Some systems consist of two separated parts, the "guitar controller" and the "interface and sound module", where the former controls the latter (as in Roland instruments). One of the challenges of guitar synths is that not all performance techniques can be smoothly translated into MIDI and resulting sound. Harmonics, palm mutes, hammer-ons and pull-offs, and pick slides are not easily picked up by guitar synths, largely due to imperfect fretting technique; with the exception of harmonics and palm mutes, these techniques can be achieved with a concentrated effort to maintain good fretting. Similarly, the synthesizer portion of a system often lacks the variety of controls (sliders, faders, knobs) for synthesis parameters that are normally available on a standard keyboard synthesizer.