 The earliest models were combination converter/synthesizers, and this type is still produced.As the early integrated models predated the MIDI standard, their components were not interchangeable; the guitarist's only option was to use whatever synthesizer came with the converter, and vice versa.By the 2000s (decade), however, all converter/synthesizers were MIDI-compatible, so any synthesizer with MIDI-input capability (the vast majority since the 1990s) can be used.