c'h~ -; SUBTRACTIVE SYNTHESIS

TEACHER UNIT GUIDE NOTES

BACKGROUND INFORMATION The word"synthesize" means"the process of combining parts or elements so as to form a whole," or more simply, "to put together." A synthesizer is an electronic musical instrument that combines separate electronic components to create a sound. The "whole" is the final waveform or sound. Early synthesizers were modular in construction. Each module had a specific function and the individual modules were connected together by the synthesist using short audio cables known as patch cords. Later synthesizers became integrated in design; that is, individual components were all pre-wired into a single case with front panel knobs and switches to modify the configuration.

This concept of modular and integrated components is still common in home stereo systems. A modular stereo system consists of separate components including the receiver, tape deck, CD player, and speakers. These components are connected together with audio patch cords. A portable stereo system or "boom box" is an integrated system providing the same components in a self-contained unit that does not require the use of patch cords.

In the mid-1960s, Robert Moog and Donald Buchla created the first widely used modular synthesizers. These early instruments used analog components which were voltage controlled. In an analog synthesizer, the components work together to create an audio signal which consists of electrical fluctuations that correspond directly and continuously with the waveform of the desired sound. (When the audio signal is sent to speakers, the speakers vibrate per those electrical fluctuations to make the sound in the air.)

A voltage control signal is used within a synthesizer module to I control' or shape the audio signal as it travels through that module. Unlike the audio Signal, we do not hear the voltage control signal, we only hear its effect on the audio Signal. The modules are adjusted by the use of knobs, switches and other devices which vary the amount of control voltage present in the component. Thus, for example, turning a certain knob on the synthesizer would result in a continuous sweep in pitch. . - .

Interestingly enough, analog synthesizers have made a comeback in recent years. A number of musicians and producers prefer the "warm" or "fat" sound of the analog components. Several companies are currently manufacturing analog synthesizers and older analog instruments are in great demand. Computer programs have also been written that create "virtual analog" synthesizers digitally. Advanced physical modeling synthesis is also being used to create "analog sounding" electronic instruments (see unit 5 for more information). In Unit Two, The Physics of Sound, we identified three components of sound: pitch, loudness, and timbre. With any musical instrument these three elements are manipulated to create the desired sound. In the case of a violin, for example, pitch is controlled by tension and length of the strings, loudness via the bow, and timbre by the materials and design in construction of the particular instrument (as well as the skill of the performer in playing the instrument to its potential). In much the same way, components of a synthesizer are combined and controlled to achieve the desired effect. We will consider each of these components individually.

AUDIO MODULES Pitch As with all musical instruments, synthesizers start with a sound source, i.e., something which makes the initial vibration. With a violin, the sound source is a string; with a clarinet, it is a reed. With a synthesizer, it is an oscillator. Oscillators create a "vibration" in electric current which is ultimately translated to a speaker so that it can be heard.

In analog synthesizers, the frequency (pitch) of the oscillator is controlled by the amount of voltage present in the oscillator; hence it is called a "voltage-controlled Fig. 3-1 oscillator (VCO)." The frequency knobs on the VCO effects the amount of voltage present; therefore, turning the knobs raises or lowers the pitch of the oscillator continuously. On the oscillator module there are usually two frequency controls, one for coarse or broad range tuning and one for fine tuning. It is also possible to use a "controller module" to vary the amount of voltage that varies the frequency of the oscillator. These controller modules will be discussed in the next section.

Loudness While adjustable in pitch, oscillators are not adjustable in loudness. Variations in Fig. 3-1 loudness are achieved with a voltage-controlled amplifier (VCA). As the signal from an oscillator is passed through a VCA, the VCA increases or diminishes the amplitude of the signal, making it louder or softer. In an analog synthesizer the VCA is voltage-controlled, so turning the knob to increase or decrease its voltage makes the sound continuously louder or softer. Controller modules can also be used to supply the voltage to control a VCA.

The amplifier module in a synthesizer should not be confused with an external , power amplifier. The VCA produces a low voltage line level signal that must be further fed into an external power amplifier. The external power amplifier boosts the line level signal to enough wattage to power the speakers in a sound system. The output of a synthesizer is only strong enough to power a set of headphones.

