The Casio HZ-600, released in 1987, represents a significant entry in Casio's Spectrum Dynamic (SD) synthesizer series. As a professional-looking bitimbral synthesizer, it embodies Casio's philosophy of making advanced technology accessible at an affordable price point. The HZ-600 served in its time as the flagship model in the SD series, which included the home keyboard-style HT-3000, HT-700, and preset MT-600, all sharing an identical sound engine. Casio had also a more advanced HT-6000 with separate filters per voice.

Design and Construction

The HZ-600 features Casio's characteristic grey plastic construction, designed for both portability and durability. The synthesizer can operate on either six D-size batteries or via a dedicated mains adapter (users are strongly advised to use only Casio's power supply for optimal performance and reliability). The instrument weighs appropriately for its portable nature while maintaining a professional appearance suitable for both studio and stage use.

The full-size, 5-octave (61-key) keyboard provides a comfortable playing experience with plastic keys featuring a light action and "soft bottom" feel. While the keyboard lacks velocity sensitivity and aftertouch, it offers quick response and smooth playability, making it suitable for fast passages. The keys are full piano-sized, addressing concerns often associated with budget synthesizers.

Sound Engine and Synthesis

The HZ-600 employs Casio's innovative Spectrum Dynamic (SD) synthesis, a digital-analog hybrid system that represents a refinement of their Phase Distortion technology. This synthesis method produces the distinctive "Casio sound" - characterized by intense, evolving harmonics and the ability to recreate acoustic instrument attack characteristics with remarkable accuracy.

The synthesizer offers 8-voice polyphony and can function as a bitimbral instrument, effectively operating as two 4-voice synthesizers simultaneously. Each voice utilizes a single digital oscillator with access to 32 digitally generated waveforms for the upper voice section (16 for the lower section). The wave memory includes classical analog waveforms such as sawtooth, square, and pulse waves, as well as white noise and various combined waveforms.

Some of the 32 upper voice waveforms feature the unique Spectrum Dynamic characteristic - they evolve over time, adding organic movement to sustained sounds. This temporal evolution is controlled by the DCA envelope, creating dynamic timbres that change throughout their duration.

Signal Path and Controls

Each voice's signal path follows a traditional subtractive synthesis model:

Oscillator Section: Single digital oscillator per voice with 32 waveforms (upper) or 16 waveforms (lower). No sub-oscillator or octave shifting capabilities are provided.

Modulation: One LFO controls vibrato exclusively, offering adjustable speed, depth, waveform selection (5 waveforms available), and delay parameters.

Filter Section: The HZ-600 features two VCFs (Voltage Controlled Filters) using NJM2090 chips - 4-pole low-pass filters with individual ADSR envelopes. Each filter offers adjustable cutoff frequency, resonance (without self-oscillation capability), and envelope depth control. This dual-filter design allows independent processing of upper and lower keyboard sections.

Amplifier Section: Each section has its own DCA (Digitally Controlled Amplifier) with ADSR envelope and adjustable depth parameter.

Programming and Interface

The HZ-600 addresses the common programming challenges of digital synthesizers through an intuitive interface design. Programming is accomplished using two switches (Parameter and Value) and a large Alpha-Dial wheel. All parameters are clearly printed on the front panel, making the programming process straightforward and accessible.

The four-character LCD display, while smaller than many competitors, features large, clear characters and provides adequate information for programming tasks. The alpha dial data entry system eliminates the tedious button-pressing associated with many synthesizers of the era.

Parameter editing involves pressing the Mode button, spinning the alpha dial to select the desired parameter, pressing the Data button, and adjusting values via the alpha dial. This streamlined approach makes sound creation and editing surprisingly user-friendly.

Memory and Storage

The HZ-600 provides comprehensive sound storage across three memory types:

ROM Preset Memory: 20 upper tones and 10 lower tones permanently stored in read-only memory. These factory presets, while distinctively "Casio" in character, serve as starting points for programming and demonstrate the synthesizer's sonic capabilities.

Internal RAM Memory: 20 upper tones and 10 lower tones in user-programmable random access memory. These locations can be overwritten with custom sounds and are initialized with more adventurous factory programming than the presets.

Optional Memory Card: The RA-100 memory card (credit card style) can store up to 120 upper tones and 60 lower tones, dramatically expanding the instrument's sound library capacity.

It's important to note that memory is volatile - the instrument requires constant power to maintain RAM contents, and there's no tape dump facility or MIDI memory dump capability.

Split and Layer Functionality

One of the HZ-600's standout features is its sophisticated split keyboard functionality. A dedicated button instantly activates keyboard split mode, with the split point clearly indicated by bright LEDs above the keyboard. Three fixed split points are available, strategically positioned to accommodate various playing styles and hand positions.

When split, the instrument operates as two independent 4-voice synthesizers with separate memory banks, parameter sets, and volume controls (via dedicated sliders). Each section can be programmed independently, though they share the same vibrato settings when active. Unfortunately, both sections appear on both stereo outputs, limiting stereo separation possibilities.

Effects and Sound Character

The HZ-600 includes a built-in chorus effect with four settings: off, slow weak, slow deep, and fast weak. The chorus setting is stored with each patch and affects both upper and lower tones simultaneously.

The overall sound character is distinctly "Casio" - somewhat tinny with limited low-end response, but capable of producing excellent bells, metallic sounds, piano-like tones, combo organ textures, string sounds, analog-style brass leads, and unique sound effects and pads. The synthesis excels at creating magical, ethereal timbres with intense harmonic content and evolving characteristics.

