Subtractive Synthesis

Subtractive Synthesis is usually associated with analog synthesizers or digital emulations of analog synthesizers. Subtractive Synthesis gets its name from how it uses filters to remove or subtract harmonic content from the sound source to create a new and sometimes evolving sound.

The basic model of a Subtractive Synthesizer is:

1. VCO –> 2. VCF –> 3. VCA

VCO = Voltage Controlled (ANALOG) Oscillator: Sound Source (Pitch)

VCF = Voltage Controlled Filter: Harmonic Content Control

VCA = Voltage Controlled Amplifier: Loudness Control (Volume)

1. Voltage Controlled Oscillator (VCO)

The VCO is the sound source and is capable of producing one or more of the following waveforms:

SINE

SAW

SQUARE

VARIABLE PULSE (PULSE WIDTH)

TRIANGLE

Some synthesizers have a noise generator that usually produces either pink or white noise. White noise contains all frequencies in the hearing spectrum at equal amplitudes. Pink noise contains all frequencies at amplitudes at a perceived amplitude of being equal. A noise generator can be useful in creating sound effects, percussive sounds and more.

The Sine wave is the simplest waveform as it only contains the fundamental with no harmonic content.  The Saw (or sawtooth) wave contains all harmonics in the following ratios:

Amplitude Pitch Ratio

Fundamental 1/1 1:1

1st Overtone 1/2 2:1

2nd Overtone 1/3 3:1

3rd Overtone 1/4 4:1

4th Overtone 1/5 5:1

Etc… to infinity 

Ramp wave (reversed sawtooth waveform) Saw and Ramp waveforms are similar in design; a saw wave falls and then returns back while a ramp wave rises then returns back to the start.

The Square wave contains the fundamental and only odd numbered harmonics in the same ratio as the saw wave.

Fundamental 1/1 1:1

1st Overtone 1/3 3:1

3rd Overtone 1/5 5:1

Etc… to infinity 

Variable Pulse is related to the square wave except that the positive and negative width of the waveform can vary creating unusual harmonics.

The Triangle wave is similar in sound to the sine wave as next to the sine wave, it contains the least amount of harmonic content.

Fundamental 1/1 1:1

1st Overtone 1/3 squared (1/9) 3:1

3rd Overtone 1/5 squared (1/25) 5:1

Etc… to infinity 

Noise Generator: pink or white noise used for sound effects and more. An oscillator is easy to emulate, as it is controlled feedback. If you have ever heard feedback from a PA system, you will recognize the piercing sine wave sound.

2. Voltage Controlled Filter (VCF)

VCF changes the harmonic content of the oscillator.

There are four main types of Filters:

LPF Low Pass Filter: Allows low frequencies to pass and attenuates the highs

HPF High Pass Filter: Allows high frequencies to pass and attenuates the lows

BP Band Pass: A combination of LPF and HPF that allows a specific band of frequencies to pass through the filter.

NOTCH Not as common as the other filter types. It is used to reduce or accent certain frequencies.

Every filter has a cutoff frequency. The cutoff frequency is the point at which a filter begins to take effect (begins to attenuate or reduce harmonic content).

A pole describes how quickly the filter removes harmonic content described in dB per octave.

Given the following poles:

1 pole filter – 6 dB/octave

2 pole filter – 12 dB/oct.

3 pole filter – 18 dB/oct.

4 pole filter – 24 dB/oct.

A 1 pole filter reduces less harmonic content than a 4 pole filter. 2 pole and 4 pole are the most common filter types.

A characteristic sound of analog subtractive is resonance. It is sometimes called emphasis, as it is an emphasis of the cutoff frequency of a filter. The filter sweep is another characteristic sound of analog subtractive synthesis. A filter sweep occurs when the cutoff frequency of a sound is varied over time (usually using an envelope or modulation wheel).

Production Tip: Some filters can be used for more than just the synthesizer’s own sounds. Putting guitars, drums, vocals and other sounds through LPF’s is used in many styles of music including Techno, Trance and Pop.

3. Voltage Controlled Amplifier (VCA)

The VCA controls the amplitude of the synthesizer.

