Channel Envelope
Channels 1, 2 and 4 have a volume envelope. The volume envelope allows for hardware volume control. The envelope is controlled by the NRx2 registers, or NR12, NR22 and NR42 for channels 1, 2, and 4, respectively. These registers have read and write access.
See the following diagrams for the format of these registers:
Bits 7-4: Initial volume (0-15)
Bit 3: Mode (0: attenuate, 1: amplify)
Bits 2-0: Period (0-7)
The upper nibble (bits 7-4) is the initial volume. The lower nibble (bits 3-0) determine the mode of operation for the envelope. There are three modes: constant, attenuate and amplify.
Modes
Constant
When the envelope register is $x0 or $x8, the volume for the channel is
constant. The channel's volume will be x, indefinitely.
Attenuate
When the lower nibble of the register is 1-7, the volume will attenuate or
decrease from the initial until it reaches 0. This mode allows for smooth
fade outs. The volume decreases by 1 every x/64 seconds, where x is the
period.
Amplify
When the lower nibble of the register is 9-F, the volume will amplify or
increase from the initial until it reaches the maximum, 15. This mode allows for
smooth fade ins. Similar to Attenuate, the volume will increase
by 1 every x/64 seconds, where x is the period.
Registers
| Name | Memory address | Description |
|---|---|---|
| NR12 | $FF12 | Channel 1 Volume Envelope (R/W) |
| NR22 | $FF17 | Channel 2 Volume Envelope (R/W) |
| NR42 | $FF21 | Channel 4 Volume Envelope (R/W) |
Writing
After writing to any of these registers, the envelope will be unstable. It is
recommended that you immediately restart the channel after doing so (write $80
to NRx4). Restarting the channel restarts the envelope. The volume will be
set to its initial and will automatically increase or decrease, depending
on the mode that was set.
The exact behavior that occurs after writing depends on the model, and in some cases, zombie mode takes effect. Zombie mode is a hardware bug that allows you to change the volume level of the envelope without a channel restart. Since Trackerboy does not utilize zombie mode, it will not be discussed here.
Channel DAC
The envelope register also controls the channel's DAC (digital to analog converter). When the upper 5 bits of the register are cleared, the DAC is off. A channel restart is not necessary to turn the DAC off, as its status is updated immediately after writing to the register.
Since the DAC has a DC offset, turning the DAC on or off will result in a change in the DC offset.