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furnace/doc/7-systems/snes.md
Electric Keet 6ede173d2c SNES docs (and other tweaks)
2023-06-27 14:06:23 -07:00

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

reference: Super Famicom Development Wiki

gain

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

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