VOPM is a Frequency-Modulation-style polyphonic synthesizer developed by Sam, and ported to linux by the Jucetice Team at Anticore.org (see link at bottom). VOPM has four operators (oscillators), and is an emulation of Yamaha's YM2151 OPM sound chip. VOPM is ideal for chiptune-style music, but can definitely create some more complex sounds as well.
The first screenshot of VOPM is using FstHost, which allows the original GUI layout and permits .fps file saving and loading. The second screenshot is VOPM in Carla (KXStudio), which utilizes its .carxs format.
VOPM includes eight algorithms for arranging the four operators, nominated C1, C2, M1 and M2. These can function as either carriers or modulators. M1 is capable of self-modulation, and C2 can produced pitched noise output.
Regarding the parameters in the GUI: TL is the maximum amplitude level, and AR indicates the Attack Rate. DL1 is the first decay level, and DR1 is the first decay rate. DR2 indicates the rate at which the sustained note should then return to zero. RR is the final release rate when the note is released. DT1 is fine detuning, and DT2 is coarse detuning. KS is the key follow parameter.
On the left side of the GUI, any operator can be omitted by clicking the respective OP-MSK button. If the noise button is clicked, C2 will produce noise rather than a sinewave. The NFRQ slider controls the noise signal frequency. The FL slider controls the feedback loop of M1. AMS and PMS control amplitude and pitch sensitivity, respectively.
A few patches of my own, in .fps format: VOPMPatches01
Some patches and an unofficial programming manual are available at One-Synth-Challenge (OSC) HERE
VOPM binary file at Anticore.org