Many companies produce effect units for the guitar that emulate synthesizer sounds. These devices can be small, effects pedal units or large, rack-mounted studio units. For example, devices such as the Electro-Harmonix MicroSynth pedal produces synth-like tones from a guitar input signal. Pedals of this nature are like most other guitar effects pedals; they are small devices triggered by footswitches. Most of these devices mimic synthesizer sound by adding a customizable filter and amplitude envelopes.
Some guitars are manufactured with independent synthesizer elements built into the guitar body. Guitars such as the Moog guitar have a wide variety of features that emulate or directly involve synthesizers. Many of these guitars involve a conversion of the frequency of the guitar sound into either a control voltage or a musical instrument digital interface value. Some synthesizers have components that respond to changes in the voltage of a circuit. In this way, voltage controls many of the synthesizer parameters. Alternatively, MIDI is a standardized protocol that allows music data to be sent across devices. Converting a guitar's signal into either voltage or MIDI allows the guitar sound to interface with conventional synthesizers.
Many synthesizers feature an audio input, which allows you to plug a guitar into the synthesizer for processing through the synthesizer's envelopes, filters and other pathways. This technique utilizes the synthesizer as an effects processor for the guitar. If you wish to do this, most synthesizers require a higher-level guitar signal than is normally produced. This can be achieved by running the guitar signal through a pre-amp or level boost device before the synthesizer.
Synthesizers themselves can be used to emulate guitar sounds, even independent of an actual guitar. You can configure the oscillators in a synthesizer to mimic the sound of a guitar. The various filters and envelopes of a synthesizer process the oscillator signals however you configure them. Certain synthesizer devices, such as the Roland VG-99 and GR-55, are pre-configured with a number of different guitar and amp models. Other synthesizers require you to build a guitar sound from the ground up. This can be a challenging but rewarding process.