The volume envelopes of the audio signals are controlled by an "Attack-Decay-Sustain-Release" generator (ADSR1). ADSR1 is triggered by the "Gate" signal from keyboard interface module.
ADSR1 is special in that in can supply four different envelopes simultaneously, albeit the timing of all four is the same. This means that the volume levels after the attack phase can be different and the sustain levels can be different for each of the outputs. This allows for some detailed sound control, when the VCOs run at different frequencies or wave forms.
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