furnace/doc/7-systems/snes.md

Super Nintendo Entertainment System (SNES) / Super Famicom

the successor to NES to compete with Genesis, packing superior graphics and sample-based audio.

its Sony-developed audio system features a DSP chip, SPC700 CPU, and 64KB of dedicated SRAM used by both. this whole system itself is pretty much a separate computer that the main CPU needs to upload its program and samples to.

Furnace communicates with the DSP directly and provides a full 64KB of memory. this memory might be reduced excessively on ROM export to make up for playback engine and pattern data. you can go to window > statistics to see how much memory your samples are using.

some notable features of the DSP are:

• pitch modulation, meaning that you can use 2 channels to make a basic FM synth without eating up too much memory.
• a built in noise generator, useful for hi-hats, cymbals, rides, effects, among other things.
• per-channel echo, which unfortunately eats up a lot of memory but can be used to save channels in songs.
• an 8-tap FIR filter for the echo, which is basically a procedural low-pass filter that you can edit however you want.
• sample loop, but the loop points have to be multiples of 16.
• left/right channel invert for surround sound.
• ADSR and gain envelope modes.
• 7-bit volume per channel.
• sample interpolation, which is basically a low-pass filter that gets affected by the pitch of the channel.

Furnace also allows the SNES to use wavetables (and the wavetable synthesizer) in order to create more 'animated' sounds, using less memory than regular samples. this however is not a hardware feature, and might be difficult to implement on real hardware.

effects

• 10xx: set waveform.
• 11xx: toggle noise mode.
• 12xx: toggle echo on this channel.
• 13xx: toggle pitch modulation. frequency modulation by the previous channel's output. no effect on channel 1.
• 14xy: toggle inverting the left or right channels. x is left, y is right.
• 15xx: set envelope mode. see gain chart below for 1 through 5.
  • 0: ADSR mode.
  • 1: gain (direct). volume holds at one level.
  • 2: linear decrement. volume lowers by subtractions of 1/64.
  • 3: exponential decrement. volume lowers by multiplications of 255/256.
  • 4: linear increment. volume rises by additions of 1/64.
  • 5: bent line (inverse log) increment. volume rises by additions of 1/64 until 3/4, then additions of 1/256.
• 16xx: set gain. 00 to 7F if direct, 00 to 1F otherwise.
• 18xx: enable echo buffer.
• 19xx: set echo delay. range is 0 to F.
• 1Axx: set left echo channel volume.
  1Bxx: set right echo channel volume.
  1Cxx: set echo feedback.
  • all of these are signed numbers.
  • 00 to 7F for 0 to 127.
  • 80 to FF for -128 to -1.
    • setting these to -128 is not recommended as it may cause echo output to overflow and therefore click.
• 1Dxx: set noise generator frequency. range is 00 to 1F. see noise frequencies chart below.
• 1Exx: set left dry / global volume.
  1Fxx: set right dry / global volume.
  • these do not affect echo.
• 20xx: set attack. range is 0 to F.
  21xx: set decay. range is 0 to 7.
  22xx: set sustain. range is 0 to 7.
  23xx: set release. range is 00 to 1F.
  • these four are only used in ADSR envelope mode. see ADSR chart below.
• 30xx: set echo filter coefficient 0.
  31xx: set echo filter coefficient 1.
  32xx: set echo filter coefficient 2.
  33xx: set echo filter coefficient 3.
  34xx: set echo filter coefficient 4.
  35xx: set echo filter coefficient 5.
  36xx: set echo filter coefficient 6.
  37xx: set echo filter coefficient 7.
  • all of these are signed numbers.
  • 00 to 7F for 0 to 127.
  • 80 to FF for -128 to -1.
  • Note: Be sure the sum of all coefficients is between -128 and 127. sums outside that may result in overflow and therefore clicking.
  • see SnesLab for a full explanation and examples.

tables

ADSR

attack	0→1 time	decay	1→S time	sustain	ratio	release	S→0 time
00	4.1s	00	1.2s	00	1/8	00	∞
01	2.5s	01	740ms	01	2/8	01	38s
02	1.5s	02	440ms	02	3/8	02	28s
03	1.0s	03	290ms	03	4/8	03	24s
04	640ms	04	180ms	04	5/8	04	19s
05	380ms	05	110ms	05	6/8	05	14s
06	260ms	06	74ms	06	7/8	06	12s
07	160ms	07	37ms	07	1	07	9.4s
08	96ms					08	7.1s
09	64ms					09	5.9s
0A	40ms					0A	4.7s
0B	24ms					0B	3.5s
0C	16ms					0C	2.9s
0D	10ms					0D	2.4s
0E	6ms					0E	1.8s
0F	0ms					0F	1.5s
						10	1.2s
						11	880ms
						12	740ms
						13	590ms
						14	440ms
						15	370ms
						16	290ms
						17	220ms
						18	180ms
						19	150ms
						1A	110ms
						1B	92ms
						1C	74ms
						1D	55ms
						1E	37ms
						1F	18ms

reference: Super Famicom Development Wiki

gain

value	linear inc.	bent line inc.	linear dec.	exponent dec.
00	∞	∞	∞	∞
01	4.1s	7.2s	4.1s	38s
02	3.1s	5.4s	3.1s	28s
03	2.6s	4.6s	2.6s	24s
04	2.0s	3.5s	2.0s	19s
05	1.5s	2.6s	1.5s	14s
06	1.3s	2.3s	1.3s	12s
07	1.0s	1.8s	1.0s	9.4s
08	770ms	1.3s	770ms	7.1s
09	640ms	1.1s	640ms	5.9s
0A	510ms	900ms	510ms	4.7s
0B	380ms	670ms	380ms	3.5s
0C	320ms	560ms	320ms	2.9s
0D	260ms	450ms	260ms	2.4s
0E	190ms	340ms	190ms	1.8s
0F	160ms	280ms	160ms	1.5s
10	130ms	220ms	130ms	1.2s
11	96ms	170ms	96ms	880ms
12	80ms	140ms	80ms	740ms
13	64ms	110ms	64ms	590ms
14	48ms	84ms	48ms	440ms
15	40ms	70ms	40ms	370ms
16	32ms	56ms	32ms	290ms
17	24ms	42ms	24ms	220ms
18	20ms	35ms	20ms	180ms
19	16ms	28ms	16ms	150ms
1A	12ms	21ms	12ms	110ms
1B	10ms	18ms	10ms	92ms
1C	8ms	14ms	8ms	74ms
1D	6ms	11ms	6ms	55ms
1E	4ms	7ms	4ms	37ms
1F	2ms	3.5ms	2ms	18ms

reference: Super Famicom Development Wiki

noise frequencies

value	freq.	value	freq.
00	0 Hz	10	500 Hz
01	16 Hz	11	667 Hz
02	21 Hz	12	800 Hz
03	25 Hz	13	1.0 KHz
04	31 Hz	14	1.3 KHz
05	42 Hz	15	1.6 KHz
06	50 Hz	16	2.0 KHz
07	63 Hz	17	2.7 KHz
08	83 Hz	18	3.2 KHz
09	100 Hz	19	4.0 KHz
0A	125 Hz	1A	5.3 KHz
0B	167 Hz	1B	6.4 KHz
0C	200 Hz	1C	8.0 KHz
0D	250 Hz	1D	10.7 KHz
0E	333 Hz	1E	16 KHz
0F	400 Hz	1F	32 KHz

reference: Super Famicom Development Wiki

resources

• SNES-format BRR samples at SMW Central