Casio's SD ("Spectrum Dynamic") Synthesizers were a late-1980s line of analog synthesizers featuring a resonant filter. SD synthesis was traditional DCO-analog synthesis, with the main difference being that some of the SD 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 insis