Audio Registers

Audio registers are named following a NRxy scheme, where x is the channel number (or 5 for “global” registers), and y is the register’s ID within the channel. Since many registers share common properties, a notation is often used where e.g. NRx2 is used to designate NR12, NR22, NR32, and NR42 at the same time, for simplicity.

As a rule of thumb, for any x in 1, 2, 3, 4:

• NRx0 is some channel-specific feature (if present),
• NRx1 controls the length timer,
• NRx2 controls the volume and envelope,
• NRx3 controls the period (maybe only partially),
• NRx4 has the channel’s trigger and length timer enable bits, as well as any leftover bits of period;

…but there are some exceptions.

One of the pitfalls of the NRxy naming convention is that the register’s purpose is not immediately clear from its name, so some alternative names have been proposed, such as AUDENA for NR52.

Global control registers

FF26 — NR52: Audio master control

• Audio on/off (Read/Write): This controls whether the APU is powered on at all (akin to LCDC bit 7). Turning the APU off drains less power (around 16%), but clears all APU registers and makes them read-only until turned back on, except NR52¹.

• CHn on? (Read-only): Each of these four bits allows checking whether channels are active². Writing to those does not enable or disable the channels, despite many emulators behaving as if.

  A channel is turned on by triggering it (i.e. setting bit 7 of NRx4)³. A channel is turned off when any of the following occurs:

  • The channel’s length timer is enabled in NRx4 and expires, or
  • For CH1 only: when the period sweep overflows⁴, or
  • The channel’s DAC is turned off. The envelope reaching a volume of 0 does NOT turn the channel off!

FF25 — NR51: Sound panning

Each channel can be panned hard left, center, hard right, or ignored entirely.

Setting a bit to 1 enables the channel to go into the selected output.

Note that selecting or de-selecting a channel whose DAC is enabled will cause an audio pop.

FF24 — NR50: Master volume & VIN panning

• VIN left/right: These work exactly like the bits in NR51. They should be set at 0 if external sound hardware is not being used.

• Left/right volume: These specify the master volume, i.e. how much each output should be scaled.

  A value of 0 is treated as a volume of 1 (very quiet), and a value of 7 is treated as a volume of 8 (no volume reduction). Importantly, the amplifier never mutes a non-silent input.

Sound Channel 1 — Pulse with period sweep

FF10 — NR10: Channel 1 sweep

This register controls CH1’s period sweep functionality.

• Pace: This dictates how often sweep “iterations” happen, in units of 128 Hz ticks⁵ (7.8 ms). Note that the value written to this field is not re-read by the hardware until a sweep iteration completes, or the channel is (re)triggered.

  However, if 0 is written to this field, then iterations are instantly disabled (but see below), and it will be reloaded as soon as it’s set to something else.

• Direction: 0 = Addition (period increases); 1 = Subtraction (period decreases)

• Individual step: On each iteration, the new period $L_{t+1}$ is computed from the current one $L_t$ as follows:

  $$L_{t+1}=L_t\pm \frac{L_t}{2^{\mathrm{step}}}$$

On each sweep iteration, the period in NR13 and NR14 is modified and written back.

In addition mode, if the period value would overflow (i.e. $L_{t+1}$ is strictly more than $7FF), the channel is turned off instead. This occurs even if sweep iterations are disabled by the pace being 0.

Note that if the period ever becomes 0, the period sweep will never be able to change it. For the same reason, the period sweep cannot underflow the period (which would turn the channel off).

FF11 — NR11: Channel 1 length timer & duty cycle

This register controls both the channel’s length timer and duty cycle (the ratio of the time spent low vs. high). The selected duty cycle also alters the phase, although the effect is hardly noticeable except in combination with other channels.

• Duty cycle (Read/Write): Controls the output waveform as follows:

  Value (binary)	Duty cycle	Waveform
  00	12.5 %
  01	25 %
  10	50 %
  11	75 %

  It’s worth noting that there is no audible difference between the 25 % and 75 % duty cycle settings.

• Initial length timer (Write-only): The higher this field is, the shorter the time before the channel is cut.

FF12 — NR12: Channel 1 volume & envelope

This register controls the digital amplitude of the “high” part of the pulse, and the sweep applied to that setting.

