Hexter is a software synthesizer that models the sound generation of
a Yamaha DX7 synthesizer. It can easily load most DX7 patch bank
files, edit those patches via a built-in editor or 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 DSSI Soft Synth Interface. DSSI
is a plugin API for software instruments (soft synths) with user
interfaces, permitting them to be hosted in-process by audio
applications.