[sdiy] Long LFSRs (Was Psych Tone)

[Richie Burnett]

2 taps. Feedback from 126 and 127 as per recommended polynomials for maximum length sequences stated in this document:

http://www.xilinx.com/support/documentation/application_notes/xapp210.pdf

-Richie,

Sent from my Xperia SP on O2

---- rsdio at audiobanshee.com wrote ----

>How many taps did you use in that 127-bit LFSR, and in which bit positions are they placed?
>
>Brian
>
>
>On Jan 5, 2019, at 2:44 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> One thing that I didn't realise is that longer LFSRs are not necessarily better for audio noise generation!  I once tried coding up a 127-bit LFSR to generate Pseudo-random Noise, thinking that it would be easy to do and that one would never hear the PN sequence repeat within a lifetime.  I was quite shocked when i heard the result though.  It didn't sound like white noise at all.  It sounded more like a dialup modem or an old home computer loading something from tape.  It was full of tonal blips, squeaks and chirps present in the output, despite having chosen the taps wisely to get a maximal length sequence.
>> 
>> Someone at work then explained to me that very long PN sequences have a large number of runs of 1's and 0's and other long sequences, and that these can sound tonal.  Because of the long sequence length, they don't come around very often, but these patterned runs don't sound like white noise at all, and are very objectionable.  They also take a very long time to break up if the register length is long!  You can obviously initialise the LFSR state to better choices that don't give you the objectionable chirps and buzzes at the start, but they will still come round again eventually!
>> 
>> Here's the sound of the first 15 seconds of a 127-bit LFSR with initial state set to one.
>> 
>> http://www.richieburnett.co.uk/temp/127-bit_LFSR.wav
>> 
>> Sound like white noise to you?! Lesson learned.
>> 
>> -Richie,
>> 
>
>
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