• Initial volume: How loud the channel initially is. Note that these bits are readable, but are not updated by the envelope functionality!
• Env dir: The envelope’s direction; 0 = decrease volume over time, 1 = increase volume over time.
• Sweep pace: The envelope ticks at 64 Hz, and the channel’s envelope will be increased / decreased (depending on bit 3) every Sweep pace of those ticks. A setting of 0 disables the envelope.

Setting bits 3-7 of this register all to 0 (initial volume = 0, envelope = decreasing) turns the DAC off (and thus, the channel as well), which may cause an audio pop.

FF13 — NR13: Channel 1 period low [write-only]

This register stores the low 8 bits of the channel’s 11-bit “period value”. The upper 3 bits are stored in the low 3 bits of NR14.

The period divider of pulse and wave channels is an up counter. Each time it is clocked, its value increases by 1; when it overflows (being clocked when it’s already 2047, or $7FF), its value is set from the contents of NR13 and NR14. This means it treats the value in the period as a negative number in 11-bit two’s complement. The higher the period value in the register, the lower the period, and the higher the frequency. For example:

• Period value $500 means -$300, or 1 sample per 768 input cycles
• Period value $740 means -$C0, or 1 sample per 192 input cycles

The pulse channels’ period dividers are clocked at 1048576 Hz, once per four dots, and their waveform is 8 samples long. This makes their sample rate equal to $\frac{1048576}{2048-\mathrm{period\_value}}$ Hz. with a resulting tone frequency equal to $\frac{131072}{2048-\mathrm{period\_value}}$ Hz.

• Period value $500 means -$300, or 1 sample per 768 input cycles
  or (1048576 ÷ 768) = 1365.3 Hz sample rate
  or (1048576 ÷ 768 ÷ 8) = 170.67 Hz tone frequency
• Period value $740 means -$C0, or 1 sample per 192 input cycles
  or (1048576 ÷ 192) = 5461.3 Hz sample rate
  or (1048576 ÷ 192 ÷ 8) = 682.67 Hz tone frequency

Period value $740 produces a higher frequency than $500. Even though the period value $740 is not four times $500, $740 produces a frequency that is four times that of $500, or two octaves higher, because ($800 - $740) or 192 is one-quarter of ($800 - $500) or 768.

FF14 — NR14: Channel 1 period high & control

• Trigger (Write-only): Writing any value to NR14 with this bit set triggers the channel.
• Length enable (Read/Write): Takes effect immediately upon writing to this register.
• Period (Write-only): The upper 3 bits of the period value; the lower 8 bits are stored in NR13.

Sound Channel 2 — Pulse

This sound channel works exactly like channel 1, except that it lacks a period sweep (and thus an equivalent to NR10). Please refer to the corresponding CH1 register:

• NR21 ($FF16) → NR11
• NR22 ($FF17) → NR12
• NR23 ($FF18) → NR13
• NR24 ($FF19) → NR14

Sound Channel 3 — Wave output

While other channels only offer limited control over the waveform they generate, this channel allows outputting any wave. It’s thus sometimes called a “voluntary wave” channel.

While the “length” of the wave is fixed at 32 “samples”, 4-bit each, the speed at which it is read can be customized. It’s possible to “shorten” the wave by either feeding it a repeating pattern, or doubling each sample and doubling the read rate. It’s also possible to artificially “increase” the wave’s length by loading a new wave as soon as the whole buffer has been read; this is sometimes used for full-on sample playback.

FF1A — NR30: Channel 3 DAC enable

This register controls CH3’s DAC. Like other channels, turning the DAC off immediately turns the channel off as well.

The DAC is often turned off just before writing to wave RAM to avoid issues with accessing it; see further below for more info.

Turning the DAC off may cause an audio pop.

FF1B — NR31: Channel 3 length timer [write-only]

This register controls the channel’s length timer.

The higher the length timer, the shorter the time before the channel is cut.

FF1C — NR32: Channel 3 output level

This channel lacks the envelope functionality that the other three channels have, and has a much coarser volume control.

• Output level: Controls the channel’s volume as follows:

  Bits 6-5 (binary)	Output level
  00	Mute (No sound)
  01	100% volume (use samples read from Wave RAM as-is)
  10	50% volume (shift samples read from Wave RAM right once)
  11	25% volume (shift samples read from Wave RAM right twice)

FF1D — NR33: Channel 3 period low [write-only]

This register stores the low 8 bits of the channel’s 11-bit “period value”. The upper 3 bits are stored in the low 3 bits of NR34.

