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