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 and ...

s, microcontrollers, and software-defined radios. The name is

analogous Analogy (from Greek ''analogia'', "proportion", from ''ana-'' "upon, according to" lso "against", "anew"+ ''logos'' "ratio" lso "word, speech, reckoning" is a cognitive process of transferring information or meaning from a particular subject ...

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 oscilla ...

." 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, 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 The Prophet '08 is a polyphonic analog synthesizer created by Dave Smith of St. Helena, California, US, for Dave Smith Instruments (DSI), released in late 2007. As with DSI's other instruments, the Prophet '08 uses analog subtractive synthesis ...

and its successor Rev2 by

Dave Smith Instruments Sequential is an American synthesizer company founded in 1974 as Sequential Circuits by Dave Smith. In 1978, Sequential released the Prophet-5, the first programmable polyphonic synthesizer; it became a market leader and industry standard, used ...

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 ( electroac ...

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 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 released 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 hardwired preset voice set ...

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 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 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 circu ...

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 Direct digital synthesis (DDS) is a method employed by frequency synthesizers used for creating arbitrary waveforms from a single, fixed-frequency reference clock. DDS is used in applications such as signal generation, local oscillators in communic ...

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 8253 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 The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...

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