APU Noise

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Revision as of 05:28, 1 July 2009 by Banshaku (talk | contribs) (The triangle section was copied inside the noise one by accident.)
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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, 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)

When the timer clocks the shift register, the following two actions occur in order:

  1. Bit 15 of the shift register is replaced with the exclusive-OR of bit 0 and one other bit: bit 6 if loop is set, otherwise bit 1.
    Shift register bits: %F-------.-6----10
  2. The shift register is shifted one bit right (bit 0 is lost).

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

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