APU Noise

The NES APU noise channel generates pseudo-random 1-bit noise at 16 different frequencies.

The noise channel contains the following: envelope generator, timer, shift register with feedback, length counter.

   Timer --> Shift Register   Length Counter
                   |                |
                   v                v
Envelope -------> Gate ----------> Gate --> (to mixer)

$400C	--le.eeee	Length counter halt and envelope (write)
$400E	L---.PPPP	Loop and period (write)
bit 7	L--- ----	Loop flag
bits 3-0	---- PPPP	The timer period is set to entry P of the following: Rate $0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $A $B $C $D $E $F -------------------------------------------------------------------------- NTSC 4, 8, 16, 32, 64, 96, 128, 160, 202, 254, 380, 508, 762, 1016, 2034, 4068 PAL 4, 7, 14, 30, 60, 88, 118, 148, 188, 236, 354, 472, 708, 944, 1890, 3778
$400F	llll.l---	Length counter load and envelope restart (write)

The shift register is 15 bits wide, with bits numbered
14 - 13 - 12 - 11 - 10 - 9 - 8 - 7 - 6 - 5 - 4 - 3 - 2 - 1 - 0

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 loop 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.

This results in a pseudo-random bit sequence, 32767 bits long when loop is clear, otherwise 93 bits long (the particular 93-bit sequence depends on where in the 32767-bit sequence the shift register was when loop was set).

The mixer receives the current envelope volume except when

• Bit 0 of the shift register is set, or
• The length counter is zero

On power-up, the shift register is loaded with the value 1.