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SD (Spectrum Dynamic) Synthesis was Casio's late-1980's hybrid digital/analogue synthesis method that followed on the heels of the more complex Phase-Distortion (PD) Synthesis employed in the more popular line of Casio CZ synthesizers. SD synthesis debuted in the HZ-600 synthesizer and is modeled on traditional subtractive analogue synthesis and uses a true analogue filter (whereas CZ's phase distortion could only emulate a filter). The distinction between SD and traditional analogue synthesis is that some of the waveforms are not static but rather change over time. In turning to SD synthesis Casio meant to create a synthesis engine that was more simple and accessible than Phase Distortion synthesis. Yamaha's then-ongoing lawsuit against Casio's PD synthesis engine (which claimed PD synthesis infringed on Yamaha's patented Frequency-Modulation or FM synthesis), may also have contributed to Casio's development of SD synthesis to diversify their offering, even while they expanded PD synthesis in the VZ line of synths. SD synthesis is used in six Casio synthesizers and home keyboards released between 1987 and 1989 and and produced until 1991, when Casio exited the synthesizer market completely and focused solely on pure consumer keyboards.

SD Synthesis Summary

SD synthesis produces moving 4-bit waveforms and digital noise from a digitally-controlled oscillator (DCO) and a noise oscillator. Most SD synthesizers use a single DCO per voice, using one of 32 possible waveforms (such as square, saw, and various pulses, some including noise). The top-of-the-line Casio HT-6000 had 64 possible waveforms and a far more sophisticated synthesis engine. In SD synthesis, a traditional voltage-controlled filter (VCF) with resonance is used to shape the DCO's sound, as is a digitally-controlled amplitude (DCA) envelope. Both the VCF and DCA are programmed with a traditional 4-stage attack/decay/sustain/release (ADSR) curve [whereas the CZ line used 8-stage envelopes and also included a pitch envelope]. Finally, a low-frequency oscillator (LFO) is programmable to alter the DCO pitch (but unfortunately cannot affect the VCF or DCA). SD synthesis is very similar to traditional subtractive analogue synthesis, with the main difference being that some of the SD digital waveforms changed dynamically over time or with different DCA envelopes.

Summary of Models

HZ-600

The HZ-600 was the first SD synthesizer, and was the only SD synthesizer cased as a professional synthesizer, i.e., without built-in speakers or auto-accompaniament. In contrast to the bulky-looking Casio CZ line, the HZ-600 was deliberately styled after the sleek-looking Roland Alpha Juno 2 right down to the inclusion of an "alpha-dial" programming wheel. The HZ-600 was a 61-key, 8-note polyphonic basic-MIDI synthesizer without inital- or after-touch. Like the Korg Poly-800, all "upper tone" voices shared a single VCF, meaning the VCF envelope would retrigger when a new note was played, affecting all previous notes still playing. The "lower tone" channel had its own separate VCF, shared by lower-tone notes. The HZ-600 included onboard analogue chorus, 3 selectable keyboard split points, 3 selectable pitch-bender ranges, modulation wheel, transpose, and a card slot for the new Casio RA-100 RAM cards, which had 8K of memory. Unlike the CZ series, portamento was not available.

MT-600

The MT-600 was a non-programmable home keyboard version of the HZ-600. While not technically a synthesizer (the tones could not be altered and new sounds could not be created), it used the SD synthesis engine and had the same preset patches as the HZ-600, though arranged in a different order. The MT-600 included a pitch-bender, which was unusual for home keyboards at the time. Unlike the HZ-600, the pitch bend range was not selectable. Auto-accompaniament used a fixed bass patch, and the "lower tone" sounds of the HZ-600 for chords. Drums were low-resolution PCM samples and resembled an expanded Casio SK-5 drumkit. The MT-600 was 3-part mutitimbral for use as a MIDI sound source.

HT-700

The HT-700 was a hybrid of the HZ-600 and the MT-600 (hence the prefix HT). It included the fully-editable SD synthesis of the HZ-600, plus it took the auto-accompaniament of the MT-600 and made it fully editable too (a very rare and powerful feature). Users could fully program their own 2-measure patterns consisting of drums, bass and chord inversions. The fill-in measure was also programmable. The HT-700 had 49 mini-keys and a pitch bender, and looked very similar to the MT-600. Unlike the MT-600, though, it included a programming wheel and a card slot for Casio RA-100 RAM cards. The HT-700 and other HT synths could not store as many patches as the HZ-600, because most of the RAM card capacity was reserved to store accompaniament patterns and chord/operation sequences. The HT-700 was also sold by Hohner in Germany as the KS-49.

HT-3000

The HT-3000 was the full-size version of the HT-700. Like the HZ-600 (but in distinction from the MT-600 and HT-700), it had 61 full-size keys, a modulation wheel, volume-pedal jack, MIDI THRU, and a 3-point splittable keyboard. It added a few other features such as an "Ending" for auto-rhythms. The HT-700 was also sold by Hohner in Germany as the KS-61.

HT-3500

The HT-3500 was not released in North America. Details are unclear but it is not believed to be substantively different from the HT-3000.

