Digitally-controlled oscillator
Encyclopedia
A digitally controlled oscillator or DCO is a hybrid digital
/analogue
electronic oscillator
used in synthesizer
s. The name is an analogy
with "voltage-controlled oscillator
". DCOs were designed to overcome the tuning stability limitations of early VCO designs.
driven by a control signal from a digital-to-analog converter
, 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
Juno-60
, Juno-106
, JX-3P
, JX-8P
, and JX-10
, the Korg
Poly-61
and Poly 800
, and some instruments by Akai
and Kawai
.
are based on a capacitor
charging linearly in an op-amp integrator configuration. 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 ramp (or sawtooth) waveform
, and this type of oscillator core is known as a ramp core.
The typical DCO design replaces the comparator in the ramp core with reset pulses generated from a counter or microprocessor. This gives the design digital frequency stability, whilst retaining a true analogue waveform output. Aside from the way reset pulses are generated, the typical VCO ramp core and the DCO are identical. Both produce a ramp waveform from which other waves are derived by waveshaping.
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, the greater simplicity and arbitrary waveforms of digital systems like direct digital synthesis
led to most later instruments adopting entirely digital oscillator designs.
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
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.
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
/analogue
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
electronic oscillator
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...
used in synthesizer
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. The name is an analogy
Analogy
Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...
with "voltage-controlled oscillator
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...
". 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 voltage-controlled oscillatorVoltage-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...
driven by a control signal from a digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
, 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 Corporation
is a Japanese manufacturer of electronic musical instruments, electronic equipment and software. It was founded by Ikutaro Kakehashi in Osaka on April 18, 1972, with ¥33 million in capital. In 2005 Roland's headquarters relocated to Hamamatsu in Shizuoka Prefecture. Today it has factories in Japan,...
Juno-60
Roland Juno-60
The Roland Juno-60 is a popular analogue 61-key polyphonic synthesizer produced by Roland Corporation in the early 1980s and a successor to the slightly earlier Juno-6. Like its predecessor, the Juno-60 has some digital enhancements, used only for clocking the oscillators and for saving and loading...
, Juno-106
Roland Juno-106
The Roland Juno-106 was a hybrid digital/analogue polyphonic synth manufactured by Roland Corporation in 1984. It featured Digitally controlled oscillators for tuning stability and digital envelope generation along with analog filters and signal path....
, JX-3P
Roland JX-3P
The Roland JX-3P is a synthesizer produced by Roland Corporation of Japan in 1983. It is notable as the one of the company's first products to incorporate a MIDI interface, limited to note-on/note-off data only.-Features:...
, JX-8P
Roland JX-8P
The Roland JX-8P was a 61-keyboard 6-note, velocity and aftertouch sensitive polyphonic synthesizer released by Roland in 1984 to compete against the Yamaha DX7. In many aspects, the JX-8P delivers classic "fat" Roland synth sounds just like instruments as Jupiter-8, Jupiter-6, MKS-80 and the...
, and JX-10
Roland JX-10
The Roland JX-10 was a 12-voice analog synthesizer produced from 1986 to 1989, and was the last true analog synthesizer made by Roland. It is essentially two Roland JX-8P synthesizers put together, along with a 76-note velocity-sensitive keyboard with aftertouch...
, the Korg
Korg
is a Japanese multinational corporation that manufactures electronic musical instruments, audio processors and guitar pedals, recording equipment, and electronic tuners...
Poly-61
Korg Poly-61
The Poly-61 is a programmable polyphonic synthesizer released by Korg in 1982, replacing the Polysix. It was notable as it was the first Korg synthesizer to feature a pushbutton user interface, dispensing from the Polysix's knobs and switches...
and Poly 800
Korg Poly-800
The Korg Poly-800 is a synthesizer released by Korg in 1983. Its initial list price of $795 made it the first fully programmable synthesizer that sold for less than $1000. It featured a 49 key non-velocity sensitive keyboard, two buttons for data entry, and a joystick controller, which could...
, and some instruments by Akai
Akai
Akai is a consumer electronics brand, founded by Saburo Akai as , a Japanese manufacturer in 1929. It is now headquartered in Singapore as a subsidiary of Grande Holdings, a Hong Kong-based conglomerate, which also owns the formerly Japanese brands Nakamichi and Sansui. The Akai brand is now used...
and Kawai
Kawai
The of Japan is best known for its grand and upright pianos, electronic keyboards and electronic synthesizers. The company was established in August 1927, and has its headquarters in Hamamatsu, Shizuoka.-Pianos:...
.
Relation to earlier VCO designs
Many voltage-controlled oscillators for electronic musicElectronic music
Electronic music is music that employs electronic musical instruments and electronic music technology in its production. In general a distinction can be made between sound produced using electromechanical means and that produced using electronic technology. Examples of electromechanical sound...
are based on a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
charging linearly in an op-amp integrator configuration. When the capacitor charge reaches a certain level, a comparator
Comparator
In electronics, a comparator is a device that compares two voltages or currents and switches its output to indicate which is larger. They are commonly used in devices such as Analog-to-digital converters .- Input voltage range :...
generates a reset pulse, which discharges the capacitor and the cycle begins again. This produces a rising ramp (or sawtooth) waveform
Sawtooth wave
The sawtooth wave is a kind of non-sinusoidal waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw....
, and this type of oscillator core is known as a ramp core.
The typical DCO design replaces the comparator in the ramp core with reset pulses generated from a counter or microprocessor. This gives the design digital frequency stability, whilst retaining a true analogue waveform output. Aside from the way reset pulses are generated, the typical VCO ramp core and the DCO are identical. Both produce a ramp waveform from which other waves are derived by waveshaping.
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 CS80 from 1977 or Roland Jupiter-8 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 organElectronic organ
An electronic organ is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally, it was designed to imitate the sound of pipe organs, theatre organs, band sounds, or orchestral sounds....
s of the time and the microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
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, the greater simplicity and arbitrary waveforms of digital systems like direct digital synthesis
Direct digital synthesis
Direct Digital Synthesizer is a type of frequency synthesizer used for creating arbitrary waveforms from a single, fixed-frequency reference clock...
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 82C53Intel 8253
The Intel 8253 and 8254 are Programmable Interval Timers , which perform timing and counting functions. They were primarily designed for the Intel 8080/8085-processors, but later used in x86-systems...
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
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
at the required frequency.
The leading edge
Leading edge
The leading edge is the part of the wing that first contacts the air; alternatively it is the foremost edge of an airfoil section. The first is an aerodynamic definition, the second a structural one....
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
- Voltage-controlled oscillatorVoltage-controlled oscillatorA 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...
- Numerically controlled oscillator
- Direct digital synthesizer