Graham s Guide to Synthesizers (part ) Analogue Synthesis Synthesizers were originally developed to imitate or synthesise the sounds of acoustic instruments electronically. Early synthesizers used analogue circuits to produce electronic sounds. The analogue synthesizer can be broken down into 3 basic areas: Oscillator (also called VCO or Voltage Controlled Oscillator) Filter (also called VCF or Voltage Controlled Filter) Envelope Generator (there may be more than one of these) Oscillator The Oscillator produces an electronic waveform. By applying different control voltages (from a keyboard) to the VCO, the user can change the frequency or note that the VCO produces. The user can select the shape of the waveform produced, and also the octave range of the oscillator. Waveforms produced in analogue synthesizers are typically: Sawtooth Triangle Square Pulse The pulse waveform is really a square wave with a variable width. If a synthesizer produces a pulse wave, there will usually be a control for varying the pulse width. These waveforms are chosen because they are simple to produce using analogue circuits, and they are harmonically rich (they sound fat). It is common for analogue synthesizers to have two oscillators. This allows two different waveforms (e.g.. square and sawtooth waves) to be added together, producing more complex waveforms. The oscillators can be tuned to the same pitch and synchronised. They can be tuned to musical (and non-musical) intervals or they can be slightly detuned to produce a doubling or chorus effect. Often, a synthesizer will have a noise generator, which produces white noise - a hissing sound, useful for simulating breath noise. Filter For a greater variety of sounds, these basic waveforms must be re-shaped into more complex waves. This is done using a circuit called a Filter. If you have used an amplifier with Bass and Treble controls, then you have used a filter. Turning just the treble down will filter out the high frequencies, allowing only the low frequencies (the Bass) to pass through the filter. This is referred to as a Low Pass Filter. High pass filters work in the opposite way (they affect the low frequencies without changing the high frequencies). Another filter is the Band Pass Filter. In this kind of filter, the user can control the
amount of cut or boost, and choose the range in the frequency spectrum where the filter will act: high frequencies, low frequencies, or somewhere in the middle. Often this kind of filter has a third control called Q, which sets the width of the frequency band. Examples of band pass filters are the mid sweep EQ controls found on mixers, or wha-wha pedals used by electric guitarists. In a wha-wha pedal, the pedal mechanism produces a voltage change, which is used to sweep the frequency of the band pass filter up and down. Envelope Generator Natural sounds are far more complex than simply passing a waveform through a filter. Sounds change over time both in tonality (the harmonic content or timbre) and also in amplitude (loudness). To achieve these changes the synthesizer uses something called an envelope generator. This produces a pattern of voltage changes, which can be used to control different aspects of the filter, the amplitude of the sound (via a Voltage Controlled Amplifier or VCA) and even the pitch of the waveform. Basic Envelope Generators usually have 4 parameters: Attack (the time the sound takes to start) Decay (the slight dying away after the attack) Sustain (the level the sound plays at while the key is held down) Release (the rate of decay after the key is released) ADSR Envelope Characteristics Amplitude Decay Sustain level Attack Release Note on Note off Time Often a synthesizer will have two envelope generators; one for the VCA, controlling the loudness of the sound; and one for the filter (VCF), usually controlling the cut-off frequency of a low pass filter or bandpass filter.
