A digitally controlled oscillator or DCO is used in

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 a ...

microcontroller A microcontroller (MCU for ''microcontroller unit'', often also MC, UC, or μC) is a small computer on a single VLSI integrated circuit (IC) chip. A microcontroller contains one or more CPUs ( processor cores) along with memory and programmabl ...

s, and

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

s. The name is analogous with "

voltage-controlled oscillator A microwave (12–18GHz) 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 oscillat ...

." 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. There are several DAC archit ...

, and is also sometimes used to describe

numerically controlled oscillator A numerically-controlled oscillator (NCO) is a digital signal generator which creates a synchronous (i.e. clocked), discrete-time, discrete-valued representation of a waveform, usually sinusoidal. NCOs are often used in conjunction with a digit ...

s. This article refers specifically to the DCOs used in many synthesizers of the 1980s . These include the

Roland Roland (; frk, *Hrōþiland; lat-med, Hruodlandus or ''Rotholandus''; it, Orlando 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 ...

Juno-6, Juno-60,

Juno-106 The Roland Juno-106 is a synthesizer released by Roland Corporation in February 1984. Features The Juno-106 is a polyphonic synthesizer with six voices. It is an analog synthesizer but with digitally controlled oscillators and chorus effect ...

, JX-3P, JX-8P, and JX-10, the

Elka Synthex The Elka Synthex is a polyphonic analog synthesizer produced by Italian music instrument manufacturer Elka from 1981 to 1985. Overview The Synthex was conceived and developed by independent Italian designer Mario Maggi, who then gained the finan ...

, the Korg Poly-61, the Oberheim Matrix-6, some instruments by

Akai Akai ( ja, 赤井, ) is a Hong Kong manufacturer of consumer electronics. It was founded as Akai Electric Company Ltd in Tokyo, Japan, in 1946. Grande Holdings in Hong Kong purchased the Akai brand, and now distributes various electronic produ ...

and

Kawai is a musical instrument manufacturing company headquartered in Hamamatsu, Shizuoka, Japan. It is best known for its grand pianos, upright pianos, digital pianos, electronic keyboards and electronic synthesizers. The company was founded in Augus ...

, and the recent Prophet '08 and its successor Rev2 by Dave Smith Instruments.

Relation to earlier VCO designs

Many voltage-controlled oscillators for

electronic music Electronic music is a genre of music that employs electronic musical instruments, digital instruments, or circuitry-based music technology in its creation. It includes both music made using electronic and electromechanical means ( electro ...

are based on a

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

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-80Yamaha 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 3300 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 A frequency divider, also called a clock divider or scaler or prescaler, is a circuit that takes an input signal of a frequency, f_, and generates an output signal of a frequency: : f_ = \frac where n is an integer. Phase-locked loop frequency ...

technology used in

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

s of the time and the

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...

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, 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 A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...

at the required frequency. The leading edge 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