What is an audio oscillator?
Audio oscillators are most commonly used in music production as tone generators and measurement tools. Technically, an oscillator circuit converts DC to AC current, which produces a continuous, repeated, alternating waveform. They are most commonly used to produce waveforms in the desired frequency, ranging between 16 Hz to 20,000 Hz. A low-frequency oscillator, or LFO, generates waveforms below 20 Hz. The electrical current alternates very quickly between two states, much like a string resonates on a guitar; this produces a waveform that can be amplified and shaped with various other audio processors. Additionally, electronic oscillators are widely used in many industries as equipment clocking, measurement, and calibration tools.
The most common types of oscillators found in analog synthesizers are voltage-controlled. A voltage-controlled oscillator, or VCO, is an oscillator in which the frequency depends on a control voltage. These oscillators will produce a different frequency depending on the control voltage it is being fed. When a key on an analog synth is pressed, it sends a specific voltage to the oscillator, which generates a waveform in the corresponding pitch. If the oscillator is tuned properly, the pitch will correspond with the note of the key that is pressed. Additionally, synthesizers will usually offer the ability to modulate the control voltage with different types of signals. For instance, adding modulation to a VCO can produce Frequency Modulation, better known as FM synthesis.
Voltage-controlled oscillators have three main parameters– frequency or pitch, amplitude or volume, and waveform or tone. A sine wave is the most basic of all waveforms; additional harmonics are added to alter its shape. The four main waveform shapes are sine, square, triangle, and sawtooth. Each type has a major impact on the tonality of the sound produced. Sine waves are usually classified as having a smooth sound, while a sawtooth is more harsh or buzzy. Another common waveform seen on oscillators is noise which most commonly comes in the white and pink variety. White noise contains all frequencies in equal proportions, while pink noise attenuates some of the higher frequencies.
Digital oscillators generate waveforms using digital signal processing or DSP. The waveforms are modeled and digitally recreated to emulate analog oscillators. Although modeling technology continues to get better and software manufacturers are making better sounding applications, most synth enthusiasts consider digital oscillators’ sound to be inferior to analog. However, the flexibility of digital gives manufacturers new opportunities to expand sonic possibilities.
Oscillators can produce a wide range of shapes, but the four most commonly found on an analog synthesizer are a Sine, Square, Triangle, and Sawtooth. The curve of the waveform line is what dictates its shape.
A sine wave is arguably the most fundamental building block of sound. It is considered pure because there are no additional harmonics added to the signal; it is only made up of the fundamental frequency.
In addition to the fundamental frequency, a Square Wave only includes odd harmonics or harmonics that occur in whole odd-number multiples of the fundamental frequency. A square wave looks like its namesake, a square.
Triangle waves look similar to a sine wave, except the curves are replaced with straight edges that connect like a triangle. Triangle waves add in odd harmonics, which diminish the further away they get from the root frequency. For example, the root note and the 3rd harmonic will be the loudest, while the 5th and 7th harmonic will be lower in level and the 11th and 13th even lower.
The Sawtooth is named after its resemblance to a sawtooth blade. In these waves, both even and odd harmonics are added, resulting in a harsh but clear tone.
History of the oscillator
In the late 1930s, audio oscillators were less than ideal for modern applications due to their complexity, instability, and cost. This prompted William Hewlett to develop his first product for the Hewlett Packard company, the model 200B variable frequency oscillator. Hewlett was inspired after seeing a seminar at Stanford University during the late 1930s by his professor Frederick E. Terman on the use of negative feedback. He was so intrigued he decided to spend an entire semester studying it for his thesis needed to complete his advanced engineering degree.
The main difference in Hewlett’s oscillator is the incandescent lamp that is used as the temperature-dependent resistor in the feedback network. The light bulb also acted as an automatic gain control that kept the oscillator’s loop gain near unity, which is a key component to achieving the lowest amount of distortion. With this design, the output could be regulated without adding distortion. This not only improved the performance but also made it much more affordable. The Model 200A at the time sold for $54.40, which was substantially less than most of the other oscillators on the market, going for anywhere between $200-600. The first big sale which launched the entire company was to Walt Disney, whose engineers used them to test channels, recording equipment, speaker systems, and other equipment needed for its new Fantasound stereo sound reproduction system that was used for their breakthrough classic Fantasia. Fantasia was the first film that was commercially released in stereo.
Hewlett’s oscillator was the first practical method for generating audio signals, used for measurement and calibration in communications, science, medicine, audio, and many more industries. Before Hewlett’s 200B, there was no easy and accurate way to produce low-frequency signals.