furnace/doc/7-systems/nes.md

# NES

the console from Nintendo that plays Super Mario Bros. and helped revive the agonizing video game market in the US during mid-80s.

also known as Famicom. it is a five-channel sound generator: first two channels play pulse wave with three different duty cycles, third is a fixed-volume triangle channel, fourth is a noise channel (can work in both pseudo-random and periodic modes) and fifth is a (D)PCM sample channel.

# effects

- `11xx`: write to delta modulation counter.
  - this may be used to attenuate the triangle and noise channels.
  - will not work if a sample is playing.
- `12xx`: set duty cycle or noise mode of channel.
  - may be 0-3 for the pulse channels and 0-1 for the noise channel.
- `13xy`: setup sweep up.
  - `x` is the time.
  - `y` is the shift.
  - set to 0 to disable it.
- `14xy`: setup sweep down.
  - `x` is the time.
  - `y` is the shift.
  - set to 0 to disable it.
- `15xx`: set envelope mode.
  - `0`: envelope + length counter (volume represents envelope duration).
  - `1`: length counter (volume represents output volume).
  - `2`: looping envelope (volume represents envelope duration).
  - `3`: constant volume (default; volume represents output volume).
  - pulse and noise channels only.
  - you may need to apply a phase reset (using the macro) to make the envelope effective.
- `16xx`: set length counter.
  - see table below for possible values.
  - this will trigger phase reset.
- `17xx`: set frame counter mode.
  - `0`: 4-step.
    - NTSC: 120Hz sweeps and lengths; 240Hz envelope.
    - PAL: 100Hz sweeps and lengths; 200Hz envelope.
    - Dendy: 118.9Hz sweeps and lengths; 237.8Hz envelope.
  - `1`: 5-step.
    - NTSC: 96Hz sweeps and lengths; 192Hz envelope.
    - PAL: 80Hz sweeps and lengths; 160Hz envelope.
    - Dendy: 95.1Hz sweeps and lengths; 190.2Hz envelope.
- `18xx`: set PCM channel mode.
  - `00`: PCM (software).
  - `01`: DPCM (hardware).
  - when in DPCM mode, samples will sound muffled (due to its nature), availables pitches are limited and loop point is ignored.
- `19xx`: set triangle linear counter.
  - `00` to `7F` set the counter.
  - `80` and higher halt it.
- `20xx`: set DPCM frequency.
  - only works in DPCM mode.
  - see table below for possible values.

# DPCM frequency table

val | NTSC      | PAL
----|-----------|-----------
 00 | 4181.7Hz  | 4177.4Hz
 01 | 4709.9Hz  | 4696.6Hz
 02 | 5264.0Hz  | 5261.4Hz
 03 | 5593.0Hz  | 5579.2Hz
 04 | 6257.9Hz  | 6023.9Hz
 05 | 7046.3Hz  | 7044.9Hz
 06 | 7919.3Hz  | 7917.2Hz
 07 | 8363.4Hz  | 8397.0Hz
 08 | 9419.9Hz  | 9446.6Hz
 09 | 11186.1Hz | 11233.8Hz
 0A | 12604.0Hz | 12595.5Hz
 0B | 13982.6Hz | 14089.9Hz
 0C | 16884.6Hz | 16965.4Hz
 0D | 21306.8Hz | 21315.5Hz
 0E | 24858.0Hz | 25191.0Hz
 0F | 33143.9Hz | 33252.1Hz

# length counter table

val | raw | NTSC  | PAL   | Dendy | NTSC 5-step | PAL 5-step | Dendy 5-step
----|-----|-------|-------|-------|-------------|------------|--------------
 00 |  10 | 83ms  | 100ms | 84ms  | 104ms       | 125ms      | 105ms
 01 | 254 | 2.1s  | 2.5s  | 2.1s  | 2.6s        | 3.2s       | 2.7s
 02 |  20 | 166ms | 200ms | 168ms | 208ms       | 250ms      | 210ms
 03 |   2 | 17ms  | 20ms  | 17ms  | 21ms        | 25ms       | 21ms
 04 |  40 | 333ms | 400ms | 336ms | 417ms       | 500ms      | 421ms
 05 |   4 | 33ms  | 40ms  | 34ms  | 42ms        | 50ms       | 42ms
 06 |  80 | 667ms | 800ms | 673ms | 833ms       | 1.0s       | 841ms
 07 |   6 | 50ms  | 60ms  | 50ms  | 63ms        | 75ms       | 63ms
 08 | 160 | 1.3s  | 1.6s  | 1.3s  | 1.7s        | 2.0s       | 1.7s
 09 |   8 | 67ms  | 80ms  | 67ms  | 83ms        | 100ms      | 84ms
 0A |  60 | 500ms | 600ms | 505ms | 625ms       | 750ms      | 631ms
 0B |  10 | 83ms  | 100ms | 84ms  | 104ms       | 125ms      | 105ms
 0C |  14 | 117ms | 140ms | 118ms | 146ms       | 175ms      | 147ms
 0D |  12 | 100ms | 120ms | 101ms | 125ms       | 150ms      | 126ms
 0E |  26 | 217ms | 260ms | 219ms | 271ms       | 325ms      | 273ms
 0F |  14 | 117ms | 140ms | 118ms | 145ms       | 175ms      | 147ms
 10 |  12 | 100ms | 120ms | 101ms | 125ms       | 150ms      | 126ms
 11 |  16 | 133ms | 160ms | 135ms | 167ms       | 200ms      | 168ms
 12 |  24 | 200ms | 240ms | 202ms | 250ms       | 300ms      | 252ms
 13 |  18 | 150ms | 180ms | 151ms | 188ms       | 225ms      | 189ms
 14 |  48 | 400ms | 480ms | 404ms | 500ms       | 600ms      | 505ms
 15 |  20 | 167ms | 200ms | 168ms | 208ms       | 250ms      | 210ms
 16 |  96 | 800ms | 960ms | 807ms | 1.0s        | 1.2s       | 1.0s
 17 |  22 | 183ms | 220ms | 185ms | 229ms       | 275ms      | 231ms
 18 | 192 | 1.6s  | 1.9s  | 1.6s  | 2.0s        | 2.4s       | 2.0s
 19 |  24 | 200ms | 240ms | 202ms | 250ms       | 300ms      | 252ms
 1A |  72 | 600ms | 720ms | 606ms | 750ms       | 900ms      | 757ms
 1B |  26 | 217ms | 260ms | 219ms | 271ms       | 325ms      | 273ms
 1C |  16 | 133ms | 160ms | 135ms | 167ms       | 200ms      | 168ms
 1D |  28 | 233ms | 280ms | 235ms | 292ms       | 350ms      | 294ms
 1E |  32 | 267ms | 320ms | 269ms | 333ms       | 400ms      | 336ms
 1F |  30 | 250ms | 300ms | 252ms | 313ms       | 375ms      | 315ms

reference: [NESdev](https://www.nesdev.org/wiki/APU_Length_Counter)