In the APU, what are the conditions to output a sample of "silence" (zero) instead of a "volume level" (non zero), including the duty cycles (0 and 1) but as negative and positive?
The initial output is always zero to positive, there's no negative. After this it goes to a highpass filter, which will centre a playing waveform around zero a little bit after its onset, and when it stops playing it will drift back up to the zero line.
The idea of +1 and -1 (positive and negative samples on square waves, duty cycles) came from the defunct Nofrendo.