Casio Collectors

Triangular wave modulation (US patent 5164530) is an FM-like Casio speciality that generates waveforms by modulating a monotonous carrier function with a sine (or other) wave and decoding the signal by mirroring it at a triangular wave. With the same count of operators the produced waveform has higher harmonics than normal FM because at high modulation the wave peaks fold back into the opposite direction. Without modulation it outputs a sine wave and so can nicely blend between very dull and bright timbres. The carrier waveform stands in ROM and so can be switched between a variety of timbres. Like with FM, operators can be combined in various ways.

https://www.google.com/patents/US5164530

- Is TM part of the "phase distortion" engine or something else?

I own a CZ-230S and revived a CZ-101 (had severe water and fire damage), but I am no PD expert and never heard about TM synthesis before (even websearch doesn't help much). Is this used in phase distortion synths (e.g. the later VZ-series) or is this only used in the Casio "Pulse Code Modulation" engine of preset sound keyboards? Patent 5164530 suggests that Casio at least planned to make dedicated TM synthesizers with a PD/FM-like user interface.


I am reworking my technical keyboard descriptions for the WarrantyVoid site. So I websearched for 1980th keyboard patents and discovered a lot of interesting info.

https://www.google.com/patents/US5319151

This is what I wrote for my SA-series page:
>>>
The sound generation is apparently based on a highly complex softsynth with many algorithms those can perform PCM, DPCM, FM and TM (triangular wave modulation) synthesis with sophisticated envelopes. These Casio PCM algorithms are described in the US patent 5319151. This rather confusing 121 page tome however is ambiguous because it covers plenty of different implementations those e.g. can employ different counts of chip-internal sub-CPU cores for sound generation in higher grade instruments. The SA-series is surely the cheapest described "First" or "Second Embodiment" which has none. The algorithms for this version even describe how shorter tasks are stuffed with blank "dummy commands" to keep the timing in sync when different sounds would need different computing time. It works indeed very VCS2600-like - a marvel of freakish realtime programming made from one big loop (plus in "First Embodiment" one timer IRQ to compute waveforms and fill the DAC output FIFO; the "Second" does even this during dummy commands).

The interpolation method with that Casio smoothly blends between wavetable sections is described in the US patent 4442745 "Long duration aperiodic musical waveform generator" It plays sections of compacted samples back and forward to implement things like long decaying cymbals. US patent 4958552 explains algorithms how envelope data is extracted from natural instrument recordings and applied on loop samples as a approximated segmented functions. The original envelope may be removed from the stored loop sample by a waveform normalizer (US patent 4691608). Most important is that these chained envelopes can have basically any length and have (unlike e.g. ADSR) no fixed count of steps. Combined with crossfading between adjacent loop samples this permits very flexible sound definition.

The US patent 5319151 "Data processing apparatus outputting waveform data in a certain interval" mentions for the "First Embodiment" that the chip size is only 5x5mm, a program word has 28 bits (including lower potion of next address) and these further hardware specs:

   "With regard to the circuit scale and the operation time of the specific embodiment (PCM sound source system capable of producing eight polyphonic sounds) the control ROM has a size of 112K bits, RAM 445.4K bits and the control data/waveform ROM (for 100 timbres) 508K bits; one machine cycle is about 276 nanoseconds with a maximum number of cycles of the interrupt program when invoked being about 150; and the executing period of the interrupting process (tone output sampling period) is about 47 microseconds."

   Expressed in KBytes this would mean 55.7KB RAM, 14KB program ROM and 63.5KB sound ROM, which isn't far away from a Commodore C64 with large ROM cartridge. Said CPU speed would be about 3.6 MIPS.
<<<

Higher grade MIDI keyboards like MT-240, MT-540 or MT-750 certainly have sub-CPU cores for 16 bit sound generation. Their external ROM is 512KB up to 1MB.

- Does anybody know more about this sound engine?


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