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ABOUT ANALOGUE SYNTHESIS

OSCILLATORS - pitch

To understand synthesis it is necessary to have some understanding about sound itself. Sound is a vibration or oscillation. These vibrations create changes in air pressure which is picked up by your ears & is perceived as sound. When dealing with musical sounds the vibrations or oscillations occur at regular intervals & are perceived as the “Pitch” or “Frequency” of a sound. The simplest musical sound is a sine wave because it contains only one “Pitch” & is perceived as a very “Pure” tone similar to a whistle. Most musical sounds consist of several different “Pitches” or “Frequencies”. The lowest is referred to as the “Fundamental” & determines the perceived “Pitch” of the note. The other frequencies present are called “Harmonics” & in musical sounds usually occur in multiples of the fundamental frequency. i.e. if the fundamental note is 440Hz then a musical harmonic series would be 2nd harmonic = 880Hz, 3rd harmonic = 1320Hz, 4th harmonic = 1760Hz, 5th harmonic = 2200hz etc. The number & loudness of these “Harmonics” determines the “Timbre” of a sound. This gives a sound character & is why a violin sounds different from a guitar & a piano sounds different again. In an Analogue synthesiser you have the choice of several different waveforms. Each waveform has different amounts of harmonics & so the “Timbre” of each one is quite different. Below are descriptions of some of the waveforms & indications on what they can be best used for.

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Sawtooth waves have all the harmonics of the fundamental frequency. As you can see every harmonic has half the amplitude of the previous one. This sound is pleasing to the ear & is useful for basses, leads, & synthesising stringed instruments.

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Square waves have only the odd harmonics present. These are at the same amplitudes as the odd harmonics in a saw wave. Square waves have a hollow / metallic sound to them & so are useful in creating unusual synthesiser sounds & oboe like sounds.

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White noise has no fundamental & all harmonics are at the same level. This wave can be used by itself to synthesise explosions or wind & when used in conjunction with other waveforms can be used to create the illusion of “Breath” in an instrument.

PWM ( PULSE WIDTH MODULATION )

The choice of waveform is important as it determines the basic “Timbre” of the sound you are making. There are additional methods of synthesis that allow more harmonics to be generated. The First of these is Pulse Width Modulation. PWM for short. Essentially the duty cycle of the normally symmetrical square wave is varied. This means the wave form goes from a Square wave to a Pulse wave like so:

This has a very pleasant “thickening” chorus like effect & is often used in Pad type & String section type sounds.

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ABOUT ANALOGUE SYNTHESIS

Additionally there is a synthesis method called Oscillator sync. This takes two oscillators & uses one ( the Master ) to reset the other ( The Slave ) each time it starts a new cycle. The effect is most noticeable when the two oscillators are out of tune as shown below.

This Sync Effect creates very piercing & metallic sounds & are used a lot as lead sounds. It is worth noting that the Nova does not require 2 oscillators to create this effect. The Sync “Effect” is created by the Analogue Sound Modelling process without the need for a Sync Oscillator, there is merely a “Sync” parameter that creates the classic Sync Effect. This means that each of the 3 oscillators in one Nova “voice” can be independently Sync’ed as if there were 3 Master & 3 Slave oscillators.

Analogue Sound Modelling technology also enables the creation of some new “Sync” related parameters that are not found on analogue synthesisers. These are “Key Sync”, “Sync Skew” & “Formant Width”

Normally in an analogue synthesiser even though the Master & Slave oscillators are detuned relative to each other, they both track keyboard pitch equally. i.e. if you play notes one octave apart, both the Master & the Slave oscillator will be transposed one octave. “Key Sync” allows the slave oscillator to have its pitch tracking adjusted independently. This means that the “Sync Effect” will change as you play different notes up & down the keyboard.

Sync Skew manipulates the frequency of the “virtual” slave oscillator within one cycle of the master oscillator. As can be seen the effect is the Sync Effect seems to have a higher frequency at the end of each cycle with positive modulation & at the start of the cycle with negative modulation. This parameter makes the sync waveform sound even harsher. This is particularly good for aggressive lead sounds.

Normal Saw Sync Waveform

Positive Skew on a Saw Sync Wave

Negative Skew on a Saw Sync Wave

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ABOUT ANALOGUE SYNTHESIS

Skew also effects the standard Square & Saw waveforms. The effect is to “squash” the waveform at the end of its cycle with positive modulation & squashing the waveform at the beginning of the cycle. On a square wave moderate amounts of this effect produce similar effects to Pulse width modulation except width modulation over 100% can be achieved allowing may cycles to be “squashed” into one original one. This can also be described as Frequency Modulation within the cycle & so mimics classic “Cross Modulation” with a Saw wave. This can produce effects similar to Sync but when this parameter is used in conjunction with Formant Width the results can be very different. Below are examples of Skew on standard Square & Saw waveforms. Note how the wave is squashed at one end & more than one cycle has been squashed into the original cycle.

