If you modulate one oscillator directly with another it creates very harsh harmonics, so you need to limit the modulation level to control the amount of modulation. In Digitone you use an envelope and a level (LEV) parameter control to attenuate the amount of how much the modulator affects the carrier. The envelopes also give the possibility to control the modulation over time. A piano for example, when first struck, the timbre is sharp but quickly fades to a softer tone. Envelopes and modulation level are vital elements in FM synthesis and are the tools that you use to shape the sounds you want to achieve.
The Digitone FM engine has two operator envelopes that are designed to be practical and easy to use. One envelope is for operator group A, and one is for group B (B1 and B2).
The envelopes are essentially expanded AD (Attack Decay) envelopes, but with an added adjustable end level (the amplitude level the sound reaches at the end of the decay stage). Normally, an AD envelope always ends at zero level, but with FM you often want to retain some modulation after a short pluck or fade in for example.
Please note that operator envelope B controls the output from both B1 and B2.
The envelopes can be either triggered or gated - making it either an ADE (Attack Decay End) or an ASDE (Attack Sustain Decay End) envelope. The sustain phase does not have an envelope level. It is instead the LEV parameter that sets the sustain level. The note length defines the length of the sustain phase.
Trigged (ATRG/BTRG ON)
Gated (ATRG/BTRG OFF)
You can also set if the envelopes should reset or not when they are retrigged.
Reset on (ARST/BRST ON)
Reset off (ARST/BRST OFF)
The LEV parameters on the SYN2 page controls the amount of modulation from the A and B operators. The LEV parameter for B is macro mapped to both operator B1 and B2 and control their modulation amount as per this graph:
Read more about the Digitone FM synthesis here:
Digitone FM synthesis overview.