Synthesizers
Synthesizers are what convert MIDI keyboards into just about any sound.
Many MIDI keyboards already have a built-in synth engine. If yours has build-in speakers (or a headphone jack), these are utilizing the internal synth.
Synth Keyboard Online
You can use Chordzy with your own synth, or with our Synth engine.
The synth inside Chordzy is powered by SoundFonts... files that define an instrument's sound. The app includes many built-in options from the open-license repository, which you can assign to your MIDI keyboard(s) in the settings. If you have a free account, your settings will sync between devices.
Read more about how Chordzy uses sound fonts, or about which synthesizer features are supported.