[Part 1 briefly reviews the differences between analogue and digital synthesis, and discusses voltage control - "one of the major innovations in the development of the synthesizer."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 synthesizer can be a learning tool for exploring some of the principles of audio and acoustics. Part 6 considers 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 synthesizer 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 synthesizers. 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 synthesizers is normally taken to refer to two different types of sequence:
- Step sequencers
- CV and gate.
1. 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 synthesizers, 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 1 volt/ 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 synthesizer 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.
2. CV and gate
CV and gate sequencing were features of some modular synthesizers (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.
This exacting process, particularly for polyphonic music, could be very time consuming, and editing was primitive with a display that showed just the note time position, pitch, gate and CV details for one note at a time. Storage was on tape cassettes.
Simpler stand-alone dedicated CV and gate sequencers followed, but difficulties with interfacing computers to CV- and gate-based analogue synthesizers meant that it was not until MIDI that general-purpose computers really started to play a role as sequencers. Once MIDI has become widely adopted, and computer-based MIDI sequencers were developed, then MIDI-to-CV/gate converters were used to enable analogue synthesizers to be controlled by a MIDI sequencer.
It is worth considering the number of cables and converters that may be encountered in an analogue synthesizer sequencing environment. The synthesizers will probably have a power supply cable, plus one or more audio output cables. CV and gate cables might be augmented with additional CVs to affect filter cutoff or envelope decay/release time.
Synchronization of a sequencer with a tape recorder, video playback, drum machine or other sequencers might require the use of standards like DIN-Sync 24, which was used before MIDI to provide synchronization with 24 pulse-per-quarter-note timing signals, plus a start/stop signal, or MIDI Time Code or conversion between them. One volt/octave and exponential CV systems might require conversion, and there were several different 'standards' for what constituted a gate signal, with corresponding converters.