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Create sounds using analogue electronics (Part 10)

Posted: 22 Aug 2012     Print Version  Bookmark and Share

Keywords:Step sequencers  analogue synthesisers  CV  gate 

Part 1 briefly looks into the differences between analogue and digital synthesis, and discusses "one of the major innovations in the development of the synthesiser"—voltage control. Part 2 begins a look at subtractive synthesis with a discussion of VCOs, waveforms, harmonic content, and filters. Part 3 discusses envelopes—the overall 'shape' of the volume of a sound, plotted against time. Part 4 looks at amplifiers as well as other modifiers, including LFOs, envelope followers, waveshapers, and modulation. Part 5 shows how a subtractive analogue synthesiser can be a learning tool for exploring some of the principles of audio and acoustics, and then looks at additive synthesis. The sixth instalment reviews the other methods of analogue synthesis. Part 7 deals with the topology of the modules that make up a typical synthesizer, and then looks at categorizing types of synthesizers. Part 8 looks at how the basic synthesiser technology has changed from valves and transistors towards microprocessors and custom ICs. Part 9 examines tape-based and other analog sampling techniques, such as 'bucket brigade' and acoustic delay lines.

Human musicians can be used for sequencing analogue synthesisers. Left-hand walking bass patterns are one example of a learned pattern that can move from a conscious control to an unconscious control. But sequencing in the context of analogue synthesisers is normally taken to refer to two different types of sequence:

 • Step sequencers
 • CV and gate

Step sequencers
Step sequencers produce pattern loops that are normally 16 notes long, with 8-, 12-, 24- or 32-note variants in some circumstances. The sequences loop continuously once started, playing 16 notes in order, although sometimes they can be stopped with CVs or gates. The typical arrangement of controls is a row of rotary (or linear slider) controls with another row of LEDs above that 'scan' across. The controls are used for setting the pitch by setting the CV that is output when the associated LED is lit.

Slider controls effectively give a 'pitch graph' or map of the notes being played. Sixteen step sequencers are often found on modular synthesisers, particularly for live performance (the scanning LEDs) and for some genres of electronic music (e.g. Tangerine Dream in the 1970s). Step sequencers are normally 1V/ octave, although there were exponential variants and converters between the two types.

The most useful musical feature is a quantiser circuit, which turns the continuous CV from the controls into discrete semitones. Without a quantiser, you should not use a step sequencer if you have perfect pitch. One feature of step sequencers is that they normally play a note for each step of the sequence: rests are unusual and usually are provided by adding a third row of switches to control the output of gate signals. If there are no gate controls, then one technique is to simulate rests by programming in very low notes.

When a modular synthesiser is being controlled by a step sequencer, it is common to patch in a keyboard and perhaps a sample/hold circuit so that notes played on the keyboard will transpose the step sequence. Without this addition, step sequencers can severely restrict the harmonic progression of the music.

CV and gate
CV and gate sequencing were features of some modular synthesisers (e.g. the large EMS systems and the EMS Poly-Synthi) and are more generic variant of the step sequencer, often using a computer to store the CVs, note durations and rest durations. One notable stand-alone example was Roland's MC-8 MicroComposer sequencer, which was introduce in 1977. This allowed the typing in of music as a series of numbers for pitch, note duration and rest duration.

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