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MechaSeb
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Date d'inscription : 14/10/2010
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DataSheet du MM5837 (Prophet5 Rev2&3, MonoPoly, OBX, 266 etc.)

le Lun 17 Déc - 11:35
Intéressant !

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This very simple digital noise source provides pure white noise over the full audio spectrum.With the additional filtering shown, it can also produce pink noise and red noise.

Digital noise sources have a bad reputation, mainly due to the infamous MM5837 chips that appeared in the Sequential Prophet 5.This chip allegedly makes a noise like a broken recording of a steam train. It uses a 17-bit LFSR at a rate which is somewhere between 24KHz and 56KHz.The datasheet doesn’t commit itself to offering a ‘typical’ rate between these extremes. Assuming the best case from an audio point of view (56KHz rate), the output of the chip repeats every 2.3 seconds! This is hardly random!

However, there is no technical reason why a digital noise source should sound bad if the sample rate is high enough and the shift register is long enough.

The Electric Druid Noise Generator uses a dual linear feedback shift register (LFSR) algorithm to generate pseudo-random bits at an output rate of around 100KHz. There is a 21-bit LFSR (with taps at 21 and 19) coupled with a 31-bit LFSR (with taps at 31 and 28).This gives an effective LFSR length of 52 bits.

The output bits from the two generators are alternated. Since the generators have different lengths, the resulting bit pattern will not repeat until the two generators both restart at the same time.This will not happen for 252 samples, which is over 142 million years!

The high output rate ensures that the noise is pure white throughout the audio spectrum. Subsequent filtering can provide other colours of noise.

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Cherche: rien du tout, j'ai plus un radis
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robonom
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Date d'inscription : 27/02/2012
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Re: DataSheet du MM5837 (Prophet5 Rev2&3, MonoPoly, OBX, 266 etc.)

le Lun 17 Déc - 20:19
Tout à fait, mon cher Mechaseb!
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