HT-6000

The HT-6000, relased in 1989, was an entirely different SD synthesizer which greatly expanded SD the synthesis engine. While designed as a consumer model, it was far more powerful than the professional-oriented HZ-600. The HT-6000 reintroduced some of the more high-end features previously only included by Casio in the CZ line (such as ring-modulation, detuning, key-follow, and initial-touch [which was only found on the CZ-1]). The HT-6000 used an impressive 4 DCO's per voice (vs. 1 on ther other SD synths, and 2 on the CZ synths). It had 64 DCO wave forms to choose from (32 basic, 16 with noise [white or metallic], and 16 with ring modulation). It had 8 independent VCF filters (1 per voice, vs. 1 per channel), and added key-follow parameters for both the DCA and VCF. The DCA also added attack and decay curves (acute and obtuse). It also added an independent ADSR envelope for noise. The 4 DCO's each use the same waveform, VCF and DCA envelopes, but can have separate tunings, velocity response curves, and relative amplitudes. Stacking the oscillators with detunes allowed the creation of phase and chorus effects, or the creation of dual-note or even triad and 4-note leads. The analogue filters could be set to respond to velocity which added some expressiveness. Because each oscillator could have separate tuning and velocity response, it was also possible to have the pitch change according to pressure, if one of two differently-tuned oscillators had an inverse velocity curve. Ring modulation used ocsillator 4 to modulate oscillator 3, and allowed the creation of metallic and pulse sounds, lower bass harmonics and even distortion. Like the HZ-600 but unlike the others, the HT-6000 had a complete parameter list silkscreened on the outer panel, somewhat alleviating the need for a manual. In terms of "home keyboard" features the HT-6000 improved the auto-accompaniament versus the others by including some additional PCM drum sounds, an additional accompaniment part ("obbligato"), 4 bass patches (versus one), additional chord inversions (including more tonic, sus4 and subdominant triads), and the addition of "Intro" and drum and chord "Variation" for auto-rhythms. The HT-6000 was also sold in Germany by Hohner as the KS-610/TR.

Criticisms of the SD Synths

Due to some limitations plus Casio's poor marketing, the SD synths never gained traction in the marketplace and are now fairly rare in the second-hand marketplace (which adds to their charm, according to some). Here are some criticisms leveled by former owners of the synths:

  • Poor bass frequencies (though the HT-6000 can do somewhat better using the ring-modulator).
  • No MIDI System Exclusive feature for editing patches via a computer program.
  • Shared VCF (except on the HT-6000).
  • Resonant filter does not seem to self-oscillate (though the manual says it does).
  • Lo-fi drums (though these are making a comeback in certain types of music).
  • LFO can only modulate pitch, but not the filter or amplitude.
  • 5-octave range even over MIDI-in (notes outside this range are octave-shifted into the range).
  • The ADSR envelopes are too fast.
  • The DCO waveform selection is too limited, and many of the waveforms sound similar.
  • RA-100 RAM cards are virtually impossible to find (though they do exist).

Praises for the SD Synths

For everybody who was disappointed in the SD synths, there seems to be others who speak very positively of them, particularly regarding:

  • The true analogue filter is better than the simulated filter of the CZ series.
  • Some sounds are very unique and difficult or impossible to replicate on other synths.
  • The programming is fairly easy even without a manual.
  • The HT-6000 with up to 32 DCO's (4 per voice x 8 voices) is reported to have the most DCO's of any analogue synthesizer.
  • If thought of as home keyboards rather than professional synths, then the SD synths were cutting edge and highly underrated.

