http://www.gamepilgrimage.com/sites/def ... apudsp.txt
https://www.nesdev.org/wiki/APU_Noise#: ... %20counter.The noise generator seems to function something like this: On reset, N=0x4000. Each update (see FLG), N=(N>>1)|(((N<<14)^(N<<13))&0x4000). And the output noise sample at any point is N (after which is volume adjustment then the left-shift to 'restore' the low bit).
If both of these are accurate, then that means that the noise channel works the same in both systems. The only differences are that NES only outputs bit 14 whereas, the SNES outputs the whole shift register, and they have a different set of sample rates.When the timer clocks the shift register, the following actions occur in order:
1) Feedback is calculated as the exclusive-OR of bit 0 and one other bit: bit 6 if Mode flag is set, otherwise bit 1.
2) The shift register is shifted right by one bit.
3) Bit 14, the leftmost bit, is set to the feedback calculated earlier.