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

reference: Super Famicom Development Wiki

gain

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

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