The wave channel’s period divider is clocked at 2097152 Hz, once per two dots, and its waveform is 32 samples long. This makes their sample rate equal to $\frac{2097152}{2048-\mathrm{period\_value}}$ Hz. with a resulting tone frequency equal to $\frac{65536}{2048-\mathrm{period\_value}}$ Hz.

• Period value $500 means -$300, or 1 sample per 768 input cycles
  or (2097152 ÷ 768) = 2730.7 Hz sample rate
  or (2097152 ÷ 768 ÷ 32) = 85.333 Hz tone frequency
• Period value $740 means -$C0, or 1 sample per 192 input cycles
  or (2097152 ÷ 192) = 10923 Hz sample rate
  or (2097152 ÷ 192 ÷ 32) = 341.33 Hz tone frequency

Given the same period value, the tone frequency of the wave channel is generally half that of a pulse channel, or one octave lower.

FF1E — NR34: Channel 3 period high & control

• Trigger (Write-only): Writing any value to NR14 with this bit set triggers the channel.

  RETRIGGERING CAUTION

  On monochrome consoles only, retriggering CH3 while it’s about to read a byte from wave RAM causes wave RAM to be corrupted in a generally unpredictable manner.

  PLAYBACK DELAY

  Triggering the wave channel does not immediately start playing wave RAM; instead, the last sample ever read (which is reset to 0 when the APU is off) is output until the channel next reads a sample. (Source)

• Length enable (Read/Write): Takes effect immediately upon writing to this register.

• Period (Write-only): The upper 3 bits of the period value; the lower 8 bits are stored in NR13.

FF30–FF3F — Wave pattern RAM

Wave RAM is 16 bytes long; each byte holds two “samples”, each 4 bits.

As CH3 plays, it reads wave RAM left to right, upper nibble first. That is, $FF30’s upper nibble, $FF30’s lower nibble, $FF31’s upper nibble, and so on.

Accessing wave RAM while CH3 is active (i.e. playing) causes accesses to misbehave:

• On AGB, reads return $FF, and writes are ignored. (Source)
• On monochrome consoles, wave RAM can only be accessed on the same cycle that CH3 does. Otherwise, reads return $FF, and writes are ignored.
• On other consoles, the byte accessed will be the one CH3 is currently reading⁶; that is, if CH3 is currently reading one of the first two samples, the CPU will really access $FF30, regardless of the address being used. (Source)

Wave RAM can be accessed normally even if the DAC is on, as long as the channel is not active. (Source) This is especially relevant on GBA, whose mixer behaves as if DACs are always enabled.

Sound Channel 4 — Noise

This channel is used to output white noise⁷, which is done by randomly switching the amplitude between two levels fairly fast.

The frequency can be adjusted in order to make the noise appear “harder” (lower frequency) or “softer” (higher frequency).

The random function that switches the output level can also be manipulated. Certain settings can cause the wave to be more regular, sounding closer to a pulse than noise.

FF20 — NR41: Channel 4 length timer [write-only]

This register controls the channel’s length timer.

The higher the length timer, the shorter the time before the channel is cut.

FF21 — NR42: Channel 4 volume & envelope

This register functions exactly like NR12, so please refer to its documentation.

FF22 — NR43: Channel 4 frequency & randomness

This register allows controlling the way the amplitude is randomly switched.

• Clock shift: See the frequency formula below.

• LFSR width: 0 = 15-bit, 1 = 7-bit (more regular output; some frequencies sound more like pulse than noise).

  LFSR lockup

  Switching from 15- to 7-bit mode when the LFSR is in a certain state can “lock it up”, which essentially silences CH4; this can be avoided by retriggering CH4, which resets the LFSR.

• Clock divider: See the frequency formula below. Note that divider = 0 is treated as divider = 0.5 instead.

The frequency at which the LFSR is clocked is $\frac{262144}{\mathrm{divider}\times 2^{\mathrm{shift}}}$ Hz.

If the bit shifted out is a 0, the channel emits a 0; otherwise, it emits the volume selected in NR42.

FF23 — NR44: Channel 4 control

• Trigger (Write-only): Writing any value to NR14 with this bit set triggers the channel.
• Length enable (Read/Write): Takes effect immediately upon writing to this register.