The whole circuit
Keyboard decoder: The PICsynth chip tests for keypresses by outputing binary onto note bus. This is decoded by decoder and fed to keyboard switches, the octave sense lines activate on keypress, which are read by the PICsynth chip.
PICsynth dual DCO (Digitally controlled oscillator) : the PICsynth chip produces 2 oscillators, in tune or detuned slightly.
Waveshape : The square waves are fed to waveshape circuit. This produces lower octaves and turns the square waves into ramp, pulse or triangle.
The VCF (Voltage controlled filter) : removes or highlights parts of the waves.
The VCA (voltage controlled amplifier) : controls the attack and decay of the notes according to control voltage provided by envelope generator.
The whole circuit