeooww effect.
A high pass filter has the opposite effect from low pass - it removes the low frequencies from the sound. This is normally less useful than a low pass frequency - you are effectively removing (or at least reducing) the fundamental frequency of the sound, leaving only the 'fizzy' harmonics. It is useful for created extremely bright, thin sounds and sound effects (particularly with white or pink noise).
This is actually a low pass & high pass filter combined - it filters out high AND low frequencies
to leave just a band of frequencies in the middle. Again, of less everyday use than a straight low
pass filter, but good for creating very thin sounds and 'cheap AM radio' effects. Note that some
synths (eg Korg MS20) have low & high pass filters available,
which of course can be used together
as a band pass filter (with the added advantage that the 'band' is user settable & the filters
can be swept independantly.
There are two settings which apply to all types of filters - cut off frequency and resonance.
Cut off frequency determines the frequency at which the filter starts to take
effect. Below this frequency (in the case of a low pass filter) the signal will not be affected.
Above it, the signal will be filtered. The lower the frequency, therefore, the more of the total
signal will be removed (if you set the filter cut off frequency below the actual fundamental
frequency of the note you are playing, the signal will be almost entirely removed.) There is
normally a knob on an analogue synthesizer to set
the base cut off frequency for the filter. This base frequency can then be changed (ie modulated)
by various control voltages
(envelope generator, LFO, etc) to give filter sweep effects.
Sweeping the filter cut off frequency can give very dramatic effects (eg the constantly changing
sound of an acid bassline, with the filter being slowly swept slowly up & down
by an LFO).
Resonance is also known as Peak or Q.
It is actually the amount of the output
of the filter that is fed back into the input. Its effect is to emphasise the frequencies just
around the
cut off frequency. Increasing the resonance makes the filter effect more 'dramatic' - particularly
with filter sweeps (they sound more like PPEEEEOOOOOWWW
than WWEEEOOOOWWW). On many synths, if you
turn the resonance right up it feeds back on itself & turns into another oscillator. Resonance is
normally controlled by a knob - it is rarely voltage controlled.
Peeooww
Weeeooww
The Voltage Controlled Amplifier (VCA)
The VCA controls the volume of the sound. It's more than just a straight volume control though. Like the VCO & VCF it can be controlled by a voltage dynamically. So it can be controlled by an envelope generator to shape the loudness contour of the sound. We can use the VCA (and envelope generator) to control whether a sound starts instantly when a key is pressed, or builds up gradually. It is also used to control how the sound dies away (quickly or slowly) when a key is released. It is usually only an envelope generator that controls the VCA, though we may want to feed some of the output of the LFO to the VCA, to produce a 'wobble' in the sound volume (called tremolo).
The Envelope Generator
The envelope generator doesn't produce any sound itself, or even process another sound directly.
Instead it produces a control voltage output when it
receives a trigger signal from a key press. This control voltage can be
fed to the VCA to control the 'shape' of the sound.
It can also be fed to a VCF
to determine how the sound's tone changes.
A good envelope generator has 4 sections: Attack, Decay,
Sustain, Release.
These each determine what happens at certain points during a note. This
is known as an ADSR envelope.
The attack portion determines how the note will start. It is the time the sound will take to go from silence to full volume. If the attack time is quite long, the sound will 'fade in' when a key is pressed (i.e. as if you are slowly turning up a volume knob). If it is set to zero time (ie instant), the sound will start the instant as you press a key. Most types of real musical sounds (piano, guitar, etc) do have a zero or very short attack time.
Once a note has initially started, it may decay in volume slightly at first - for example a piano note starts instantly with a very loud, percussive bit, then drops quickly to a quieter volume while the note sustains (as the key is held down). The time the note takes to fade from the initial peak to the sustain level is called the decay time. This is very useful when the envlope generator is controlling a filter - by having a short Decay time & a lowish Sustain level, we can have a percussive 'twang' at the start of the note.
