Oscillators

Oscillators are fundamental entities in the universe. Wind over water produces oscillations on the surface. Water waves are good models for all manner of waves. Musical instruments are oscillators that create waves in air that act much like waves in water but are invisible. Electronic oscillators with a frequency of 20 to 20,000 cycles per second (Hertz) produce sound waves when played through an amplifier and speakers. In synthesizers the primary oscillators produce both the pitch and the timbre of instrument sounds.

In the human larynx, the vocal chords are oscillators that vibrate when air passes over them. The pitch of the voice is determined by the tension applied to the vocal chords by muscles and the rate of flow of air. A well trained singer will practice breath control creating notes in a range of about two octaves that are accurate to pitch and have a pleasing quality. Vibrato is added by oscillating laryngeal muscles.

Oscillator frequencies are pitches, described by frequency in cycles per second (Hertz = Hz). Pitch differences greater than 2 Hz are perceived as different notes. The audio frequency range is 20 to 20,000 Hz. Voice perception depends on a smaller range of about 40 to 5000 Hz. A pitch of frequency 440 Hz is described as concert A, the A above middle C on the piano. Multiples of that frequency are also A. Higher As are 880 Hz, 1760 Hz and lower As, descending, are 220 Hz, 110 Hz, and 55 Hz. An A octave spans the intervals between between two adjacent As.

The first analogue synthesizer oscillators tended to drift and did not stay in tune. The next innovation used digital oscillators based on vibrating quartz crystals; this method of time keeping is now ubiquitous on the planet running radios, watches, computers, navigation devices, and synthesizers. Digital synthesizers are now the standard of pitch accuracy and stability. If you want, you can fine tune pitches by thousands of a cycle per second. A keyboard can a be transformed in a few programming steps to transpose and play any sequence so of pitches you can imagine.

In synthesizers, low frequency oscillators, LFO, operated in a range of about 1 to 60 HZ are used to modify the main frequency of sound. If the main pitch is modified by an LFO, the effect is tremolo. If the amplitude of the pitch is modified, the effect is vibrato. A violinist changes the note pitch by finger pressure on a string and then oscillates his finger to produce vibrato. Sustained notes on the violin are less pleasing without vibrato.

Low Frequency Oscillators available in the Proteus 2500 and Emulator X2

LFO waveforms available in Proteus 2500 and Emulator X2 synthesizers

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Topics presented at Persona Digital Studio are from the book, The Sound of Music by Stephen Gislason. Click the Download button to download the eBook from Persona Digital Online.

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