otomatalabs/furnace
7
0
Fork 0
Code Issues Pull requests Actions Packages Projects Releases Wiki Activity
furnace/doc/7-systems/nes.md
Electric Keet 8925e29a41 Many system/instrument doc updates.
2025-07-30 14:00:45 -07:00

9.8 KiB
Raw Blame History

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 Family Computer (Famicom), especially in Japan.

the console is powered by the Ricoh 2A03, a CPU with sound generator built-in. it has five channels: 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. range is 00 to 7F.
    • this may be used to attenuate the triangle and noise channels; see "non-linear mixing" below.
    • will not work if a sample is playing.
  • 12xx: set duty cycle or noise mode of channel.
    • may be 0 to 3 for the pulse channels:
      • 0: 12.5%
      • 1: 25%
      • 2: 50%
      • 3: 75%
    • may be 0 or 1 for the noise channel:
      • 0: long (15-bit LFSR, 32767-step)
      • 1: short (9-bit LFSR, 93-step)
        • short noise may sound very different depending on its initial LFSR state. more info can be found at NESdev.
  • 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 value. 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) and available pitches are limited. see "DPCM samples" below.
  • 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.

info

this chip uses the NES instrument editor.

chip config

the following options are available in the Chip Manager window:

  • Clock rate: sets the rate at which the chip will run.
  • DPCM channel mode: allows you to set which mode to use for the DPCM channel.
    • DPCM: the default mode, playing 1-bit DPCM samples as supported by the hardware.
    • PCM: this mode provides crisper 7-bit samples by writing to the delta counter directly. uses a lot of CPU time in console.

non-linear mixing

the triangle, noise, and PCM channels are mixed together non-linearly such that the level of the DMC (the "delta modulation counter", representing the PCM channel's amplitude) inversely affects the volume of the others. this can be intentionally used with effect 11xx to hold the DMC at a level that lowers the volume of the triangle channel in a controlled fashion; at maximum DMC level (117F) the triangle plays at about 57% of full volume.

DPCM samples

due to hardware limitations, a loop in a DPCM sample must start on a multiple of 512 samples (512, 1024, 1536...) and have a length that is a multiple of 128 plus 8 samples (136, 264, 392...)

NES DPCM only has 16 preset sample rates, shown in a table below. for help adapting samples to work with this, see the limited samples guide.

envelope mode

both pulse channels and the noise channel have a simple hardware envelope generator with two modes: one that holds the volume at whatever the software sets, and one that creates a downward ramp of volume starting from maximum and counting down to 0. the volume range of the ramp can't be controlled (similar to the triangle wave, for the same reason). the envelope can be set to loop each time it reaches 0.

frame counter

the frame counter is a global counter that controls the speed of hardware envelopes and sweeps. the resulting speed depends on both the chip's clock rate and whether it's set to 4-step or 5-step mode.

length counter

setting the length counter will automatically stop a note after the specified amount of time. note that the effect values aren't arranged in a convenient way for their lengths; for convenience, they are shown in a table below.

short noise frequencies (NTSC)

note arpeggio fundamental MIDI note pitch
C-0 @0 4.7 Hz -9.47 d_1 + 53¢
C#0 @1 9.5 Hz 2.53 D-0 + 53¢
D-0 @2 18.9 Hz 14.55 D-1 + 55¢
D#0 @3 25.3 Hz 19.53 G-1 + 53¢
E-0 @4 37.9 Hz 26.55 D-2 + 55¢
F-0 @5 50.6 Hz 31.57 G-2 + 57¢
F#0 @6 75.8 Hz 38.55 D-3 + 55¢
G-0 @7 95.3 Hz 42.51 F#3 + 51¢
G#0 @8 120.3 Hz 46.55 A#3 + 55¢
A-0 @9 150.4 Hz 50.41 D-4 + 41¢
A#0 @10 200.5 Hz 55.39 G-4 + 39¢
B-0 @11 300.7 Hz 62.41 D-5 + 41¢
C-1 @12 601.4 Hz 74.41 D-6 + 41¢
C#1 @13 1202.8 Hz 86.41 D-7 + 41¢
D-1 @14 2405.6 Hz 98.41 D-8 + 41¢
D#1 @15 4811.2 Hz 110.41 D-9 + 41¢

reference: NESdev

length counter table

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

reference: NESdev

DPCM frequency table

value tracker NTSC freq NTSC pitch PAL freq PAL pitch
00 C-3 4181.7Hz C-8 - 2¢ 4177.4Hz C-8 - 4¢
01 D-3 4709.9Hz D-8 + 4¢ 4696.6Hz D-8 - 1¢
02 E-3 5264.0Hz E-8 - 3¢ 5261.4Hz E-8 - 4¢
03 F-3 5593.0Hz F-8 + 2¢ 5579.2Hz F-8 - 3¢
04 G-3 6258.0Hz G-8 - 4¢ 6023.9Hz G-8 - 70¢
05 A-3 7046.4Hz A-8 + 2¢ 7044.9Hz A-8 + 1¢
06 B-3 7919.4Hz B-8 + 4¢ 7917.2Hz B-8 + 3¢
07 C-4 8363.4Hz C-9 - 2¢ 8397.0Hz C-9 + 5¢
08 D-4 9419.9Hz D-9 + 4¢ 9446.6Hz D-9 + 9¢
09 F-4 11186.1Hz F-9 + 2¢ 11233.8Hz F-9 + 9¢
0A G-4 12604.0Hz G-9 + 8¢ 12595.5Hz G-9 + 7¢
0B A-4 13982.6Hz A-9 - 12¢ 14089.9Hz A-9 + 1¢
0C C-5 16884.6Hz C-10 + 15¢ 16965.4Hz C-10 + 23¢
0D E-5 21306.8Hz E-10 + 17¢ 21315.5Hz E-10 + 18¢
0E G-5 24858.0Hz G-10 - 16¢ 25191.0Hz G-10 + 7¢
0F C-6 33143.9Hz C-11 - 18¢ 33252.1Hz C-11 - 12¢

reference: NESdev