High Quality Content by WIKIPEDIA articles! Casio's SD ("Spectrum Dynamic") Synthesizers were a late-1980s line of hybrid digital-analog synthesizers featuring a resonant analog filter. SD synthesis was very similar to traditional DCO-analog synthesis, with the main difference being that some of the SD digital waveforms' harmonic spectrums changed temporally, or dynamically in relation to the amplitude envelope. SD synthesis is used in six Casio synthesizers and home keyboards released in 1987 and produced until 1991, when Casio exited the synthesizer market completely and focused solely on pure consumer keyboards. Due to some programming limitations plus Casio's poor marketing, the SD synths never gained wide popularity and are now fairly rare in the second-hand marketplace (which adds to their charm, according to some). There still exists a small but devoted fanbase who insist that SD synthesis, particularly as expanded in the high-end model HT-6000, was overlooked and highly...