APU Noise

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

The noise channel contains the following: envelope generator, timer, Linear Feedback Shift Register, length counter.

   Timer --> Shift Register   Length Counter
                   |                |
                   v                v
Envelope -------> Gate ----------> Gate --> (to mixer)
$400C --lc.vvvv Length counter halt, constant volume/envelope flag, and volume/envelope divider period (write)
 
$400E M---.PPPP Mode and period (write)
bit 7 M--- ---- Mode 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, 8, 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 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.

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

The mixer receives the current envelope volume except when

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

The earliest revisions of the 2A03 CPU ignored the Mode flag, treating it as always 0. These CPUs were used in the first batch of Famicom consoles that were recalled, in Vs. System boards, and in the arcade games that used the 2A03 as a sound coprocessor.[1]

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