VRC6 audio

Konami's VRC6 mapper provided 3 extra channels for sound: two pulse waves, and one sawtooth. All channels operate similarly to the native channels in the NES APU.

On some boards, the A0 and A1 lines were switched, so for those boards, registers will need adjustment when emulating ($x001 will become $x002 and vice versa). Registers listed here are how they are for 悪魔城伝説 (Akumajou Densetsu, iNES mapper 024). For Madara and Esper Dream 2 (iNES mapper 026), you will need to adjust the registers.

Registers

Frequency Control ($9003)

Normally you should write $00 to this register on startup to initialize it, and not make any further writes to it. This is what all three original VRC6 games do.

$9003 controls the overall frequency of the VRC6 audio.

7  bit  0
---- ----
.... .ABH
      |||
      ||+- Halt
      |+-- 16x frequency (4 octaves up)
      +--- 256x frequency (8 octaves up)

H - halts all oscillators, stopping them in their current state
B - 16x frequency, all oscillators (4 octave increase)
A - 256x frequency, all oscillators (8 octave increase)

• The halt flag overrides the other flags.

• The 256x flag overrides the 16x flag.

• The 16x/256x flags effectively control a 4-bit and 8-bit right shift of the 12-bit period registers.

Pulse Control ($9000,$A000)

$9000 controls Pulse 1

$A000 controls Pulse 2

7  bit  0
---- ----
MDDD VVVV
|||| ||||
|||| ++++- Volume
|+++------ Duty Cycle
+--------- Mode (1: ignore duty)

Saw Accum Rate ($B000)

7  bit  0
---- ----
..AA AAAA
  ++-++++- Accumulator Rate (controls volume)

Freq Low ($9001,$A001,$B001)

$9001 controls Pulse 1

$A001 controls Pulse 2

$B001 controls Saw

7  bit  0
---- ----
FFFF FFFF
|||| ||||
++++-++++- Low 8 bits of frequency

Freq High ($9002,$A002,$B002)

$9002 controls Pulse 1

$A002 controls Pulse 2

$B002 controls Saw

7  bit  0
---- ----
E... FFFF
|    ||||
|    ++++- High 4 bits of frequency
+--------- Enable (0 = channel disabled)

Pulse Channels

The VRC6 pulse channels operate similarly to the NES's own pulse channels. The CPU clock rate (1.79 MHz) drives the 12-bit divider 'F'. Every cycle the divider counts down until it reaches zero, at which point the divider resets and the duty cycle generator is clocked.

The duty cycle generator takes 16 steps. When the current step is greater than than the given duty cycle 'D' the current channel volume 'V' is output, otherwise, 0 is output. (The first part of the duty cycle outputs 0, the second half outputs 'V'.)

When the mode bit 'M' is true, the channel ignores the duty cycle generator and outputs the current volume regardless of the current duty.

Therefore, 'D' and 'M' values provide the following duty cycles:

D value	Duty (percent)
0	1/16	6.25%
1	2/16	12.5%
2	3/16	18.75%
3	4/16	25%
4	5/16	31.25%
5	6/16	37.5%
6	7/16	43.75%
7	8/16	50%
M	16/16	100%

When the channel is disabled by clearing the 'E' bit, output is forced to 0, and the duty cycle is immediately reset and halted; it will resume from the beginning when 'E' is once again set. Thus it is possible to reset phase by clearing and immediately setting 'E'.

Sawtooth Channel

For the sawtooth, the CPU clock rate drives a 12-bit divider 'F'. Every cycle, the divider counts down until it reaches zero, at which point it reloads and clocks the accumulator. However, it seems that the accumulator only reacts on every 2 clocks.

When clocked, the rate value 'A' is added to an internal 8-bit accumulator. The high 5 bits of the accumulator are then output (provided the channel is enabled by having the 'E' bit set). After 'A' has been added 6 times, on the 7th clock, instead of 'A' being added, the internal accumulator is reset to zero.

For an example, assume an 'A' value of $08

Step	Accumulator	Output	Comment
0	$00	$00
1	$00	$00	(odd step, do nothing)
2	$08	$01	(even step, add 'A' to accumulator)
3	$08	$01
4	$10	$02
5	$10	$02
6	$18	$03
7	$18	$03
8	$20	$04
9	$20	$04
10	$28	$05
11	$28	$05
12	$30	$06
13	$30	$06
0	$00	$00	(14th step, reset accumulator)
1	$00	$00
2	$08	$01

If A is more than 42 (floor(255 / 6)), the accumulator will wrap, resulting in distorted sound.

If E is clear, the accumulator is forced to zero until E is again set. The phase of the saw generator can be reset by clearing and immediately setting E.

References

• VRCVI Chip Info by Kevin Horton