MIDI Implementation

The HZ-600 features comprehensive MIDI connectivity with In, Out, and Thru ports. The basic MIDI channel is adjustable between 1 and 15 for the upper section, with the lower section automatically assigned to the next higher channel. The instrument recognizes and transmits Note On/Off, Pitch Bend, Modulation (when switched on), Program Change, and Sustain Pedal messages.

While the HZ-600 cannot transmit or receive velocity information, it serves as an effective master keyboard capable of controlling other MIDI devices on two channels simultaneously.

Controls and Connectivity

Front Panel: Two standard wheels for pitch bend (spring-centered) and modulation, two volume sliders (main and lower), four-character LCD display, and comprehensive programming controls.

Rear Panel: Dual quarter-inch stereo outputs, sustain pedal input, foot volume control input, MIDI In/Out/Thru ports, DC 9V power input, power switch, and recessed pitch adjustment knob (±50 cents range).

Additional Features: Headphone output on the front panel, auto-off function (6-minute timeout), pitch bend range selection (2, 3, or 5 semitones), keyboard transpose function (+6/-5 semitones), and basic channel assignment.

Target Market and Applications

The HZ-600 serves multiple markets effectively. For beginners, it provides an accessible introduction to synthesis with simplified programming and comprehensive features at an affordable price. The instrument's user-friendly interface and immediate sound results make it ideal for those new to electronic music production.

Professional musicians will find the HZ-600 valuable as a distinctive sound source capable of cutting through dense mixes with unique lead lines and textural elements. Its bitimbral operation and MIDI implementation make it suitable for both studio recording and live performance applications.

The synthesizer particularly excels in electronic and experimental music contexts where its characteristic "magical" sound qualities can be fully appreciated, rather than applications requiring realistic acoustic instrument emulation.

Technical Specifications

General

• Type: Digital-Analog Hybrid Synthesizer
• Release Year: 1987

Sound Generation

• Synthesis Method: Spectrum Dynamic (SD) - Phase Distortion variant
• Polyphony: 8 voices
• Oscillators: 1 digital oscillator per voice
• Waveforms:
  • Upper section: 32 waveforms (including SD evolutionary waves)
  • Lower section: 16 waveforms
  • Includes: Sine, sawtooth, square, pulse, white noise, combined waves
• Sub-oscillators: None
• Octave Switching: None available
• Multitimbral: Bitimbral (Upper/Lower sections)
• Tuning: ±50 cents via rear panel control

Modulation

• LFO: 1 LFO per voice
• LFO Waveforms: 5 selectable waveforms
• LFO Destinations: Vibrato only
• LFO Parameters: Speed, Depth, Waveform, Delay

Filters

• Type: 2 × 4-pole Low-pass VCF (NJM2090)
• Configuration: One filter per section (Upper/Lower)
• Parameters: Cutoff frequency, Resonance (no self-oscillation), ADSR envelope
• Envelope: Individual ADSR with adjustable depth per filter

Amplification

• Type: 2 × DCA (Digitally Controlled Amplifier)
• Configuration: One DCA per section
• Envelope: ADSR with adjustable depth per section

Memory

• ROM Presets: 20 Upper tones, 10 Lower tones
• Internal RAM: 20 Upper tones, 10 Lower tones (user programmable)
• Memory Card: Optional RA-100 card
  • 120 Upper tones
  • 60 Lower tones
• Memory Type: Volatile RAM (requires constant power)
• Backup: Battery backup only, no tape interface

Keyboard

• Keys: 61 full-size keys (5 octaves)
• Action: Light action with soft bottom feel
• Velocity Sensitivity: None
• Aftertouch: None
• Split Points: 3 fixed positions
• Split Operation: Independent 4-voice sections

Controls and Interface

• Programming: 2 switches (Parameter/Value) + Alpha-Dial wheel
• Display: 4-character LCD with large characters
• Wheels: Pitch Bend (spring return), Modulation
• Sliders: Main Volume, Lower Volume
• Split Control: Dedicated split button with LED indicators

Effects

• Chorus: Built-in with 4 settings
  • Off
  • Slow Weak
  • Slow Deep
  • Fast Weak
• Storage: Chorus setting stored per patch

MIDI Implementation

• Ports: In, Out, Thru (5-pin DIN)
• Channels:
  • Basic channel: 1-15 (Upper section)
  • Lower section: Basic channel +1
• Messages Supported:
  • Note On/Off
  • Pitch Bend
  • Modulation Wheel
  • Program Change
  • Sustain Pedal
• Velocity: Not supported (transmit or receive)
• Memory Dump: Not supported

Audio Connections

• Outputs: 2 × 1/4" stereo outputs (both sections mixed)
• Headphone: 1 × 1/4" front panel jack
• Pedal Inputs:
  • Sustain pedal (1/4")
  • Foot volume control (1/4")

Construction:

• Grey plastic housing

Power

• Power Options:
  • DC 9V adapter (Casio recommended)
  • 6 × D-size batteries
• Auto-Off: 6-minute timeout (defeatable)

Additional Features

• Pitch Bend Range: Selectable 2, 3, or 5 semitones
• Transpose: +6/-5 semitones in semitone steps
• Parameter Labeling: All parameters printed on front panel
• Split Volume: Independent volume controls for each section
• Programming Access: All synthesis parameters user-accessible