The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in Frequency Modulation (FM) synthesis is not a fixed value and can vary depending on the specific circuit design and components used in the synthesizer.
In FM synthesis, the modulation index (ratio of modulating frequency to carrier frequency) and the amplitude of the modulation signal determine the amount of frequency modulation applied to the carrier signal. The voltage applied to the VCO's control input determines the frequency deviation from the carrier frequency, which corresponds to the strength of the modulation.
The actual voltage threshold can be influenced by factors such as the VCO's design, the specific electronic components used (like transistors or integrated circuits), the power supply voltage, and the intended range of modulation. Generally, VCOs used in FM synthesis are designed to respond to a range of input voltages to provide a controllable and expressive modulation. The voltage range might be specified in the user manual or technical documentation of the synthesizer.
If you're working with a specific synthesizer or circuit, it's best to refer to its documentation or specifications to find out the precise voltage range required to trigger the VCO effectively for frequency modulation.