The sustain level is the volume of the note while a key is being held down (after the initial attack/decay portion). If you set the sustain level to maximum, then normally you won't be able to hear the decay part of the envelope - after the attack, the volume is at maximum, but if sustain is set at maximum too then there is nothing to decay down to. If you set sustain at zero, then after the sound has peaked following the attack portion, it will fade to nothing, even if you continue to hold down the key. In this case, the Decay time will determine how fast the sound decays down to nothing. Note that the most common reason for not being able to get a sound out of a synth is because you have the sustain level (and attack + decay times) set to zero!
After the key has been released, the release time determines how long the sound will take to
fade from the sustain level down to nothing. If the release time is set to zero, the sound
will stop instantly as soon as you release the key. If you set it to a high value, the note
will continue to sound, fading away, after you release the key.
Note that if you now play another key on a monphonic synth, the original note will be cut off and
the new note will start its attack. On a polyphonic synth, when you press a new key the new
note will start, while the the original note would be left to finish its release (if there
are enough free voices).
Not all envelope generators have the full ADSR sections. Some simpler synths have only Attack,
Sustain and Release (which may be called Decay).
The envelope generator output does not just have to be applied to the VCA. It can also be applied
to other parts of the synth. It is normal to allow the VCF to be modulated by the main envelope
generator (or even to have a separate envelope generator dedicated to the VCF). This allows you
to make tonal changes to the note while it plays. A great example is the 'squelchy' bass sounds
used for most electronic dance music. By having a zero attack, short decay and zero sustain level,
on the VCF envelope generator, you get a sound that starts with the filter wide open, then quickly
sweeps down to fully closed. This gives a dramatic 'peow' sound (particularly if you have some
resonance on the filter).
The Low Frequency Oscillator (LFO)
The LFO produces an output frequency, as
does the VCO.
However, while the VCO produces an
audible frequency (from, say, 20 Hz up to 20 Kilohertz), the LFO produces a signal with a relatively
low frequency - it may range from 1 cycle a minute up to maybe 10 times a second (10 Hz).
The waveform is usually a sine or triangle wave, though some synths let you vary this.
The LFO is not used to generate a signal that you can hear, but is used to modulate the other
parts of the synth to generate effects.
It is normally used to modulate:
By applying a fairly fast (say 5Hz) sine wave voltage from the LFO to the main
oscillator, we can
produce vibrato. This is a 'wobbling' of the frequency which can sound really nice if used
sparingly. You get a similar effect when you 'wobble' your finger on a guitar fret when playing
a note. The output of the LFO is usually fed to a Modulation Wheel before going
to the
oscillator. This allows you to control the vibrato level in real time - great for solos.
Lower frequencies and more modulation can be used to give siren type effects (e.g. Throbbing
Gristle's Hamburger Lady).
A fairly fast LFO can be used to give a sort of bubbling effect when applied to the filter. A very slow (0.1hz) LFO sweep can be used (particularly with a lot of resonance on the filter) to give a constant swooshing up/down (eg the TH303/Bassline bass sounds on House records).
Modulating the VCA a little with a fairly fast LFO gives a tremolo effect (eg as used on electric pianos). It's not normal to use a slow LFO on the VCA 'cos all you'll get is a slowly changing volume!
The Modulation Wheel
The pitch bend wheel is simply a convenient method of controlling the level of the LFO signal sent to the oscillator, filter or VCA. It allows you to 'wang' the vibrato right up at a particularly expressive point in your solo, or fade in the deep filter sweep as and when you need it. It depends on the make of synth whether or not the modulation wheel will allow you to control modulation of the filter or VCA - many wheels are hard coded to only allow oscillator modulation (ie vibrato). Note that on some synths (eg Roland) there is not a separate modulation wheel. Instead, the pitch bend lever can be pushed forward to produce LFO modulation. Some synths (eg Korg MS10/MS20) do not have a modulation wheel at all - there is a control wheel but you will have to patch it yourself to control the level of LFO modulation. For vibrato, this would involve using the wheel output to control the level of the separate VCA, then feeding the LFO output to input of the VCA and taking the VCA output to the VCO modulation input. So, when you push the wheel up, you 'open up' the VCA and let more of the LFO signal through to the VCO. Complicated! But that's the price you pay for having so much flexibility!