Timbre Timbre is primarily affected by two factors. First is the oscillator itself. The oscillator creates a pattern of electrical fluctuations in a waveform that parallels the waveform of the sound desired. Typical waveforms in analog synthesizers include the sine wave trian Ie wave sawtooth wave s uare wave and ulse wave. Each has its own unique harmonic content. (See Unit 2.) Noise is also available as a choice on most synthesizers, creating a non-pitched sound source - i.e., there is no prevailing fundamental pitch; all harmonics are randomly present. Noise is useful for many effects such as wind, surf, or explosive sounds. The waveform of the oscillator is selected with a switch on the module.

Fig. 3-1 Timbre is also varied by a voltage-controlled filter (VCF). As with any filter, which allows certain things to pass through while holding others back (a coffee filter, for example), so the VCF allows certain frequencies to pass through while holding back others. The most common type of filter used in analog synthesizers Fig. 3 - 2 is a low-pass filter. It filters out higher harmonics while allowing the lower harmonics to pass through. The point at which frequencies are allowed to pass or not is called the cut-off frequency, and is controlled by the amount of voltage present in the filter module. Thus, turning the cut-off frequency knob on the filter allows more or fewer harmonics to pass through, making the sound brighter or duller in timbre. This method of starting with a harmonically rich waveform and then using filters to subtract out certain harmonics is what gives this approach to synthesis its name- subtractive synthesis. Since the filter and amplifier change or modify the timbre and loudness of the sound source (oscillator), they are often referred to as modifiers.

There are other types of filters available on some synthesizers. One other fairly Fig. 3 - 3 common filter is the high-pass filter, which allows higher frequencies to pass through while blocking out lower harmonics. Another somewhat less common Fig. 3 - 4 filter is the band-pass filter. This filter removes the frequencies above and below the cut-off frequency. The band-pass filter functions like the mid-range EQ on a mixer.

Fig. 3 - 5 Resonance is another important aspect of the filtering process. Resonance provides a boost to a small band of frequencies centered around the filter's cut-off frequency. Resonance is also called Peak or Q on some synthesizers. The effect is similar to the way that certain acoustic instruments have an emphasis on a small band of harmonics. Resonance is used to obtain the characteristic "wOw" sound popular in early analog synthesizers.

CONTROLLER MODULES What makes most acoustic sounds interesting is that they change over time. Consider a piano note:- If a~lower note on the piano is struck firmly and-held down, many changes can be heard. First, the sound becomes softer and softer with time. Also, you will notice that the higher harmonics die out first, so the sound begins bright in timbre but becomes duller over time.

In order to make synthesized sounds interesting, they too must change over time. These changes are achieved with controller modules. These modules can be programmed to send a voltage that changes over time to the VCO, VCA, and VCF in order to control their operation automatically. It's important to remember that controller modules do not roduce an audio si al. The roduce control voltages that modify the modules producing the audio signal. We will examine three of the most common controller modules used in subtractive synthesis.

Envelope Generator To begin with, consider the characteristic "shape" of sounds over time. As an example, let's reexamine the loudness of the piano note. When the note is firmly struck, it sounds immediately and audibly loud. Within a few seconds, the loudness decays considerably and then reaches a point where it sustains at a slightly declining level for as long as we hold down the key. When the key is released, the sound dies almost immediately. Figure 3-6 is a graph of the piano Fig. 3 - 6 sound's loudness over time.

Fig. 3 - 7 The envelope generator (EG) is used to give synthesized sounds their shape. On early synthesizers, the envelope generator had four stages: Attack, Decay, Sustain, Release. (The initials ADSR are sometimes used to refer to envelopes.) Figure 3-8 Fig.3 8 is a graph of a four-stage envelope that is similar in shape to the piano sound described above.

On early analog synths, the attack, decay, and release are adjusted in terms of time- the higher the number, the longer the time and the slower each section is completed. Sustain is defined by a level setting that is maintained as long as the key is depressed. While early envelope generators were usually four-stage, more modem synthesizers often include more stages that are each adjustable by both Fig. 3 - 9 level and time (except sustain which is still only a level). See Figure 3-9 for an example of a six-stage envelope.

