The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in Frequency Modulation (FM) synthesis can vary depending on the specific design of the synthesizer and the characteristics of the VCO being used. FM synthesis involves modulating the frequency of one oscillator (the carrier) with the output of another oscillator (the modulator). The modulator's output voltage, when applied to the VCO, causes the carrier oscillator's frequency to change.
In most cases, the voltage range used to modulate a VCO in FM synthesis typically falls within the standard control voltage (CV) range of modular synthesizers, which is often between -5V and +5V. However, the actual voltage threshold required to trigger the VCO into frequency modulation can vary based on factors such as the specific VCO's design, the desired modulation depth, and the scaling and calibration of the modulation input.
It's important to consult the documentation or specifications provided by the manufacturer of the synthesizer or VCO module you are using to get accurate information about the voltage threshold for triggering FM modulation. Additionally, experimenting with different modulation voltages and observing the resulting changes in frequency can help you understand and fine-tune the behavior of the FM synthesis in your specific setup.