maanantai 8. joulukuuta 2014

Buchla Quantized Random Voltages

This is my first publication in this forum. I have many ongoing DIY-projects and I will report about them here as they proceed...

Buchla has a module called 266 Source of Uncertainty. It outputs random voltages with adjustable distribution probability. Doepfer has a really nice (and cheap) implementation of this module called A149. I started to build a prototype of my version of the quantized random voltages part of Buchlas module (picture shows the board with all the mechanical parts and potentiometers in place).
There is only one random voltage output but you can morph it from N+1 to 2^N states with six potentiometers. The potentiometer near the jacks adjusts N from 1 to 6.
Random voltages are generated with a 17-stage linear feedback shift register (LFSR) so the length of the sequence is 131071 states. If we clock this LFSR with 1Hz clock it starts to repeat after 36 hours. LFSR's are pseudo random not 100% random as is transistor created white noise. With a switch you can choose from 2 or 4 feedback tapping points (EXOR taps in LFSR). Buchla used also a 17-stage LFSR but the organisation of shift register was different and there was only one fixed feedback point. These differences can be described in terms of the amplitude distribution and the degree of low-pass filtering in the random voltages.
Bernie Hutchins from Electronotes has generously shared interesting articles about randomness, noise and LFSR's (EN#64,EN#76,EN#212,EN#402). These are freely downloadable from:

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