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S-DSP/Noise: Difference between revisions

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[[File:LSFR noise.png|thumb]]
Noise clock table
Noise clock table


Note that the noise sampling frequency is the perceived pitch (due to overtones) even though it is double (Nyquist) that of the fundamental.
Hardware noise on the SNES is generated by a 15-bit Fibonacci [https://www.eetimes.com/tutorial-linear-feedback-shift-registers-lfsrs-part-1/ linear-feedback shift register]. <sup>[1]</sup>  Unlike the NES which has two hardware noise timbres, the SNES only has one.  The taps are located at bits 0 & 1, which are XOR'd together and fed back into bit 14 while the whole register shifts to the right (dividing everything else by two). <sup>[2]</sup>  Its initial seed value is 4000h and its period is 32,767 (2^15 - 1), after which the noise audibly repeats.  [[S-DSP/Noise/Values|See all of the values here.]]
 
Note that the noise sampling frequency is the perceived pitch (due to overtones) even though it is double ([https://www.techtarget.com/whatis/definition/Nyquist-Theorem Nyquist]) that of the fundamental.  It is selected via bits 0 to 4 of the [[FLG]] register in [[DSPRAM]] (see the table below).  On reset, the noise frequency is reset to zero.
 
The [[S-DSP/Gaussian Filter|Gaussian interpolator]] does not operate on the hardware noise generator.


{| class="wikitable"
{| class="wikitable"
|-
|-
! Value !! Sampling Frequency !! Pitch
! FLG.0-4 !! Sampling Frequency !! Pitch !! Example
|-
|-
| 00 || 0 Hz || off
| 00 || 0 Hz || off ||
|-
|-
| 01 || 16 Hz || C0 - 38¢
| 01 || 16 Hz || C0 - 38¢ || [[File:noise 01.wav]]
|-
|-
| 02 || 21 Hz || E0 + 33¢
| 02 || 21 Hz || E0 + 33¢ || [[File:noise 02.wav]]
|-
|-
| 03 || 25 Hz || G0 + 35¢
| 03 || 25 Hz || G0 + 35¢ || [[File:noise 03.wav]]
|-
|-
| 04 || 31 Hz || B0 + 7¢
| 04 || 31 Hz || B0 + 7¢ || [[File:noise 04.wav]]
|-
|-
| 05 || 42 Hz || E1 + 33¢
| 05 || 42 Hz || E1 + 33¢ || [[File:noise 05.wav]]
|-
|-
| 06 || 50 Hz || G1 + 35¢
| 06 || 50 Hz || G1 + 35¢ || [[File:noise 06.wav]]
|-
|-
| 07 || 63 Hz || B1 + 35¢
| 07 || 63 Hz || B1 + 35¢ || [[File:noise 07.wav]]
|-
|-
| 08 || 83 Hz || E2 + 12¢
| 08 || 83 Hz || E2 + 12¢ || [[File:noise 08.wav]]
|-
|-
| 09 || 100 Hz || G2 + 35¢
| 09 || 100 Hz || G2 + 35¢ || [[File:noise 09.wav]]
|-
|-
| 0A || 125 Hz || B2 + 21¢
| 0A || 125 Hz || B2 + 21¢ || [[File:noise 0A.wav]]
|-
|-
| 0B || 167 Hz || E3 + 23¢
| 0B || 167 Hz || E3 + 23¢ || [[File:noise 0B.wav]]
|-
|-
| 0C || 200 Hz || G3 + 35¢
| 0C || 200 Hz || G3 + 35¢ || [[File:noise 0C.wav]]
|-
|-
| 0D || 250 Hz || B3 + 21¢
| 0D || 250 Hz || B3 + 21¢ || [[File:noise 0D.wav]]
|-
|-
| 0E || 333 Hz || E4 + 18¢
| 0E || 333 Hz || E4 + 18¢ || [[File:noise 0E.wav]]
|-
|-
| 0F || 400 Hz || G4 + 35¢
| 0F || 400 Hz || G4 + 35¢ || [[File:noise 0F.wav]]
|-
|-
| 10 || 500 Hz || B4 + 21¢
| 10 || 500 Hz || B4 + 21¢ || [[File:noise 10.wav]]
|-
|-
| 11 || 667 Hz || E5 + 20¢
| 11 || 667 Hz || E5 + 20¢ || [[File:noise 11.wav]]
|-
|-
| 12 || 800 Hz || G5 + 35¢
| 12 || 800 Hz || G5 + 35¢ || [[File:noise 12.wav]]
|-
|-
| 13 || 1.0 kHz || B5 + 21¢
| 13 || 1.0 kHz || B5 + 21¢ || [[File:noise 13.wav]]
|-
|-
| 14 || 1.3 kHz || E6 - 24¢
| 14 || 1.3 kHz || E6 - 24¢ || [[File:noise 14.wav]]
|-
|-
| 15 || 1.6 kHz || G6 + 35¢
| 15 || 1.6 kHz || G6 + 35¢ || [[File:noise 15.wav]]
|-
|-
| 16 || 2.0 kHz || B6 + 21¢
| 16 || 2.0 kHz || B6 + 21¢ || [[File:noise 16.wav]]
|-
|-
| 17 || 2.7 kHz || E7 + 41¢
| 17 || 2.7 kHz || E7 + 41¢ || [[File:noise 17.wav]]
|-
|-
| 18 || 3.2 kHz || G7 + 35¢
| 18 || 3.2 kHz || G7 + 35¢ || [[File:noise 18.wav]]
|-
|-
| 19 || 4.0 kHz || B7 + 21¢
| 19 || 4.0 kHz || B7 + 21¢ || [[File:noise 19.wav]]
|-
|-
| 1A || 5.3 kHz || E8 + 9¢
| 1A || 5.3 kHz || E8 + 9¢ || [[File:noise 1A.wav]]
|-
|-
| 1B || 6.4 kHz || G8 + 35¢
| 1B || 6.4 kHz || G8 + 35¢ || [[File:noise 1B.wav]]
|-
|-
| 1C || 8.0 kHz || B8 + 21¢
| 1C || 8.0 kHz || B8 + 21¢ || [[File:noise 1C.wav]]
|-
|-
| 1D || 10.7 kHz || E9 + 25¢
| 1D || 10.7 kHz || E9 + 25¢ || [[File:noise 1D.wav]]
|-
|-
| 1E || 16 kHz || B9 + 21¢
| 1E || 16 kHz || B9 + 21¢ || [[File:noise 1E.wav]]
|-
|-
| 1F || 32 kHz || B10 + 21¢
| 1F || 32 kHz || B10 + 21¢ || [[File:noise_1F.wav]]
|}
|}
=== See Also ===
* [[The Infamous Bit-Of-Confusion]]
* [[Polynomial Counter]]


