Oscillator sync
Encyclopedia
Oscillator sync is a feature in some synthesizer
s with two or more VCO
s (or digital emulations of VCOs). One oscillator will restart the period of another oscillator, so that they will have the same base frequency
. The timbre
can be altered on the synched oscillator by varying its frequency input.
A synched oscillator that resets the other oscillator(s) is called the master, and any synched oscillator that is reset by another oscillator is called a slave.
There are two common forms of oscillator sync which appear on synthesizers: Hard Sync and Soft Sync. Soft Sync is a term used for a variety of mechanisms. Other sync mechanisms exist which are also discussed.
), and is arbitrary. The slave oscillator's pitch may be tuned
to (or detuned
from) this frequency, or may remain constant. Every time the master oscillator's cycle
repeats, the slave is retriggered, regardless of its position. If the slave is tuned to a lower frequency than the master 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 slave oscillator often causes unnatural timbres and the impression of harmony
.
This effect may be achieved by measuring the zero axis crossings of the master oscillator and retriggering the slave oscillator after every other crossing.
This form of oscillator sync is more common than soft sync, but is prone to generating aliasing
in naive digital implementations.
, which often generates asymmetrical
shapes.
In some usage, Soft Sync means a sync intended to nudge and lock the slave oscillator into the same or an integer or fractional multiple of the master oscillator frequency when they both have similar phases. See also Phase locked loop
Soft Sync may accurately refer to any of these, depending on the synthesizer or manufacturer in question.
.
Methods such as additive synthesis
, 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 slave will not be reset to the identical phase each, but to a phase advanced by an equivalent time to the phase of the master at the reset. This prevents jitter in the slave frequency and provides truer synchronization.
For digital oscillators, Reversing Synch may less frequently generate aliasing
. This effect may be naively implemented by measuring the zero axis crossings of the master oscillator and reversing the slope of the slave 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 (An analog implementation could be a highly damped sine oscillator excited by the reset pulse.)
, Frequency modulation
or Phase modulation
. Such architectures include
Synthesizer
A synthesizer is an electronic instrument capable of producing sounds by generating electrical signals of different frequencies. These electrical signals are played through a loudspeaker or set of headphones...
s with two or more VCO
Voltage-controlled oscillator
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation or phase...
s (or digital emulations of VCOs). One oscillator will restart the period of another oscillator, so that they will have the same base frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. The timbre
Timbre
In music, timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices and musical instruments, such as string instruments, wind instruments, and percussion instruments. The physical characteristics of sound that determine the...
can be altered on the synched oscillator by varying its frequency input.
A synched oscillator that resets the other oscillator(s) is called the master, and any synched oscillator that is reset by another oscillator is called a slave.
There are two common forms of oscillator sync which appear on synthesizers: Hard Sync and Soft Sync. Soft Sync is a term used for a variety of mechanisms. Other sync mechanisms exist which are also discussed.
Hard Sync
The master oscillator's pitch is generated by user input (typically the synthesizer's keyboardMusical keyboard
A musical keyboard is the set of adjacent depressible levers or keys on a musical instrument, particularly the piano. Keyboards typically contain keys for playing the twelve notes of the Western musical scale, with a combination of larger, longer keys and smaller, shorter keys that repeats at the...
), and is arbitrary. The slave oscillator's pitch may be tuned
Musical tuning
In music, there are two common meanings for tuning:* Tuning practice, the act of tuning an instrument or voice.* Tuning systems, the various systems of pitches used to tune an instrument, and their theoretical bases.-Tuning practice:...
to (or detuned
Musical tuning
In music, there are two common meanings for tuning:* Tuning practice, the act of tuning an instrument or voice.* Tuning systems, the various systems of pitches used to tune an instrument, and their theoretical bases.-Tuning practice:...
from) this frequency, or may remain constant. Every time the master oscillator's cycle
Periodic function
In mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...
repeats, the slave is retriggered, regardless of its position. If the slave is tuned to a lower frequency than the master 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 slave oscillator often causes unnatural timbres and the impression of harmony
Harmony
In music, harmony is the use of simultaneous pitches , or chords. The study of harmony involves chords and their construction and chord progressions and the principles of connection that govern them. Harmony is often said to refer to the "vertical" aspect of music, as distinguished from melodic...
.
This effect may be achieved by measuring the zero axis crossings of the master oscillator and retriggering the slave oscillator after every other crossing.
This form of oscillator sync is more common than soft sync, but is prone to generating aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
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 slave oscillator is forced to reset to some level and phase (for example, zero) with every cycle of the master regardless of position or direction of the slave waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
, which often generates asymmetrical
Asymmetry
Asymmetry is the absence of, or a violation of, symmetry.-In organisms:Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension....
shapes.
In some usage, Soft Sync means a sync intended to nudge and lock the slave oscillator into the same or an integer or fractional multiple of the master oscillator frequency when they both have similar phases. See also Phase locked loop
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 slave crosses a user-defined threshold.
- Hard Sync which is disabled when the frequency of the slave extends too high above or too far below the frequency of the master.
- Hard Sync which is disabled when the frequency of the slave is lower than the frequency of the master.
Soft Sync may accurately refer to any of these, depending on the synthesizer or manufacturer in question.
Phase Advance 'Sync'
The phase of the slave is advanced by some amount when the master oscillator level crosses some threshold. Used for audio synthesis, this may give an audible effect similar to Soft Sync.Reset Inhibit Sync
When the master oscillator crosses some threshold, the normal reset of the slave is disabled: it will stick at its final level, positive or negative. When the master crosses back over some threshold, the slave 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 aliasingAliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
.
Methods such as additive synthesis
Additive synthesis
Additive synthesis is a technique of sound synthesis that creates musical timbre by explicitly adding sinusoidal overtones together.The timbre of an instrument is composed of multiple harmonic or inharmonic partials , of different frequencies and amplitudes, that change over time...
, 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 slave will not be reset to the identical phase each, but to a phase advanced by an equivalent time to the phase of the master at the reset. This prevents jitter in the slave frequency and provides truer synchronization.
For digital oscillators, Reversing Synch may less frequently generate aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
. This effect may be naively implemented by measuring the zero axis crossings of the master oscillator and reversing the slope of the slave 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 (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 modulationAmplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
, Frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
or Phase modulation
Phase modulation
Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...
. Such architectures include