Фазалық бұрмалау синтезі: Casio CZ сериясының әдісі

Electronic sound synthesis method
Phase distortion (PD) synthesis is a synthesis method introduced in 1984 by Casio in its CZ range of synthesizers. In outline, it is similar to phase modulation synthesis as championed by Yamaha Corporation (under the name of frequency modulation), in the sense that both methods dynamically change the harmonic content of a carrier waveform by influence of another waveform (modulator) in the time domain. However, the application and results of the two methods are quite distinct. Casio made five different synthesizers using their original concept of PD synthesis (with variations). The later VZ 1 and co's synthesis method Interactive phase distortion is much more similar to the aforementioned phase modulation, rather than a direct evolution of phase distortion; see below.

Casio's implementation of PD used oscillators generated by modulator and carrier waveforms, synchronised to each other per cycle. The modulators were various angular waves that could 'distort' the carrier's sine into other shapes, to a degree derived from the "DCW" envelope. In doing so, many harmonics were created in the output. As modulators were rich in harmonic content, they could create spectra more linear, i. e. more similar to traditional subtractive spectra, than Yamaha's phase modulation (PM/FM) synthesis. PM does not require oscillator sync but was for a long time limited to sine waves, which meant output spectra bore the non linear hallmark of Bessel functions. PD is a different type of PM whose very different modulators caused significant difference in operation and sound between PD and PM. Thus the two aren't directly equivalent. The phase transforms are all assembled from piecewise linear functions under binary logic control and shows characteristic sharp knees (and for some transforms, even sudden jumps) as they move from minimum to maximum, where the frequency counter's accumulator wraps around and starts over. The sharp knees are smoothed by the roundness of the modulated sine wave and not too noticeable in the resulting signal.

As outlined above, phase distortion broadly applies similar mathematical concepts to phase modulation synthesis, but their implementation and results are not equivalent. Whereas PM pioneered by John Chowning and commercially used by Yamaha uses an oscillating modulator that can have its own period, PD applies an angular modulator of straight line segments hard synchronised to the same period as its corresponding carrier, i. e. modulating each cycle identically. PM/FM produces Bessel function derived spectra unless linearised by the application of feedback, whereas PD produces more linear spectra. This manifests in PD synths' reputation for being easier to produce traditional subtractive sounds, such as those typically associated with analogue synths, which are characterised by linear spectra. These facts demonstrate how although the broad concept alteration of phase is the same, implementation and results differ greatly. Casio's own later engine named Interactive Phase Distortion (iPD), which featured in their VZ synths (VZ 1, VZ 10M, and VZ 8M; the first two also rebadged by Hohner as the HS 2 resp. HS 2/E), actually bears very little resemblance to 'actual' PD, being based around an idiosyncratic type of PM instead. In iPD, multiple oscillators are combined in various configurable routings (similar to Yamaha's "algorithms") and can modulate each other using PM or ring modulation (the latter not available in Yamaha's system). Casio's options for combining and routing oscillators are more flexible than Yamaha's. For example, the VZ10 M is capable of 90 unique combinations of oscillators and modulations compared to the 32 algorithms of the DX7. Additionally the VZ 10M oscillators offer 8 different waveforms, allowing more complex sound generation than the DX7 which uses only sine waves.