Description:

hexter is a software synthesizer that models the sound generation of a Yamaha
DX7 synthesizer. It can easily load most DX7 patch bank files, accept patch
editing commands via MIDI sys-ex messages (ALSA systems only), and recreate the
sound of the DX7 with greater accuracy than any previous open-source emulation
(that the author is aware of....)

hexter operates as a plugin for the Disposable Soft Synth Interface (DSSI).
DSSI is a plugin API for software instruments (soft synths) with user
interfaces, permitting them to be hosted in-process by audio applications.