MODULATORS:

ENVELOPES

Envelopes change sound over time. The most basic envelope is an ADSR, which stands for Attack Decay Sustain Release. Envelopes can be applied to several sections of a synthesizer but the most common use of an envelope is for the VCA.

ADSR + VCO = change in pitch over time.

ADSR + VCF = change in harmonic content over time.

ADSR + VCA = change in amplitude over time.

LFOs

Low Frequency Oscillator: an oscillator that produces sound below human hearing (20 Hz) and can be used to modulate any of the three basic Subtractive Synth Modules. The LFO is capable of producing all the waveforms of the VCO plus some more including sometimes a random generator.

LFO + VCO = vibrato

LFO + VCF = wah

LFO + VCA = tremolo

Sample and Hold is a LFO waveform type that takes snapshots of the LFO at set intervals at different rate than the LFO frequency and holds that value for the modulator assignment until the next snapshot is taken. Sample and hold can provide rhythmic effects and can sound really cool when synced to a songs tempo.

Oscillator Sync is an option on some synthesizers (usually in the LFO section) that forces the oscillator to start at the beginning of the waveform each time a new sound is triggered.

Software Virtual Instruments have certainly improved upon the features and tuning issues of the hardware synthesizers of the past. The classic synthesizers, as with most hardware, are easier and faster to program as the interface is tactile instead of mouse operated. But the jury is still out on the stability of the software environment for music production.

By comparison, hardware musical instruments rarely have the compatibility and stability issues that software instruments have. Hardware from thirty years ago is still in use, but software written just ten or five years ago is considered outdated and incompatible with current systems. (Examples: Pro Tools 9 plug-ins are not compatible with version 11, the move from 32 bit to 64 bit). This is not to say that hardware doesn’t need maintenance or repair, as it does.

Comparison between hardware ANALOG synthesizers and virtual ANALOG instruments:

Hardware:

• True analog voltage controlled warm sound
• Oscillator stability and tuning issues
• Tactile user interface Faster to program
• More of a performance instrument as most controls are accessible
• Added benefit of not needing an upgrade as often as software
• Possibility of becoming a lifelong instrument
• Has to be physically connected (cables everywhere)
• Bulky Harder to integrate into a setup

Software:

• As software is updatable, not all software releases are bug free or function as to be expected
• Can be upgraded inexpensively
• If support is dropped, newer OS versions of software may not be compatible
• Steep learning curve
• Have to relearn an instrument with upgrades
• Shelf life dependent on manufacturer support and updates
• With thousands of presets, users tend to dial up presets in a playback mode instead of interacting with the instrument in a performance mode Interface issues
• Dependent upon a computer
• Sterile digital sound – no digital software can sound the same as voltage controlled electronics
• Annoying piracy protection software (key dongles, serial numbers, registration) that seems to punish legit users

Paradox:

#1 The same thing that makes hardware appealing also makes it a pain to use (oscillator instability)

#2 The same thing that makes software appealing also makes it a pain to use (updatability)

Ring Modulation: Ring modulation is a process on some early analog synthesizers that combines two oscillators or sound sources so that the output is both the sum and difference of the harmonic content of the two source sounds. Ring Modulation produces bell-like sounds.

Examples of classic analog subtractive synthesizers:

• Moog MiniMoog
• Sequencial Prophet 5
• Oberheim OB8
• Oberheim Matrix 12
• Roland Jupiter 8

Monophonic vs. Polyphonic Synthesizers

A monophonic synthesizer can only produce one sound at a time. A polyphonic synthesizer can produce several different pitches of the same synth sound by holding more than one key at a time. Monophonic synthesizers are typically used for lead sounds and bass sounds as only one note at a time is needed to play a bass or lead part. The Moog MiniMoog is a classic example of a monophonic subtractive synthesizer. Search for “minimoog” on youtube.com for video examples the mini in action.

Subtractive Synthesis in Sound Design:

Subtractive synthesis was the primary form of sound design for early gaming systems such as the Commodore 64, the original Nintendo Entertainment System, CollecoVision, Atari 2600, any Z80 base video arcade system and more. It is still used today for sound design. The retro appeal of these sounds are making appear in more modern pop/dance songs.

Subtractive Synth Sound Design Examples: Bomb drop, Laser beams, R2D2 robot, Star Trek bridge sounds etc.