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.