Analog sequencer

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Synthesizers.com Q960 Sequential Controller module

An analog sequencer sequentially steps through a series of adjustable CV/gate signals to control synthesizer modules, e.g. oscillators from the control voltage and envelope shapers from the gate signal.[1] Typically it is used to produce repeating 4 to 16 note tonal patterns or arpeggios.[2]

Description

The simplest form of analogue sequencer succesively progresses through a number of steps. The active step is indicated by an illuminated LED and a potentiometer or slide-fader at each step determines the output voltage. An internal or external clock generator causes the sequencer to step from one pot to the next, and from the final step back to the first. If the CV output is between 0V and +2V and is patched into the pitch CV input of a 1V/octave oscillator, the pitch can be set anywhere in a two-octave range. This makes it possible to construct simple melodies that repeat continuously while the clock is running. The configuration of the pots or faders on the sequencer exactly mimics the notes as they would appear on a musical staff.[3]

A more complex sequencer will offer further features e.g. multiple rows of sequences, multiple outputs, individual variable clock inputs for the rows, CVs for the clocks themselves, range controls for the CVs, note-skip capabilities, variable sequence lengths, and others. However the underlying concept remains the same.[3]

Voltage Quantisation

The CV pots are capable of producing any voltage within their range, including values that lie between the notes in a conventional musical scale. For precise tuning a Voltage quantiser, or simply a quantiser is required. This device rounds every voltage passing through it up or down to that which produces an exact semitone. No matter what the input voltage may be, the quantiser output is the nearest 1/12th Volt defined by the well-tempered scale for a 1V/Octave synthesizer. This makes it much easier to create recognisable melodies or bass lines.[3]

Circuitry

Traditionally most analog synths sequencers relied upon CMOS counter chips, working like simple shift registers which just pass a value along. More recent CMOS analog sequencers such as the Klee or CGS13 use actual shift register ICs instead. With these since the bits are accessible, logic, switching, and other functions can be applied within the sequence.[4]

Programmable analog sequencers use a microprocessor or microcontroller to set the count. These are capable of more modes than simple logic chips.[4]

Compared to MIDI

Analog sequencers have no audible difference to MIDI. Neither generate audio both pass data but the data is encoded differently, CV/gate vs. encoded 0s and 1s. Analog sequencers will never be as precise nor as algorithmically versatile as digital sequencers. However they're said to be more intuitive, better for hands-on interaction or live performance and feature different functions such as:

  • Controlling modules or devices with CV/gate interfaces.
  • Not just oscillators but sequencing other modules such as filters, LFOs or envelopes.
  • The ability to vary tones without regard to a standard musical scale.
  • To control parameters such as the tempo of the sequence from the output of other modules, to create interacting sequences.
  • Triggering the sequencer at audio rates thus using the sequencer to create unique waveforms.[4][5][6]

DIY analog sequencers

See also

References

  1. ^ Electronic Synthesiser Projects by M.K. Berry, Bernard Babani, 1981, ISBN 0-85934-056-2, Ch. 4 Programmable Sequencer
  2. ^ MFOS 16 Step Sequencer
  3. ^ a b c Synth Secrets, Part 16: From Sample & Hold To Sample-rate Converters by Gordon Reid, SOS August 2000
  4. ^ a b c Sequencer Analog / Digital, Muff Wiggler forum, December 2013
  5. ^ Best analog sequencers and why?, Gearslutz.com forum, Feb. 2010
  6. ^ What do analog sequencers offer over midi seq's?, Gearslutz.com forum, Jan 2011

Further reading

  • Electronic Music Circuits by Barry Klein, 1996, ISBN 0-672-21833-X, pp. 64-66, p. 210. Two circuits one based on CMOS 4017B decade counter and another only with 555 timer ICs.
  • Electronic Synthesiser Construction by R.A. Penfold, Bernard Babani, 1986, ISBN 0-85934-159-3, Ch. 3 Sequencing. Circuits for 4017BE based ten step gate sequencer or CV/gate sequencer and 4067BE sixteen way analogue switch based CV/gate sequencer.
  • Electronic Synthesiser Projects by M.K. Berry, Bernard Babani, 1981, ISBN 0-85934-056-2, Ch. 4 Programmable Sequencer. Construction details, CV/Trigger, two synchronised channels of up to ten steps each, decade counter based.
  • Music Synthesizers: A Manual of Design & Construction by Delton T. Horn, TAB Books, 1984, ISBN 0-8306-1565-2, pp. 209-213. 555 timer based circuits.

External links

Schematics

PCBs and kits

An incomplete list:

Music theory