In order for an envelope generator to affect the loudness of a sound as described, it would be used to control a VCA. It is also possible to use envelopes to control oscillators which would then affect the pitch of the sound over time. Or, one can shape the timbre of a sound over time by using an envelope to control a filter.

Low-Frequency Oscillator Fig. 3-7 The Low-Frequency Oscillator (LFO) is used as a controller to create certain effects. Like a regular oscillator, the LFO creates a waveform; however, the frequency is below the range of human hearing-between about 0.1 Hz and 20 Hz. Although we cannot actually hear the LFO directly, we can hear its effect on another module. When an LFO is set to a sine wave and used to control a VCO, the result is a continuousflse and fall mthe pitch of theVCO. This effect isKnown as vibrato. VVhen the LFO controls a VCA, itcreates a tremolo, which is a continuous rise and fall in loudness. An LFO can also control a filter, causing a continuous fluctuation in timbre between brighter and duller. Typically, LFOs are set to either sine waves or triangle waves, but interesting effects can be gained by using other waveforms as welL For example, using square wave on the LFO to modulate the sound source (VCO) will result in trill effect rather than vibrato.

There are commonly three main controls used for the LFO. The rate determines the frequency of the LFO and the depth sets the amount of modulation. The third variable, delay, allows the LFO modulation to be faded in to the audio module over time.

When an LFO is used to control or change the character of a sound source or modifier it is called modulation. Vibrato and tremolo are two of the most common modulation effects. Almost allkeyboard synthesizers have a wheel or lever on the left side that is used for modulation purposes. It is generally referred to as the mod wheel or mod lever (see Unit 10).

Keyboard On an analog synthesizer the keyboard sends out a voltage to the veo. The higher the key, the higher the voltage and thus the higher the pitch. A feature called key follow or keyboard tracking can also send this control signal to the filter. This will cause the sound to get brighter as the pitch rises in much the same way many acoustic instruments do.

The keyboard also sends out trigger signals which can be routed to an envelope generator. These trigger signals are used to tell the envelope generator to begin its attack each time a key is pressed. (For more on keyboards, refer to Unit 10 on Keyboard Features.)

Fig. 3-10 For a diagram of the typical signal routing in a subtractive synthesizer, see Figure 3-10. Sound Source Modifiers

Voltage Voltage Voltage Controlled Controlled Controlled Oscillator Filter Amplifier ® ® Frequency Cut-off Coarse Frequency

® ® Level Frequency Resonance Fine BandPass N'-®rtJ1 LOw~ High Pass \t:::} Pass Waveform Mode @@ @ @@ Audio Audio Audio Output Audio Audio Input Output Input Output @@@ @@@ @@@ Control Voltage Control Voltage Control Voltage Input Input Input

Figure 3 -1. A Voltage-Controlled Oscillator, Filter and Amplifier. These are the three primary audio modules in an analog synthesizer.

Cut-off Frequency -, A m p I i t Frequencies u that are heard d e

20 Hz Frequency 20KHz Cut-off Frequency +

Frequencies that are heard

20 Hz Frequency 20KHz

Figure 3 - 3. The effect of a high-pass filter. Frequencies below the cut-off point are filtered out or rejected.

u t =c._. • ''fstttiw'" t mt&-.."----t r¥HttxifrntX Y--'?t' ' @ , i ~rd

A Cut-off Frequency m p + I i t u Frequencies d that are heard e

20 Hz Frequency 20KHz

Figure 3 - 4. The effect of a band-pass filter. Frequencies above and below the cut-off point are filtered out or rejected.

Cut-off Frequency ., A m p + I i t u d e Frequencies that are heard

20 Hz Frequency 20KHz A m p I i t Key release u d e /

Time

Figure 3 - 6. Amplitude (loudness) of a piano sound over time.

"~--~------'We"

Envelope Low Generator Frequency Oscillator

®~ olHZ®20HZ Attack Rate ®~ ® Decay Depth ®~ ® Sustain Delay ®~ f\f\J®11Jl Release Waveform @@@ @@@ Control Voltage Control Voltage Output Output A m p I Key release i t / u Sustain d e

Time

Figure 3 - 8. A four-stage envelope.

