The voltage threshold for triggering a voltage-controlled oscillator (VCO) in frequency modulation (FM) synthesis is not typically defined by a specific voltage level, as it can vary depending on the design of the synthesizer and the specific VCO being used. Instead, the triggering or modulation of a VCO in FM synthesis is often determined by the modulation index and the modulation source's characteristics.
In FM synthesis, the frequency of a VCO is modulated by another signal (the modulation source), which is often another VCO or an envelope generator. The modulation index determines the amount of frequency modulation applied to the VCO. The greater the modulation index, the more the VCO's frequency will deviate in response to the modulation source.
So, in FM synthesis, it's more about the modulation index and the characteristics of the modulation source (such as its frequency and amplitude) rather than a specific voltage threshold for triggering the VCO. The voltage level that triggers or affects the modulation source is usually determined by the synthesizer's circuitry and settings, and it might not directly correspond to a threshold for the VCO.
It's worth noting that specific synthesizers and VCOs can have different design choices, so there isn't a universally applicable voltage threshold for triggering a VCO in FM synthesis. If you're working with a particular synthesizer or module, you should refer to its documentation to understand how its FM synthesis parameters work and how modulation sources affect the VCOs.