Other Features In addition to the regular oscillators or VCOs, there is normally another oscillator called the Low Frequency Oscillator or LFO. The LFO has a frequency range of between 0. Hz ( wave cycle every 5 seconds) and 0 Hz (0 cycles per second). This is used for producing vibrato effects, and police sirens! There are usually controls to vary the amount of Envelope Generator/VCO interaction with the filter, and there may also be a Keyboard Tracking control, which can vary the envelope amount or envelope time over the length of the keyboard - so that higher notes could have faster attack and decay times than lower notes. Other controls allow interaction between the LFO and the filter section allowing for the creation of some fairly complex sounds. Some models of synthesizer used a series of jack sockets and patch leads to connect different sections of the synthesizer together in different ways (hence the term Patch Name ). Manufacturers of early synths supplied factory pre-set sounds in the form of large paper cards with holes cut out to correspond to the controls on the synthesizer. These cards were dropped over the main panel so that controls could be turned to the marked positions. Most synthesizers between the 960 s and the early 980 s were Monophonic - they only played one note at a time. Some famous synthesizers of this period are the Moog MiniMoog, Roland SH0 and RS0 and Korg MS0, MS0. Oktave Kat, and the rather horrid Wasp synthesizer. Some major drawbacks of analogue technology were tuning stability (as the circuits warmed up the instruments went out of tune) and the complex electronics required to produce polyphonic instruments (the PolyMoog was for very rich people only). In the early 980 s digital technology became more affordable, and companies such as Roland and Korg began producing 3 and 4 note polyphonic synthesizers. Sequential Circuits began producing the Prophet synthesizer (Prophet 5 being their best known). All of the analogue designs described here use a system called Subtractive Synthesis to produce sound - take a complex waveform and remove some of the harmonic structure (by filtering). The advent of cheaper digital technology in the early 980 s was set to revolutionise synthesizer design.
Graham s Guide to Synthesizers (part ) The Digital Revolution The early 980s saw several changes in the way that electronic instruments produced sound. FM sysnthesis, low cost digital sampling, and sample & synthesis or S&S all radically changed the way that keyboards sounded. FM Synthesis In about 98, Yamaha released a new breed of synthesizer using FM synthesis - their best selling model being the DX7. This machine offered 6 note polyphony (previous synthesizers offering between and 8 note polyphony), velocity sensitive keys and MIDI connections. Instead of using subtractive synthesis based on square & saw waves, filters & ADSR envelopes, the DX7 used something called FM synthesis, which combined sinewaves in something called algorithms, using complex envelope generators. FM stands for Frequency Modulation, which is where the frequency of one oscillator is used to vary the frequency of another. Here s a simple explaination of how this works. In an analogue synthesizer, the low frequency oscillator (LFO) can be used to vary the pitch of the main oscillator - producing a vibrato effect: Main oscillator (VCO) Low frequency oscillator (LFO) Combined where LFO varies the frquency of VCO The LFO uses frequencies typically between 0.5 Hz and 0 Hz - frequencies just below most people s hearing. We hear the effect that the low frequency modulation has on the higher frequency, but we don t perceive this sound as being a different waveform; we just hear the original sound with some vibrato. In this example the VCO acts as a carrier wave for the LFO frequency - allowing us to hear it. If both oscillators were to use the same frequency, the resulting waveforms become extremely complex. In the following examples, two sinewaves are combined. (The frequency of sinewave is double that of sinewave.) In example a, the of the two sinewave oscillators is combined in parallel. No frequency modualtion is taking place, this is simply two notes an octave apart playing at the same time. Note that if the two oscillators were tuned to the same frequency, the from this example would be a slightly louder sinewave.
Initial Waveforms Combining Algorithm Output Waveform + a Sine mod b mod Sine c mod d In example b, the frequency of one oscilator is being modulated by an identical frequency. The waveform produced here is no longer a sinewave. In example c the frequency of the second oscillator is double that of the first, and in example d the positions are reversed, so that the first oscillator has a frequency double that of the second. As you can see, the waveforms produced are much more complex than the original sinewaves, having a much more complex harmonic structures. Being built from sinewaves, these waveforms can be used to produce very natural sounding synthesized instruments - the DX7 was particularly good at producing tuned percussion sounds such as bells and marimbas, as well as very convincing flute and brass sounds. This is a very simplified example. The DX7 didn t simply use two sinewaves, it used six. These could be arranged in one of thirty-two algorithm shapes which combined diferent numbers of series and parallel oscillators. Each oscillator had it s own six stage envelope generator which controlled it s amplitude, and thus the amount of FM interaction between oscillators. Each oscillator also had it s own pitch envelope generator, and the initial sinewaves could be set at any frequency. The DX7 was an incredibly complex machine to programme, but still produces some of the best synthesized sounds to date.