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Message
From: Attila Kinali<attila@k...>
Date: Wed Apr 25 22:24:09 CEST 2007
Subject: [oc] White Noise generator
On Fri, 20 Apr 2007 21:53:17 +0200 (CEST)
jeanpaul.milani@a... wrote:
> I'm approcing to realize a white noise generator
> using a Lattice fpga and a pcm1793 dac @ 192KHz
> I've found many solutions on the web but many of them
> are too slice's expencive for me.
> I've try with a lfsr using this code:
[...]
> when i've try the machine i've notice that the power spectrum isn't flat
> but it's quite gaussian with a peak @ 20KHz.
> I hope someone can help me
This is not a problem of your code, but one of the
design. LSFR produce a gausian distribution because
of their structure. A simple (and most
probably not fully true) explanation would be
that the lsfr produces more or less random values
that are evenly distributed. But the frequency
component of the output signal depends on the
dy/dt of the signal which is dependent on
the difference of two successive samples.
Now, because the samples are evenly distributed
it is much more likely to find sample differences
around 2^(wordlength/2) than of 0 (two times the
same sample) and 2^(wordlength) (0 and max value).
I'm sure, someone with a real understanding
of signal theory could give you a better explanation,
but this whole thing boils down to that you are
not looking for a random number generator that
produces a evenly distributed numbers, but evenly
distributed differences of consecutive values.
One way to achieve this would be to treat the output
of an LFSR as a signed number and just add it up.
But then you have to deal with probable over- and
underflows.
Attila Kinali
--
Linux ist... wenn man einfache Dinge auch mit einer kryptischen
post-fix Sprache loesen kann
-- Daniel Hottinger
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