Postive Skew on a Saw Wave

Positive Skew on a Square Wave

Formant width is a parameter that controls the level of the cycles of the “virtual” slave oscillator. This can be used to simulate resonance within the oscillator itself by using the “Soften” parameter to smooth out the sharp edges of this wave form. As can be seen the effect is to reduce the level of every successive slave cycle. Additionally this parameter has an effect on the normal Saw & Square waveforms. The effect is to boost the treble content of these waves.

Both Skew & Formant Width can be used in conjunction to create yet even more waveforms. Below are examples.

Negative Skew & Formant Width on a Saw Wave Negative Skew & Formant Width on a Saw Wave

Analogue Sound Modelling technology allows even more control over the waveform. Once you have selected your basic oscillator waveforms you can further modify then using a “Softening” process. This “Softening” rounds off all the “Sharp” edges of the waveform, thereby reducing it’s harmonic content. Below is an example of the “Soften” parameter on a Square wave.

The Soften parameter is completely variable & as can be seen can reduce a square wave to only one harmonic producing a Sine wave. The Soften process can also be applied to the noise generator providing control over the harmonic content of the noise. Below is an example of the “Soften” parameter on Pink Noise.

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ABOUT ANALOGUE SYNTHESIS

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Harmonics

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Harmonics

Finally there is Ring modulation. This uses two oscillators but instead of adding them together as would happen in a mixer, they are multiplied together. This is very similar to FM & produces the kind of effect shown below:

The Ring Mod effect creates metallic & bell like sounds & is used generally for lead sounds but if used subtly can produce Electric Pianos etc & if used radically can produce unusual sound effects.

All these methods further enhance the basic Oscillator waveforms to include many more or a useful mix of harmonics. Once the waveforms have been selected you can then “fine tune” the harmonic content of the mixture of different waveforms by passing them through a “Filter” to remove unwanted harmonics. The filter in an Analogue synthesiser is a very powerful “Tone Control”. Like the tone control on a stereo, the filter can alter how things sound but it cannot change the style of music being played on the record, & so the filter in a synthesiser can alter the “tone” of a sound but is restricted by the basic “Timbre” of the waveforms. For this reason, several waveforms are available at once & you can “Mix” them together to provide more harmonically rich waveforms. Below is a diagram showing the signal path in the Nova & all the waveforms at various locations.

As can be seen different waveforms are being produced by different oscillators using different techniques. The Oscillators, Ring Modulators & the Noise Generator are all being Mixed together & feed to the filter. The signal is then in turn fed to the Amplifier. Oscillator 1 is using a Square wave modulated by Skew & then Softened. To create a sine-like wave except it has an extra bump in it this produces a Whistle like sound. Oscillator 2 is using a Saw wave modulated by Skew & Sync producing a Harsh sound & Oscillator 3 is using a Square wave modulated by Skew & Formant width to produce a bright PWM like waveform. The 1*3 Ring modulator & the 2*3 Ring modulator are producing complex waveforms & these along with all the Oscillators & the Noise generator are fed to the Mixer.

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ABOUT ANALOGUE SYNTHESIS

FILTER - tone

There are several different types of filter. These are Low Pass Filter, High Pass Filter & Band Pass Filter. The Low Pass Filter allows harmonics below a set frequency to pass through the filter. Hence the name Low Pass Filter. The High Pass Filter allows harmonics above a set frequency to pass through the filter. Hence the name High Pass Filter. The Band Pass Filter allows harmonics at a set frequency to pass through the filter, the harmonics above & below the set frequency do not pass through the filter. Hence the name Band Pass Filter. Below are the frequency response curves of the three types of filters.

Additionally the slope of the curve at which the filter rejects unwanted harmonics can be altered. The effect is similar to a “Q” control on a parametric EQ. In the 12dB position the Cutoff Frequency slope is less steep so the higher frequencies are not attenuated as much as they are in the 24 or 18dB positions. This makes the resulting filtering in the 12dB position more subtle than the 24 or 18dB positions which you should select if you want the Cutoff Frequency to be more obvious. The slope is measured in dB per Octave & below are the response curves of a Low Pass Filter with 24, 18 & 12 dB per Octave slopes.

All these filters have a Resonance parameter. This has the effect of emphasising harmonics at the cutoff frequency of the filter. This is very useful for creating large tonal differences to a basic waveform. The effect is shown below as both frequency response curves when resonance is applied in a the Filter.

Frequency Frequency Frequency

Band Pass Filter with Resonance Responce Curves

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