A digitally controlled oscillator or DCO is used in

synthesizer A synthesizer (also synthesiser or synth) is an electronic musical instrument that generates audio signals. Synthesizers typically create sounds by generating waveforms through methods including subtractive synthesis, additive synthesis a ...

microcontroller A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...

s, and

software-defined radio Software-defined radio (SDR) is a radio communication system where components that conventionally have been implemented in analog hardware (e.g. mixers, filters, amplifiers, modulators/ demodulators, detectors, etc.) are instead implemented ...

s. The name is

analogous Analogy is a comparison or correspondence between two things (or two groups of things) because of a third element that they are considered to share. In logic, it is an inference or an argument from one particular to another particular, as oppose ...

with "

voltage-controlled oscillator A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used for fre ...

". DCOs were designed to overcome the tuning stability limitations of early VCO designs.

Confusion over terminology

The term "digitally controlled oscillator" has been used to describe the combination of a

driven by a control signal from a

digital-to-analog converter In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function. DACs are commonly used in musi ...

, and is also sometimes used to describe numerically controlled oscillators. This article refers specifically to the DCOs used in many synthesizers of the 1980s . These include the

Roland Roland (; ; or ''Rotholandus''; or ''Rolando''; died 15 August 778) was a Frankish military leader under Charlemagne who became one of the principal figures in the literary cycle known as the Matter of France. The historical Roland was mil ...

Juno-6, Juno-60, Juno-106, JX-3P, JX-8P, and JX-10, the Elka Synthex, the

Yamaha DX7 The Yamaha DX7 is a synthesizer manufactured by Yamaha Corporation from 1983 to 1989. It was the first successful digital synthesizer and is one of the best-selling synthesizers in history, selling more than 200,000 units. In the early 1980s, th ...

, the Oberheim Matrix-6, some instruments by

Akai Akai (, ) is a Japanese brand & former electronics manufacturer, established as Akai Electric Company Ltd in Tokyo in 1929. It was best known outside Japan for its tape recorders during the 1960s and 1970s. The company became bankrupt in 2000 ...

and Kawai, and the recent

Prophet '08 The Prophet '08 is a polyphonic analog synthesizer released by Dave Smith Instruments (DSI) in 2007. As with DSI's other instruments, the Prophet '08 uses analog subtractive synthesis. Similar in functionality to the renowned Sequential Circuit ...

and its successor Rev2 by Dave Smith Instruments.

Relation to earlier VCO designs

Many voltage-controlled oscillators for

electronic music Electronic music broadly is a group of music genres that employ electronic musical instruments, circuitry-based music technology and software, or general-purpose electronics (such as personal computers) in its creation. It includes both music ...

are based on a

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

charging linearly in an op-amp integrator configuration. When the capacitor charge reaches a certain level, a

comparator In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals V_+ and V_- and one binary digital output V_\text. The output is ideally ...

generates a reset pulse, which discharges the capacitor and the cycle begins again. This produces a rising ramp (or sawtooth) waveform, and this type of oscillator core is known as a ramp core. A common DCO design uses a programmable counter IC such as the

8253 The Intel 8253 and 8254 are programmable interval timers (PITs), which perform timing and counting functions using three 16-bit counters. The 825x family was primarily designed for the Intel 8080/ 8085-processors, but were later used in x86 c ...

instead of a comparator. This provides stable digital pitch generation by using the leading edge of a square wave to derive a reset pulse to discharge the capacitor in the oscillator's ramp core.

Historical context

In the early 1980s, many manufacturers were beginning to produce polyphonic synthesizers. The VCO designs of the time still left something to be desired in terms of tuning stability. Whilst this was an issue for monophonic synthesizers, the limited number of oscillators (typically 3 or fewer) meant that keeping instruments tuned was a manageable task, often performed using dedicated front panel controls. With the advent of polyphony, tuning problems became worse and costs went up, due to the much larger number of oscillators involved (often 16 in an 8-voice instrument like the

Yamaha CS-80 The Yamaha CS-80 is an analog synthesizer introduced by Yamaha Corporation in 1977. It supports true 8-voice polyphony, with two independent synthesizer layers per voice each with its own set of front panel controls, in addition to a number of ha ...

Yamaha CS-80
''Vintage Synth Explorer'' from 1977 or

Roland Jupiter-8 The Jupiter-8, or JP-8, is an eight-voice polyphonic analog subtractive synthesizer introduced by Roland Corporation in early 1981. The Jupiter-8 was Roland's flagship synthesizer for the first half of the 1980s. Approximately 3,300 units have ...

Roland Jupiter 8
''Vintage Synth Explorer'' from 1981). This created a need for a cheap, reliable, and stable oscillator design. Engineers working on the problem looked to the

frequency division Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

technology used in

electronic organ An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the pump organ, harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has si ...

s of the time and the

microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...

s and associated chips that were starting to appear, and developed the DCO. The DCO was seen at the time as an improvement over the unstable tuning of VCOs. However, it shared the same ramp core, and the same limited range of waveforms. Although sophisticated analogue waveshaping is possible,STG Wavefolder
''STG Soundlabs website'' the greater simplicity and arbitrary waveforms of digital systems like direct digital synthesis led to most later instruments adopting entirely digital oscillator designs.

Operation

A DCO can be considered as a VCO that is synchronised to an external frequency reference. The reference in this case is the reset pulses. These are produced by a digital counter such as the

chip. The counter acts as a

frequency divider Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

, counting pulses from a high frequency master clock (typically several MHz) and toggling the state of its output when the count reaches some predetermined value. The frequency of the counter's output can thus be defined by the number of pulses counted, and this generates a

square wave Square wave may refer to: *Square wave (waveform) A square wave is a non-sinusoidal waveform, non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same ...

at the required frequency. The

leading edge The leading edge is the part of the wing that first contacts the air;Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, 1997. alternatively it is the foremost edge of an airfoil sectio ...

of this square wave is used to derive a reset pulse to discharge the capacitor in the oscillator's ramp core. This ensures that the ramp waveform produced is of the same frequency as the counter output.

Problems with the design

For a given capacitor charging current, the amplitude of the output waveform will decrease linearly with frequency. In musical terms, this means a waveform an octave higher in pitch is of half the amplitude. In order to produce a constant amplitude over the full range of the oscillator, some compensation scheme must be employed. This is often done by controlling the charging current from the same microprocessor that controls the counter reset value.

References