Digitally controlled oscillator
A digitally controlled oscillator or DCO is a hybrid digital/analogue electronic oscillator used in synthesizers, microcontrollers, and software-defined radios. The name is 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 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 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, the Oberheim Matrix-6, some instruments by Akai and Kawai, and the recent Prophet '08 by Dave Smith Instruments.
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.[1] 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[2] from 1977 or Roland Jupiter-8[3] 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 organs of the time and the microprocessors 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,[4] the greater simplicity and arbitrary waveforms of digital systems like direct digital synthesis led to most later instruments adopting entirely digital oscillator designs.
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
References
- ^ Russ, Martin (2004). "Early versus modern implementations". Sound Synthesis and Sampling. section 2.6.1, p.137.
{{cite book}}: Unknown parameter|nopp=ignored (|no-pp=suggested) (help) - ^ Yamaha CS-80, Vintage Synth Explorer
- ^ Roland Jupiter 8, Vintage Synth Explorer
- ^ STG Wavefolder, STG Soundlabs website