SD Synthesis Details
SD (Spectrum Dynamic) Synthesis was modeled on traditional DCO-VCF-DCA analog synthesis but used waveforms that included predefined variations over time. The SD sound source is a 4-bit digitally controlled oscillator (DCO) that uses preprogrammed additive synthesis to offer waveforms including triangle, sawtooth, squares of different widths, and some unusual pulses, plus various combinations thereof. The DCO has preprogrammed control of the individual harmonics within the waveform, and some (but not all) of the available waveforms are "moving", meaning that the amplitudes of the various harmonics are designed to change as the DCA envelope progresses. For example, one waveform has an octave-unison effect where the higher harmonics fade in over time. This predefined temporal motion of the harmonic spectrum yields the term "Spectrum Dynamic." The user has very limited influence over the spectrum dynamic using the DCA envelope, and doing so is a trial-and-error process, that remains somewhat of an undocumented mystery (it may be that Casio was deliberately vague about the workings of SD, and the extent to which it actually affected the overall waveform, to protect against another lawsuit like the Yamaha lawsuit against Casio's phase-distortion synthesis, but it is also plausible that Casio wanted to enshroud the fact that these waveforms are based on multipulse squarewave, which was already considered technically outdated when SD synths came out). Basically each so-called "waveform" of an SD synth consists of 2 layered subvoices with independent preset volume envelopes (that can not be changed by the user). This way some "waveforms" crossfade between timbres without filter sweep to simulate e.g. the brighter attack phase of metallic clangs or picked strings.
Most SD synthesizers use a single DCO (plus a digital noise generator for certain waveforms) per voice, and offer 32 possible waveforms. The top-of-the-line Casio HT-6000 offered 64 possible waveforms, 4 DCO's per voice, velocity, detuning, ring-modulation, and an expanded SD parameter set. In SD synthesis, an analog voltage-controlled resonant filter (VCF) is used to shape the DCO's waveform (whereas on Casio CZ Synthesizers the phase distortion engine could only emulate a resonant filter). The SD sound is further shaped by a digitally controlled amplitude (DCA) envelope. Both the VCF and DCA are programmed with traditional 4-stage attack/decay/sustain/release (ADSR) curves . Finally, like on the CZ's, a low-frequency oscillator (LFO) is programmable to modulate the DCO pitch, but unfortunately it cannot modulate the VCF or DCA. Nearly all of the SD synth parameters had 5-bit precision, allowing a stepwise range of 0-31.
Read more about this topic: Casio SD Synthesizers
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