Pseudo Random Noise

[Don Tillman]

   Date: Fri, 15 Sep 1995 18:01:44 -0700
   From: chordman at ix.netcom.com (Scott Gravenhorst, Synthaholic)

   I have logic maps for up to 31 bit pseudo random noise circuits.  I am 
   interested in larger yet circuits (up to and beyond 64 bits).

What's a "logic map"?

   Do any of you have circuit data or better yet an insight into the 'math 
   behind' ?

Besides pseudo random noise circuits, feedback shift registers are
also used for error detection and correction (like on the disk drive
on the computer you're using now, as well as your cd player),
logic signature testing, and encryption.

The math behind behind them is hairy as all hell, and is not for the
timid.  If you're into it serious textbooks on cyclic redundancy
checking error correcting codes will cover the math, where they prove
that a certain feedback shift register will always be able to correct
a certain number of incorrect bits in a row.

For more instant gratification, feedback shift registers are trivial
to simulate with a simple program in your favorite computer language
or spreadsheet.  They're actually a lot of fun to explore without the
math. 

For the simple basic case, say with two feedback taps at bits N and
N-1, you'll get a cycle (2^N)-1 states long, with one last "stable"
state that just sits there.

  -- Don