S-DSP/Noise

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

• The Infamous Bit-Of-Confusion
• Polynomial Counter

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