Hardware synthesizers use a variety of synthesis types to generate, adjust and mix waveforms to produce sound. The most common types of synthesis are subtractive, additive, granular, frequency modulation, physical modeling and sample-based. Although synthesizers are usually fitted with a keyboard similar to those found on pianos, other user control interfaces are often included as well such as ribbon and infrared controllers.
The internal components found in synthesizers vary depending on the type or types of synthesis the synth uses, however a few subsystems are present in almost all synthesizers. Some kind of oscillator (or multiple oscillators), either voltage-controlled (analog) or digital, is more often than not used as the primary sound source. An envelope modulator regulates the speed at which the sound "fades" in and out and the length of time during which it is sustained when played. Filters remove or reduce user-defined frequencies and LFOs, or "Low Frequency Oscillators," are used to automatically control other components rhythmically.
Early synthesizers were large modular setups, in which the configuration of individual components was left up to the user. Such modular synths are still produced though most modern synthesizers are self-contained. As such, the amount to which the internal connections can be modified by the user is limited.
During the 1980s, digitally-controlled oscillators were often replaced unpredictable analog oscillators and the first synthesizers with built-in effects processors were released. The introduction of MIDI allowed for a standard way to connect synths, samplers and other music equipment together to be controlled from a single master device. Recent innovations include combination devices, which feature both hardware synthesis as well as computer audio interfaces that facilitate using music hardware from within digital audio workstation software.