Editing Digitally controlled oscillator

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A '''digitally controlled oscillator''' or '''DCO''' is used in [[synthesizer]]s, [[microcontroller]]s, and [[software-defined radio]]s. The name is [[analogy|analogous]] with "[[voltage-controlled oscillator]]". DCOs were designed to overcome the tuning stability limitations of early VCO designs.

== Confusion over terminology ==
The term "digitally controlled oscillator" has been used {{Citation needed|date=August 2020}} to describe the combination of a [[voltage-controlled oscillator]] driven by a control signal from a [[digital-to-analog converter]], and is also sometimes used to describe [[numerically controlled oscillator]]s.

This article refers specifically to the DCOs used in many synthesizers of the 1980s {{why?|date=August 2020}}. These include the [[Roland Corporation|Roland]] Juno-6, [[Roland Juno-60|Juno-60]], [[Roland Juno-106|Juno-106]], [[Roland JX-3P|JX-3P]], [[Roland JX-8P|JX-8P]], and [[Roland JX-10|JX-10]], the [[Elka Synthex]], the [[Yamaha DX7]], the [[Oberheim Matrix synthesizers|Oberheim Matrix-6]], some instruments by [[Akai]] and [[Kawai Musical Instruments|Kawai]], 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]] are based on a [[capacitor]] charging linearly in an [[Operational_amplifier_applications#Integrator|op-amp integrator]] configuration.<ref name="Chamberlin">{{cite book | chapter = Basic Analog Modules, Voltage-Controlled Oscillator | first = Hal | last = Chamberlin | title = Musical Applications of Microprocessors | pages = section 6, p.181 | year = 1985 | no-pp = true}}</ref> When the capacitor charge reaches a certain level, a [[comparator]] generates a reset pulse, which discharges the capacitor and the cycle begins again. This produces a rising [[sawtooth wave|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.<ref name="Russ">{{cite book | chapter = Early versus modern implementations | first = Martin | last = Russ | title = Sound Synthesis and Sampling | pages = section 2.6.1, p.137 | year = 2004 | no-pp = true}}</ref> 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]]<ref name="CS80">[http://www.vintagesynth.com/yamaha/cs80.shtml Yamaha CS-80], ''Vintage Synth Explorer''</ref> from 1977 or [[Jupiter 8|Roland Jupiter-8]]<ref name="Jupiter8">[http://www.vintagesynth.com/roland/jup8.shtml Roland Jupiter 8], ''Vintage Synth Explorer''</ref> 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]]s of the time and the [[microprocessor]]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,<ref name="Waveshaper">[http://stgsoundlabs.com/products/wave_folder_fu.htm STG Wavefolder], ''STG Soundlabs website''</ref> 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 [[Intel 8253|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 (waveform)|square wave]] at the required frequency.
The [[signal edge|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.

== See also ==
* [[Direct digital synthesizer]]
* [[Numerically controlled oscillator]]
* [[Voltage-controlled oscillator]]

== References ==
{{reflist}}

{{DEFAULTSORT:Digitally Controlled Oscillator}}
[[Category:Electronic oscillators]]
[[Category:Synthesizer electronics]]

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