1"t ceM ~ '@ wrff

A m p I Key release i t L4 / U d e .,

T1 T2 T3 T4 Time

Figure 3 - 9. A 6-stage envelope. Stages are defined by both amplitude levels (e.g., LI, L2, etc.) and times necessary to reach each level (e.g., TI, T2, etc.). Notice that the sustain stage is defined by only a level (L4), since the time is determined by how long the key is held down. Notice also that the release stage is defined only by a time (TS), which is the time needed to reach zero amplitude after the key is released. Oscillator Filter Amplifier (Pitch) (Timbre) (Loudness) Output ...... Vibrato Tremolo : ...... 0 ...... "" ••

Low-Freq. Envelope Oscillator Generator (1 or more) (1 or more) A O~~Volt~el" In' InImIn IminImfooer

Figure 3 - 10. Typical routing of subtractive synthesis modules. The audio signal is displayed as moving from left to right, and the controller signals are shown moving from bottom to top. This is the standard format for mapping signal routings in analog subtractive synthesis. (Note: envelopes can control oscillators and LFOs can control filters, but these routings are not as typicaL)

" SUBTRACTIVE SYNTHESIS

ADDITIONAL RESOURCES

LISTENING: Switched-On Bach Wendy Carlos CBS Records MK-7194 Th.is recording is considered to be the classic that made the Moog Synthesizer a household word. Recorded in 1968, itcontains transcriptions of several works by J. S. Bach. Carlos recently redid the album under the title Switched on Bach 2000 (Telarc CD-80323).

Secrets ofSynthesis Wendy Carlos CBS Records MK42333 This album demonstrates a variety of synthesis techniques with dialog provided by the composerI author.

READING: What's a Synthesizer? Jon F. Eiche Hal Leonard Books. 1987.

The Secrets ofAnalog and Digital Synthesis Steve De Furia Hal Leonard Books. 1987.

Synthesizer Basics Derived from Keyboard Magazine Hal Leonard Publishing. 1998.

Vintage Synthesizers Mark Vail GPE Books/Miller Freeman Books. 1993. A great source of info on analog synths, it includes street prices and sources for buying and repairs.

Introduction to MIDI/Synthesis Matthew Nicholl CPP Belwin, Inc. 1993.

Synthesizer Performance and Real-time Techniques Jeff Pressing A-R Editions. 1992. I COMPUTER SOFIWARE SYNTHESIZERS: Turbosynth™ Digidesign This is a commercially available program for the Macintosh. The software literally turns the Macintosh screen into a modular synthesizer. Italso works well for demonstration of other approaches such as FM synthesis, additive synthesis, and sampling. A free, demo version of the program is available that will not let you save your work, but is great for demos and experimentation.

Retro AS-l Analog Synthesizer BitHeadz (http://www.bitheadz.com/Retro_AS1.html) This full-featured professional software synthesizer requires a Power Mac. The user interface and graphics are incredible and 1,000 analog sounds are included. A free demo version is available from the web site.

Audio Architect http://www.audioarchitect.com/ A software program for PC (Windows) with a sound card that creates the sounds of many classic synthesizers. Its screen is similar to Turbosynth. A free demo version of the program is available.

Nord Modular Synthesizer Armadillo Enterprises (http://www.c1avia.se/nordmodular/concept.html) Nord utilizes a unique combination of hardware and software to create a virtual synthesizer. The hardware interface is a two-octave keyboard complete with knobs for realtime control. The sounds are generated in software with a PC (Windows)..

Koblo http://www.koblo.com/ This com pany has several analog style software synths for the Mac. The Vibra1000 is a great freeware synthesizer for those who want to get started. Several other models are shareware.

HARDWARE SYNTHESIZERS: FatMan Analog MIDI Synth P AiA Electronics (http://www.qns.com/paia/fatman.htm) The FatMan is two space rack mount MIDI controlled analog synthesizer. Itcomes in a kit form that you assemble yourself (a wonderfulleaming experience). Itis an inexpensive way to purchase an analog synth.

