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This channel is used to output white noise. This is done by randomly switching the amplitude between high and low at a given frequency. Depending on the frequency the noise will appear 'harder' or 's ofter'. It is also possible to influence the function of the random generator, so the that the output becomes more regular, resulting in a limited ability to output Tone instead of Noise. 4000078h - SOUND4CNT_L (NR41, NR42) - Channel 4 Length/Envelope (R/W) Bit Expl. 0-5 W Sound length; units of (64-n)/256s (0-63) 6-7 - Not used 8-10 R/W Envelope Step-Time; units of n/64s (1-7, 0=No Envelope) 11 R/W Envelope Direction (0=Decrease, 1=Increase) 12-15 R/W Initial Volume of envelope (1-15, 0=No Sound) 16-31 - Not used The Length value is used only if Bit 6 in NR44 is set. 400007Ch - SOUND4CNT_H (NR43, NR44) - Channel 4 Frequency/Control (R/W) The amplitude is randomly switched between high and low at the given frequency. A higher frequency will make the noise to appear 'softer'. When Bit 3 is set, the output will become more regular, and some frequencies will sound more like Tone than Noise. Bit Expl. 0-2 R/W Dividing Ratio of Frequencies (r) 3 R/W Counter Step/Width (0=15 bits, 1=7 bits) 4-7 R/W Shift Clock Frequency (s) 8-13 - Not used 14 R/W Length Flag (1=Stop output when length in NR41 expires) 15 W Initial (1=Restart Sound) 16-31 - Not used Frequency = 524288 Hz / r / 2^(s+1) ;For r=0 assume r=0.5 instead Noise Random Generator (aka Polynomial Counter) Noise randomly switches between HIGH and LOW levels, the output levels are calculated by a shift register (X), at the selected frequency, as such: 7bit: X=X SHR 1, IF carry THEN Out=HIGH, X=X XOR 60h ELSE Out=LOW 15bit: X=X SHR 1, IF carry THEN Out=HIGH, X=X XOR 6000h ELSE Out=LOW The initial value when (re-)starting the sound is X=40h (7bit) or X=4000h (15bit). The data stream repeats after 7Fh (7bit) or 7FFFh (15bit) steps.
