V/Oct vs Hz/V Conversion for Vintage Synthesizers

Integrating modern MIDI equipment with vintage analog synthesizers requires converting digital note data into analog control voltages (CV) to dictate pitch. This article breaks down the primary differences between the two dominant vintage pitch standards: Volts per Octave (V/Oct) and Hertz per Volt (Hz/V). Understanding their distinct mathematical curves, hardware origins, and triggering methods will help you select the right MIDI-to-CV converter and ensure your classic instruments track pitch accurately across the keyboard.

The Basic Concept of Pitch CV

MIDI transmits pitch as discrete digital values from 0 to 127. Vintage analog synthesizers, however, rely on continuous electrical voltage to adjust the frequency of an analog Voltage-Controlled Oscillator (VCO). A MIDI-to-CV converter bridges this gap by reading digital Note On/Off commands and outputting a corresponding voltage. The two standards define how that voltage translates into musical pitch.

Volt per Octave (V/Oct)

Popularized by Bob Moog and Don Buchla, and later adopted by Roland, Sequential Circuits, Oberheim, and the modern Eurorack format, 1V/Oct is the most common analog pitch standard.

• How It Works: The relationship between voltage and musical intervals is linear. Every one-volt increase raises the pitch by exactly one octave (12 semitones). Therefore, each semitone corresponds to approximately 0.0833 volts (1/12th of a volt).
• Internal Architecture: Because musical pitch doubles in frequency every octave (e.g., 220 Hz to 440 Hz is an exponential change), synthesizers built for V/Oct require an internal exponential converter circuit. This circuit takes the linear voltage increase and translates it into an exponential frequency curve to drive the oscillator.
• Advantages: Transposition and summing are straightforward. Adding a fixed voltage (such as +1V) automatically shifts the pitch by a musical octave regardless of the starting note.

Hertz per Volt (Hz/V)

Pioneered primarily by Japanese manufacturers—most notably Korg (e.g., MS-10, MS-20) and Yamaha (e.g., the CS-series)—Hz/V takes a different engineering approach.

• How It Works: The relationship between voltage and frequency is directly linear. A fixed increase in voltage results in a fixed increase in frequency (Hertz). To raise the pitch by one octave, the voltage must double. For example, if 1V produces 100 Hz, 2V produces 200 Hz (one octave up), and 4V produces 400 Hz (another octave up).
• Internal Architecture: Because the frequency scales linearly with the input voltage, Hz/V synthesizers do not need an internal exponential converter circuit. This made early Hz/V synthesizers less prone to pitch drift caused by temperature variations in analog exponential converters.
• Disadvantages: Pitch transposition and interval scaling cannot be handled using simple analog mixing or summing.

Why They Are Incompatible

Attempting to control a Hz/V synthesizer directly with a standard 1V/Oct CV output results in severely distorted tuning. In a V/Oct system, the step size between notes is uniform across the keyboard. In a Hz/V system, the voltage step size between notes must grow exponentially as you move higher up the keyboard. Sending a linear voltage sequence to a Hz/V synthesizer causes the notes to compress in pitch at the lower end and spread unnaturally wide at the higher end.

Gate and Trigger Standards

When adapting MIDI to vintage gear, gate and trigger formats must also match:

• V-Trigger (Voltage Trigger): Common on 1V/Oct instruments (Roland, modern Eurorack). The gate line stays at 0V when a key is released and jumps to a positive voltage (typically +5V to +12V) when a key is pressed.
• S-Trigger (Switch Trigger): Frequently used alongside Hz/V synthesizers (such as Korg), as well as vintage Moog synths. The circuit sits at a positive voltage while idle and drops to ground (0V) when a key is pressed.

Bridging the Standards

Direct passive cable adapters cannot convert V/Oct to Hz/V or vice versa due to the non-linear math involved. Controlling Hz/V vintage gear with modern MIDI setups requires:

1. A Configurable MIDI-to-CV Converter: Units from manufacturers like Kenton, Doepfer, or Expert Sleepers feature dedicated settings that recalculate digital MIDI note messages using the Hz/V logarithmic voltage curve.
2. Selectable Gate Types: The converter must support switching between standard V-Trig and inverted S-Trig outputs to properly trigger the synthesizer's envelopes.