Novation BassStation Novation (http://www.novationusa.com/brskr.htm) The BassStation is a MIDI compatible analog synthesizer that is available either as a single space rack mount module or with a two octave keyboard. The compact keyboard version would be particularly useful for demos and student projects.

Rave-o-Iution 309 Quasimidi (http://www.quasimidLcom/english/qm-309.htm) The Rave-o-Iution 309 is a two-in-one rhythm unit combines a polyphonic drum module with a monophonic bass/lead synthesizer. The unit uses an Analogue Emulation Synthesis method and also features an integrated sequencer. Roland JP-8000 Keyboard Synthesizer & JP-8080 Sound Module Roland US (http://www.rolandUS.com/) The JP-8000 an advanced"analog modeling" (physical modeling) synthesizer with a 61 note keyboard and 38 onboard knobs and sliders. The realtime knobs and sliders let the user edit and create sounds much in the same way that vintage analog synthesizers did.

Yamaha ANIX Synthesizer Yamaha (http://www.yamaha.com/) The AN1X an "analog" physical modeling synthesizer with a 61 note keyboard. Itis one of the first generation of physical modeling based synthesizers that specifically emulates analog synthesis. The unit also has numerous realtime performance features.

INTERNET RESOURCES: Analogue Synthesizers: What is Analogue Synthesis? http://www.warwick.ac.uk/%7Eesaag/analog.htm This site provides a detailed discussion of analog synthesis and includes online audio examples.

Digital Oscilloscope 2.1 http://karserve.ethz.ch/AdS/ mac/oscilloscope/ digital_oscilloscope210.sit. bin Digital Oscilloscope enables you to use a Macintosh computer as an oscilloscope and a frequency meter. This is a great freeware program for analyzing waveforms and sounds.

The Internet Synthesizer http://www.wfmu.org/-jhhl/sinth.html This is an online synth that creates audio samples based on the responses from a form filled out by the user. It simulates the experience of patching together analog synthesizer modules. The synthesizer appears to be somewhat challenging to use and requires that the computer be able to play real audio samples.

Sound View http://www.physics.swri.edu/soundview/soundview.html A freeware realtime sound analyzer for the Macintosh computer that allows you to record, play, view, and analyze a sound in a variety of ways.

The Synthesizer Picture Archive http://www.code404.com/synths/ This site serves as a repository for images of various synthesizers and other electronic instruments.

Synthmuseum.com http://www.synthmuseum.com/ This virtual synthesizer museum is the largest online museum of its kind, displaying vintage electronic musical instruments from all over the world.

SynthZone http://www.synthzone.com/analogue.htm This site provides numerous links to sites related to analog and vintage synthesizers.

Vintage Synth Explorer http://www.vintagesynth.com/ HPTP l'ITP vJ'lriow:; J'lrchi . . . A Module by any other name... The chart below gives the name of common analog modules or terms and similar or equivalent terms found in other analog or digital synthesizers.

Common Analog Synthesizer Term Alternative or Equivalent Term

Voltage-Controlled Oscillator (VCO) Oscillator (OS C) Digitally Controlled Oscillator (DCO) Waveform I Wave

Voltage-Controlled Filter (VCF) Digitally Controlled Filter (DCF) Time Variant Filter (TVF) Variable Digital Filter (VDF)

Digitally Controlled Amplifier (DCA) Voltage-Controlled Amplifier (VCA) Time Variant Amplifier (TVA) Variable Digital Amplifier (VDA)

Low Frequency Oscillator (LFO) Modulation Generator (MG)

Envelope Generator ADSR, EG

Resonance Peak, Q SUBTRACTIVE SYNTHESIS

VOCABULARY LIST attack, decay, sustain, release (ADSR) audio signal band-pass filter cut-off frequency envelope generator (EG) high-pass filter integrated low-frequency oscillator (LPO) low-pass filter modifiers modular modulation mod wheel (mod lever) noise patch cord resonance sound source subtractive synthesis synthesize tremolo vibrato voltage controlled voltage-controlled amplifier (VeA) voltage-controlled filter (VeF) voltage-controlled oscillator (VeO)