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Oscillator sync is a feature in some
synthesizer A synthesizer (also spelled synthesiser) is an electronic musical instrument that generates audio signals. Synthesizers typically create sounds by generating waveforms through methods including subtractive synthesis, additive synthesis and ...
s with two or more VCOs, DCOs, or "virtual" oscillators. As one
oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
finishes a cycle, it resets the period of another oscillator, forcing the latter to have the same base frequency. This can produce a
harmonically In music, harmony is the process by which individual sounds are joined together or composed into whole units or compositions. Often, the term harmony refers to simultaneously occurring frequencies, pitches ( tones, notes), or chords. However, ...
rich sound, the
timbre In music, timbre ( ), also known as tone color or tone quality (from psychoacoustics), is the perceived sound quality of a musical note, sound or musical tone, tone. Timbre distinguishes different types of sound production, such as choir voice ...
of which can be altered by varying the synced oscillator's frequency. A synced oscillator that resets other oscillator(s) is called the master; the oscillators which it resets are called slaves. There are two common forms of oscillator sync which appear on synthesizers: Hard Sync and Soft Sync. According to ''
Sound on Sound ''Sound on Sound'' is an independently owned monthly music technology magazine published by SOS Publications Group, based in Cambridge, United Kingdom. The magazine includes product tests of electronic musical performance and recording devices, a ...
'' journalist Gordon Reid, oscillator sync is "one of the least understood facilities on any synthesizer".


Hard Sync

The leader oscillator's pitch is generated by user input (typically the synthesizer's
keyboard Keyboard may refer to: Text input * Keyboard, part of a typewriter * Computer keyboard ** Keyboard layout, the software control of computer keyboards and their mapping ** Keyboard technology, computer keyboard hardware and firmware Music * Musi ...
), and is arbitrary. The follower oscillator's pitch may be tuned to (or detuned from) this frequency, or may remain constant. Every time the leader oscillator's cycle repeats, the follower is retriggered, regardless of its position. If the follower is tuned to a lower frequency than the leader it will be forced to repeat before it completes an entire cycle, and if it is tuned to a higher frequency it will be forced to repeat partway through a second or third cycle. This technique ensures that the oscillators are technically playing at the same frequency, but the irregular cycle of the follower oscillator often causes complex timbres and the impression of
harmony In music, harmony is the process by which individual sounds are joined together or composed into whole units or compositions. Often, the term harmony refers to simultaneously occurring frequencies, pitches ( tones, notes), or chords. However ...
. If the tuning of the follower oscillator is swept, one may discern a harmonic sequence. This effect may be achieved by measuring the zero axis crossings of the leader oscillator and retriggering the follower oscillator after every other crossing. This form of oscillator sync is more common than soft sync, but is prone to generating
aliasing In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when a ...
in naive digital implementations.


Soft Sync

There are several other kinds of sync which may also be called Soft Sync. In a Hard Sync setup, the follower oscillator is forced to reset to some level and phase (for example, zero) with every cycle of the leader regardless of position or direction of the follower
waveform In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electronic ...
, which often generates
asymmetrical Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...
shapes. In some cases, Soft Sync refers to a process intended to nudge and lock the follower oscillator into the same or an integer or fractional multiple of the leader oscillator frequency when they both have similar phases, similar to a
phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
.


Reversing Sync

This form of oscillator sync is less common. This form is very similar to Hard Sync, with one small difference. In Reversing Soft Sync, rather than resetting to zero, the wave is inverted; that is, its direction is reversed. Reversing Soft Sync is more associated with analog triangle core oscillators than analog sawtooth core oscillators.


Threshold or Weak Sync

Several kinds of Soft Sync use comparison thresholds: * Hard Sync which is disabled when the frequency or amplitude of the follower crosses a user-defined threshold. * Hard Sync which is disabled when the frequency of the follower extends too high above or too far below the frequency of the leader. * Hard Sync which is disabled when the frequency of the follower is lower than the frequency of the leader. Soft Sync may accurately refer to any of these, depending on the synthesizer or manufacturer in question.


Phase Advance 'Sync'

The phase of the follower is advanced by some amount when the leader oscillator level crosses some threshold. Used for audio synthesis, this may give an audible effect similar to Soft Sync.


Reset Inhibit Sync

When the leader oscillator crosses some threshold, the normal reset of the follower is disabled: it will stick at its final level, positive or negative. When the leader crosses back over some threshold, the follower is reset.


Overlap Sync

In this method, the current wave completes but a new waveform is generated at the sync pulse. The tail of the old wave and the new wave are output summed if they overlap.


Digital Implementation Aspects

Naive approaches to sync in digital oscillators will result in
aliasing In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when a ...
. To prevent this,
band-limited Bandlimiting is the limiting of a signal's frequency domain representation or spectral density to zero above a certain finite frequency. A band-limited signal is one whose Fourier transform or spectral density has bounded support. A bandlimi ...
methods such as
additive synthesis Additive synthesis is a sound synthesis technique that creates timbre by adding sine waves together. The timbre of musical instruments can be considered in the light of Fourier series, Fourier theory to consist of multiple harmonic or inharmoni ...
, BLIT (Band-Limited Impulse Train) or BLEP (Band-Limited Step) must be adopted to avoid aliasing. In a digital oscillator, best practice is that the follower will not be reset to the identical phase each cycle, but to a phase advanced by an equivalent time to the phase of the leader at the reset. This prevents jitter in the follower frequency and provides truer synchronization. For digital oscillators, Reversing Sync may less frequently generate
aliasing In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when a ...
. This effect may be naively implemented by measuring the zero axis crossings of the leader oscillator and reversing the slope of the follower oscillator after every other crossing. For digital implementation, note that none of the Threshold or Weak Sync methods actually synthesize the waveform in a way different from Hard Sync (rather, they selectively deactivate it). Overlap sync is primarily a digital technique with simple implementation, such as used in FOF;http://www.gersic.co
Physical Modeling Synthesis
an analog implementation could be a highly damped sine oscillator excited by the reset pulse.


Sync-based Architectures

A variety of synthesis architectures are based on sync, often used in conjunction with
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
,
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, or
phase modulation Phase modulation (PM) is a modulation pattern for conditioning communication signals for transmission. It encodes a message signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal forms ...
. Such architectures includ
VOSIM
and
physical modelling synthesis Physical modelling synthesis refers to sound synthesis methods in which the waveform of the sound to be generated is computed using a mathematical model, a set of equations and algorithms to simulate a physical source of sound, usually a musical i ...
.


References

{{DEFAULTSORT:Oscillator Sync Synthesizers Oscillators Synchronization