APU Length Counter

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The length counter provides automatic duration control for the NES APU waveform channels. Once loaded with a value, it can optionally count down (when the length counter halt flag is clear). Once it reaches zero, the corresponding channel is silenced.

Address Bitfield Description
$4015 ---d.nt21 DMC control and length counter enabled flags (write)
 
$4000 ssHc.vvvv Pulse channel 1 duty cycle, length counter halt, constant volume flag, and volume/envelope (write)
$4004 ssHc.vvvv Pulse channel 2 duty cycle, length counter halt, constant volume flag, and volume/envelope (write)
$400C --Hc.vvvv Noise channel length counter halt, constant volume flag, and volume/envelope (write)
bit 5 --H- ---- Halt length counter (this bit is also the envelope's loop flag)
 
$4008 Hlll.llll Triangle channel length counter halt and linear counter load (write)
bit 7 H--- ---- Halt length counter (this bit is also the linear counter's control flag)
 
$4003 LLLL.Lttt Pulse channel 1 length counter load and timer (write)
$4007 LLLL.Lttt Pulse channel 2 length counter load and timer (write)
$400B LLLL.Lttt Triangle channel length counter load and timer (write)
$400F LLLL.L--- Noise channel length counter load (write)
bits 7-3 LLLL L--- If the enabled flag is set, the length counter is loaded with entry L of the length table:
     |  0   1   2   3   4   5   6   7    8   9   A   B   C   D   E   F
-----+----------------------------------------------------------------
00-0F  10,254, 20,  2, 40,  4, 80,  6, 160,  8, 60, 10, 14, 12, 26, 14,
10-1F  12, 16, 24, 18, 48, 20, 96, 22, 192, 24, 72, 26, 16, 28, 32, 30
Side effects The envelope is restarted, for pulse channels phase is reset, for triangle the linear counter reload flag is set.

Clocking

When the enabled bit is cleared (via $4015), the length counter is forced to 0 and cannot be changed until enabled is set again (the length counter's previous value is lost). There is no immediate effect when enabled is set.

When clocked by the frame counter, the length counter is decremented except when:

  • The length counter is 0, or
  • The halt flag is set

Table structure

The structure of the length table becomes clearer when rearranged like in the following index to length map (which corresponds to the order in which the values appear in the internal APU lookup table):

Legend:
<bit pattern> (<value of bit pattern>) => <note length>

Linear length values:
1 1111 (1F) => 30
1 1101 (1D) => 28
1 1011 (1B) => 26
1 1001 (19) => 24
1 0111 (17) => 22
1 0101 (15) => 20
1 0011 (13) => 18
1 0001 (11) => 16
0 1111 (0F) => 14
0 1101 (0D) => 12
0 1011 (0B) => 10
0 1001 (09) => 8
0 0111 (07) => 6
0 0101 (05) => 4
0 0011 (03) => 2
0 0001 (01) => 254

Notes with base length 12 (4/4 at 75 bpm):
1 1110 (1E) => 32  (96 times 1/3, quarter note triplet)
1 1100 (1C) => 16  (48 times 1/3, eighth note triplet)
1 1010 (1A) => 72  (48 times 1 1/2, dotted quarter)
1 1000 (18) => 192 (Whole note)
1 0110 (16) => 96  (Half note)
1 0100 (14) => 48  (Quarter note)
1 0010 (12) => 24  (Eighth note)
1 0000 (10) => 12  (Sixteenth)

Notes with base length 10 (4/4 at 90 bpm, with relative durations being the same as above):
0 1110 (0E) => 26  (Approx. 80 times 1/3, quarter note triplet)
0 1100 (0C) => 14  (Approx. 40 times 1/3, eighth note triplet)
0 1010 (0A) => 60  (40 times 1 1/2, dotted quarter)
0 1000 (08) => 160 (Whole note)
0 0110 (06) => 80  (Half note)
0 0100 (04) => 40  (Quarter note)
0 0010 (02) => 20  (Eighth note)
0 0000 (00) => 10  (Sixteenth)

With the least significant bit set, the remaining bits select a linear length (with the exception of the 0 entry). Otherwise, we get note lengths based on a base length of 10 (MSB clear) or 12 (MSB set).

Length counter internals

In the actual APU, the length counter silences the channel when clocked while already zero (provided the length counter halt flag isn't set). The values in the above table are the actual values the length counter gets loaded with plus one, to allow us to use a model where the channel is silenced when the length counter becomes zero.

The triangle's linear counter works differently, and does silence the channel when it reaches zero.