What is an Analog Synthesizer?

 

Most modern synthesizers use some form of digital synthesis method, where sounds are created as a series of numbers and 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 and 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 and 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 were really big, often they took up entire walls! (look at the modular Moog synths used by Keith Emerson, Walter/Wendy Carlos, etc.)

Later, the key modules were put together in one box, with a standard connection hard wired in and with keyboard attached. These synthesizers were less flexible than the big modular monsters, but were a lot more portable and easier to use (the Minimoog is the classic example here).

 

The components of an Analog Synthesizer

An analogue synthesizer essentially consists of 3 main components – a voltage controlled oscillator (VCO), a voltage controlled filter (VCF) and a voltage controlled amplifier (VCA).
The VCO produces the 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, this four behaviours are often referred as ADSR (Attack, Decay, Sustain, Release). 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).

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.

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 and 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 and 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 and 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:

• Sawtooth Wave
  It’s a very bright sounding waveform, good for trebly sounds.
• Square Wave
  Good for bass and deep bass sounds. In many synthesizers you can vary the pulse width of the wave.
• Noise
  It’s a random mixture of all frequencies, it sounds like the hiss you get on a radio when your are not tuned into a station. The noise waveform can be of two types:
  – White Noise : which is truly a random waveform
  – Pink Noise : in which the lower frequencies get more amphasis, producing a heavier hiss.
  Noises are good to generate percussive sounds like snares and handclaps, but are also good to generate sounds effects like the wind sound, the sea, the breath effects and other wind instruments.

 

The Voltage-Controlled Filter (VCF)

The filter is the module That shapes the tonal qualities of the sounds, it works by removing or simply reducing certain frequencies from the sound. There are three types of filters:

• Low Pass Filter
  Removes or reduces high frequencies from the sounds, “Low Pass” infact is referred to the fact that only low frequencies are allowed to pass throught the filter, the cut frequency  (the frequency at which the filter starts to remove or reduce the upper frequencies) can be adjusted by the user.
• High Pass Filter
  Opposite to the Low pass Filter, the High Pass Filter is used to remove or reduce the frequencies lower than a certain frequency.
• Band Pass Filter
  Is a combination of the low and High Pass Filters: you define the cut frequency and the lower and upper frequencies will be removed or reduced.

Filter Settings

Filters have two main parameters which may be used to shape the sound:

• Cut off frequency
• Resonance

Cut off frequency determines the frequency at which the filter starts to take effect, this base frequency can be changed (modulated) by variuos control voltages like envelope generators and LFOs, for example, to give filter sweep effects.

Resonance is also known as Peak or Q. It is the amount of the output of the filter that is fed back into the input. Its effect is to emphasize the frequencies just around the cut off frequency.

 

The Voltage Controlled Amplifier (VCA)

The VCA controls the volume of the sound, but it’s more than a straight volume control. Like VCP & VCF it can be controlled by a voltage dynamically like an envelope generator to shape the liudness of contour of the sound.

You can generate interesting effects with a VCA and an envelope generator, for example you can define how the sound volume should increase the a note is generated or how the the volume should decrease when the note is released, you can also create effects like the tremolo and many other sound behaviours.

 

The Envelope Generator

The Envelope Generator does not produce any sound by itself, instead it is used to control other modules as stated above.

A good envelope generator has at least 4 sections:

• Attack
• Decay
• Sustain
• Release

These sections determines what happens at certain points during the “life” of a note, often this sections are called ADSR (Attack, Decay, Sustain, Release).

Attack Time

The attack portion determines how the note will start. It is the time the sound will take to go from silence to full volume.

Decay 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.

Sustain Level

The sustain level is the volume of the note while a key is being held down (after the initial attack/decay portion).

Release Time

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.

The Low Frequency Oscillator

The LFO produces an output frequency like the VCO but while a VCO produces and audible frequency the LFO produces a signal with a relatively low frequency usually ranging from 1 cycle a minute up to 10 times a second (10Hz). The waveform is usually a sine or a triangle wave but it depends on the synth itself. The LFO is used to modulate the other parts of the synth mainly to generate effects.

That’s all for now! Have fun with your analog synthesizer!