=== References ===
=== References ===
* https://archive.org/details/SNESDevManual/book1/page/n174
# https://github.com/ares-emulator/ares/blob/master/ares/sfc/dsp/dsp.hpp#L76
# https://github.com/ares-emulator/ares/blob/master/ares/sfc/dsp/misc.cpp#L26
# Table 3-7-4 on [https://archive.org/details/SNESDevManual/book1/page/n174 page 3-7-8] of the official Super Nintendo development manual


[[Category:Audio]]
[[Category:Audio]]
[[Category:SNES Hardware]]
[[Category:Registers]]

Latest revision as of 08:45, 1 December 2023

LSFR noise.png

Noise clock table

Hardware noise on the SNES is generated by a 15-bit Fibonacci linear-feedback shift register. [1] Unlike the NES which has two hardware noise timbres, the SNES only has one. The taps are located at bits 0 & 1, which are XOR'd together and fed back into bit 14 while the whole register shifts to the right (dividing everything else by two). [2] Its initial seed value is 4000h and its period is 32,767 (2^15 - 1), after which the noise audibly repeats. See all of the values here.

Note that the noise sampling frequency is the perceived pitch (due to overtones) even though it is double (Nyquist) that of the fundamental. It is selected via bits 0 to 4 of the FLG register in DSPRAM (see the table below). On reset, the noise frequency is reset to zero.

The Gaussian interpolator does not operate on the hardware noise generator.

FLG.0-4 Sampling Frequency Pitch Example
00 0 Hz off
01 16 Hz C0 - 38¢
02 21 Hz E0 + 33¢
03 25 Hz G0 + 35¢
04 31 Hz B0 + 7¢
05 42 Hz E1 + 33¢
06 50 Hz G1 + 35¢
07 63 Hz B1 + 35¢
08 83 Hz E2 + 12¢
09 100 Hz G2 + 35¢
0A 125 Hz B2 + 21¢
0B 167 Hz E3 + 23¢
0C 200 Hz G3 + 35¢
0D 250 Hz B3 + 21¢
0E 333 Hz E4 + 18¢
0F 400 Hz G4 + 35¢
10 500 Hz B4 + 21¢
11 667 Hz E5 + 20¢
12 800 Hz G5 + 35¢
13 1.0 kHz B5 + 21¢
14 1.3 kHz E6 - 24¢
15 1.6 kHz G6 + 35¢
16 2.0 kHz B6 + 21¢
17 2.7 kHz E7 + 41¢
18 3.2 kHz G7 + 35¢
19 4.0 kHz B7 + 21¢
1A 5.3 kHz E8 + 9¢
1B 6.4 kHz G8 + 35¢
1C 8.0 kHz B8 + 21¢
1D 10.7 kHz E9 + 25¢
1E 16 kHz B9 + 21¢
1F 32 kHz B10 + 21¢

See Also

References

  1. https://github.com/ares-emulator/ares/blob/master/ares/sfc/dsp/dsp.hpp#L76
  2. https://github.com/ares-emulator/ares/blob/master/ares/sfc/dsp/misc.cpp#L26
  3. Table 3-7-4 on page 3-7-8 of the official Super Nintendo development manual
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