[sdiy] Analysis of frequency variation in analogue synths

[ASSI]

On Sonntag, 6. Mai 2007 23:30, Eric Brombaugh wrote:
> True. What will that correlation look like on the output of the
> averager/filter? Given that the LFSR is 32-bits, how narrow would the
> filter have to be (how many cycles) before the effects of the
> correlation are negligible? I'd think anything > 32 cycles would
> approximate the same results you'd get by just clocking the LFSR 32x
> faster. Also, what would be the effects of scrambling the LFSR bits
> when forming the output word?

Good question.  Off the top of my head (I haven't checked this 
thoroughly, so take it with a grain of salt): Shifting bits into the 
binary output words forms exponential pulses.  The spectrum of a 
periodic expo pulse sequence is a mixture of square- and triangle like 
components phase shifted by 90 degrees (~coskx/k² and ~sinkx/k).  
Instead of a white spectrum you will get something with an 
(asymptotical) 9db to 6db per octave droop.  Since the correlation is 
basically a delay line, averaging will produce combing in the spectrum, 
depending on the spacing of the taps into the state of the LFSR.  
Scrambling of the bits does not change the delay, but produces folding 
of the exponential pulse, thus shifting energy from lower frequencies 
to higher parts of the spectrum before the "comb filter".  With the 
right combination of tap spacing and scrambling you can probably 
produce "formant-like" noise.


Achim.
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