Feature Comparison Matrix

Casio SD Synth Features MT-600 HZ-600 HT-700 HT-3000/3500 HT-6000
Keys 49 Mini 61 Full 49 Mini 61 Full 61 Full
Built-in Speakers Yes (1W+1W) No Yes (1W+1W) Yes (2W+2W) Yes (2W+2W)
Velocity (Initial Touch) Sensitive No No No No Yes
Polyphony 8 8 8 8 8
Multi-timbral MIDI Channels 3 2 3 3 4
Programmable SD Synthesis No Yes Yes Yes Yes, Expanded
Headphones Jack Rear, 1/8" Front, 1/4" Rear, 1/8" Front, 1/4" Front, 1/4"
Volume Pedal/Line-in Jack No Yes, 1/4" Stereo No Yes, 1/4" Stereo Yes, 1/4" Stereo
Line-out Jacks None L & R 1/4" RCA L & R 1/4" L & R 1/4"
MIDI Jacks In, Out In, Out, Thru In, Out In, Out, Thru In, Out, Thru
Transpose No -5 to +6 -5 to +6 -5 to +6 -5 to +6
Chorus No Analogue, 3 levels Analogue, 3 levels Analogue, 3 levels Analogue, 3 levels
RAM Card Patch Capacity (Upper/Lower) None 120 / 60 20 / 10 20 / 10 20 / 10
Accompaniament Sequencer Memory (Chords/Operations) 1304/395 None 2x 640/198 2x 640/198 2x 427/198
Pitch Bender Range (Semitones) Fixed @ 2 Selectable (2, 3, 5) Fixed @ 2 Fixed @ 2 Fixed @ 2
Modulation Wheel No Yes No Yes Yes
Programming Wheels None 1 1 1 2
Auto-shutoff override No Yes No No Yes
Keyboard Split None 3 split points 2 (only for auto-accompaniament) 3 split points 3 split points
Preset Patches Piano, Harpsichord, Jazz Organ, Brass Ens, Symph Ens, Synth Bells, Magical Wind, Blues Harmonica, Light Harp, Plunk Extend, Elec Piano, Vibraphone, Synth Clavi, Strings, Synth Bass, Synth Celesta, Pearl Drop, Synth Reed, Fantasy, Typhoon Sound.( Same as MT-600 Same as MT-600 Same as MT-600 Synth Ens 1, Cosmic Dance, String Ens, Brass Ens, Pipe Organ, Piano, Harpsichord, Guitar, Trumpet, Vibraphone, Synth Ens 2, Cathedral, Symphonic Ens, Synth Brass, Jazz Organ, Elec Piano, Harp, Funky Clavi, Flute, Synth Bells
"Internal" Patch Defaults (Rewritable except on the MT-600) Piano 2, Marimba, Pipe Organ, Strings 2, Synth Ens 1, Synth Vib 1, Koto, Double Reed, Clarinet, Miracle, Elec Piano, Vibraphone 2, Violin, Synth Strings, Synth Ens 2, Synth Vib 2, Synth Harp, Slash Reed, Synth Guitar, Explosion (Internal sounds common to all 4 models.) Same as MT-600 Same as MT-600 Same as MT-600 Synth Ens, Space Fantasy, Chorus, Synth Harp 1, Pipe Organ 2, Piano 2, Harpsichord 2, Harmonica, Synth Reed, Steel Drum, Brass Ens 2, Metallic Sound, Synth Sound, Fantasy, Jazz Organ 2, Synth Celesta, Synth Harp 2, Clarinet, Synth Guitar, Marimba
Auto accompaniment patterns 20 presets, non-programmable None 20 presets, 10 internal, +10 card 20 presets, 10 internal, +10 card 20 presets, 10 internal, +10 card
Volume Sliders Master, Accompaniment Master, Lower Tone Master, Accompaniment, Drums Master, Accompaniment/Lower Tone, Drums Master, Accompaniment/Lower Tone, Drums
Accompaniment Parts 2 (Chord, Bass) 0 2 (Chord, Bass) 2 (Chord, Bass) 3 (Chord, Bass, Obligatto)
Bass Patches 1 0 1 1 4 (Wood, Elec, Slap, Synth)
Obligatto Patches 0 0 0 0 6
Programmable PCM Percussion Sounds No programmable percussion. Preset rhythms use HT-700/3000 drums. None 15 (bass, snare, rim-shot, elec. toms (hi, low), ride, claps, hi-hat (open, closed), bongos (hi, low), timbales (hi, low), agogos (hi, low), claps) Same as HT-700 18 (same as HT-700/3000 but adds gated snare, timpani, orchestra hit, cowbell, and drops the claps.)
Chord Inversions available for Custom Accompaniment Not programmable No accompaniment 8 8 14
Rhythm Intro No No No No Yes
Rhythm Ending No No No Yes Yes
Rhythm Variation No No No No Yes
Accompaniment Variation No No No No Yes
Synth Oscillators/Voice 1 1 1 1 4 for upper tones, 2 for lower tones
Available DCO Waveforms DCO-based presets uneditable. 32 (Some including noise oscillation) 32 (Some including noise oscillation) 32 (Some including noise oscillation) 64 (32 basic, 16 including noise oscillation and 16 including ring modulation)
Programmable LFO Settings None Pitch only. Delay, speed, depth, wave (saw up/down, sine, square, random) Pitch only. Delay, speed, depth, wave (saw up/down, sine, square, random) Pitch only. Delay, speed, depth, wave (saw up/down, sine, square, random) Pitch only. Delay, speed, depth, wave (saw up/down, sine, square, random)
Programmable VCF Settings None ADSR w/ cutoff frequency, resonance, depth ADSR w/ cutoff frequency, resonance, depth ADSR w/ cutoff frequency, resonance, depth ADSR w/ cutoff frequency, resonance, depth, key-follow, velocity curve
Number of VCF's 1 shared/channel 1 shared/channel 1 shared/channel 1 shared/channel 8 independent (1/voice of polyphony)
Programmable DCA Settings None ADSR, depth ADSR, depth ADSR, depth ADSR, attack and decay curves (acute or obtuse), key-follow, plus velocity curves and depths for each of 4 oscillators
Independent DCA for Noise Oscillator No No No No Yes (ADSR)
Detuning No No No No Yes, higher than fundamental, fine or coarse (but not both)
Ring Modulation No No No No Yes, oscillator 4 can modulate oscillator 3.
"Line Editor" Programming No No No No Yes: simultaneous side-by-side view of velocity, amplitude or detune for all 4 oscillators, with separate increment/decrement buttons.
Recent E-Bay Sales Prices $30 USD (circuit-bending has increased demand) $80 USD $50 USD $100 USD $250 USD
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