APU Noise

The NES APU triangle channel generates a pseudo-triangle wave. It has no volume control; the waveform is either cycling or suspended. It includes a linear counter, an extra duration timer of higher accuracy than the length counter.

The triangle channel contains the following: timer, linear counter, length counter, halt flag, sequencer. The linear counter contains an internal halt flag and counter.

      Linear Counter   Length Counter
            |                |
            v                v
Timer ---> Gate ----------> Gate ---> Sequencer ---> (to mixer)

$4008	CRRR.RRRR	Linear counter setup (write)
bit 7	C---.----	Control flag (this bit is also the length counter halt flag)
bits 6-0	-RRR RRRR	Counter reload value
$400A	LLLL.LLLL	Timer low (write)
bits 7-0	LLLL LLLL	Timer low 8 bits
$400B	llll.lHHH	Length counter load and timer high (write)
bits 2-0	---- -HHH	Timer high 3 bits
Side effects		Sets the halt flag

The timer's period is the 11-bit value (%HHH.LLLLLLLL) formed by timer high and timer low, plus one.

When the frame counter generates a linear counter clock, the following actions occur in order:

1. If the halt flag is set, the linear counter is reloaded with the counter reload value, otherwise if the linear counter is non-zero, it is decremented.
2. If the control flag is clear, the halt flag is cleared.

The sequencer is clocked by the timer except when

• The linear counter is zero, or
• The length counter is zero

The sequencer sends the following looping 32-step sequence of values to the mixer:

15, 14, 13, 12, 11, 10,  9,  8,  7,  6,  5,  4,  3,  2,  1,  0
 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15

At the lowest two timer periods ($400B = 0 and $400A = 0 or 1), the resulting frequency is so high that the mixer effectively receives a value half way between 7 and 8.