APU Pulse: Difference between revisions
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Each of the two [[APU|NES APU]] pulse (square) wave channels generate a pulse wave with variable duty. | Each of the two [[APU|NES APU]] pulse (square) wave channels generate a pulse wave with variable duty. | ||
Each pulse channel contains the following: | Each pulse channel contains the following: |
Revision as of 02:55, 12 June 2022
Each of the two NES APU pulse (square) wave channels generate a pulse wave with variable duty.
Each pulse channel contains the following:
Sweep -----> Timer | | | | | v | Sequencer Length Counter | | | | | | v v v Envelope -------> Gate -----> Gate -------> Gate --->(to mixer)
Registers
Note: the addresses below are write-only! Reading from these addresses exhibits open-bus behavior.
Address | Bitfield | Description |
---|---|---|
$4000 | DDlc.vvvv | Pulse 1 Duty cycle, length counter halt, constant volume/envelope flag, and volume/envelope divider period |
$4004 | DDlc.vvvv | Pulse 2 Duty cycle, length counter halt, constant volume/envelope flag, and volume/envelope divider period |
Side effects | The duty cycle is changed (see table below), but the sequencer's current position isn't affected. | |
$4001 | EPPP.NSSS | See APU Sweep |
$4005 | EPPP.NSSS | See APU Sweep |
$4002 | LLLL.LLLL | Pulse 1 timer Low 8 bits |
$4006 | LLLL.LLLL | Pulse 2 timer Low 8 bits |
$4003 | llll.lHHH | Pulse 1 length counter load and timer High 3 bits |
$4007 | llll.lHHH | Pulse 2 length counter load and timer High 3 bits |
Side effects | The sequencer is immediately restarted at the first value of the current sequence. The envelope is also restarted. The period divider is not reset.[1] |
Sequencer behavior
The sequencer is clocked by an 11-bit timer. Given the timer value t = HHHLLLLLLLL formed by timer high and timer low, this timer is updated every APU cycle (i.e., every second CPU cycle), and counts t, t-1, ..., 0, t, t-1, ..., clocking the waveform generator when it goes from 0 to t. Since the period of the timer is t+1 APU cycles and the sequencer has 8 steps, the period of the waveform is 8*(t+1) APU cycles, or equivalently 16*(t+1) CPU cycles.
Hence
- fpulse = fCPU/(16*(t+1)) (where fCPU is 1.789773 MHz for NTSC, 1.662607 MHz for PAL, and 1.773448 MHz for Dendy)
- t = fCPU/(16*fpulse) - 1
Note: A period of t < 8, either set explicitly or via a sweep period update, silences the corresponding pulse channel. The highest frequency a pulse channel can output is hence about 12.4 kHz for NTSC. (TODO: PAL behavior?)
Duty Cycle Sequences
Duty | Output waveform |
---|---|
0 | 0 1 0 0 0 0 0 0 (12.5%) |
1 | 0 1 1 0 0 0 0 0 (25%) |
2 | 0 1 1 1 1 0 0 0 (50%) |
3 | 1 0 0 1 1 1 1 1 (25% negated) |
Notice that a few Famiclone units have swapped APU duty cycles, as 12.5 [0], 50 [1], 25 [2] and 25 negated [3] instead.
Implementation details
The reason for the odd output from the sequencer is that the counter is initialized to zero but counts downward rather than upward. Thus it reads the sequence lookup table in the order 0, 7, 6, 5, 4, 3, 2, 1.
Duty | Sequence lookup table | Output waveform |
---|---|---|
0 | 0 0 0 0 0 0 0 1 |
0 1 0 0 0 0 0 0 (12.5%) |
1 | 0 0 0 0 0 0 1 1 |
0 1 1 0 0 0 0 0 (25%) |
2 | 0 0 0 0 1 1 1 1 |
0 1 1 1 1 0 0 0 (50%) |
3 | 1 1 1 1 1 1 0 0 |
1 0 0 1 1 1 1 1 (25% negated) |
Pulse channel output to mixer
The mixer receives the pulse channel's current envelope volume (lower 4 bits from $4000 or $4004) except when
- The sequencer output is zero, or
- overflow from the sweep unit's adder is silencing the channel, or
- the length counter is zero, or
- the timer has a value less than eight (t<8, noted above).
If any of the above are true, then the pulse channel sends zero (silence) to the mixer.
Pulse channel 1 vs Pulse channel 2 behavior
The behavior of the two pulse channels differs only in the effect of the negate mode of their sweep units.