[FFmpeg-devel] [PATCH] normal PRNG using ziggurat method
Tue Sep 9 13:58:40 CEST 2008
On Mon, Sep 08, 2008 at 11:04:48PM -0400, Justin Ruggles wrote:
> Michael Niedermayer wrote:
> > On Mon, Sep 08, 2008 at 08:05:34PM -0400, Justin Ruggles wrote:
> >> Hi,
> >> This is a patch to add a PRNG which generates numbers with standard
> >> normal distribution. It uses the ziggurat method, which is quite fast
> >> compared to other methods.
> >> I was working on another implementation based on the original paper,
> >> which had sample code in it that I was using as a starting point. Then
> >> I noticed the teeny tiny print at the bottom of the journal's download
> >> page which states that the code is under GPL... :( I found another
> >> implementation which was based on the paper without using the
> >> accompanying code and is under a BSD-style license. I trimmed it down,
> >> cleaned-up it up, and adapted it for FFmpeg.
> >> I need this for spectral extension processing in the E-AC-3 decoder.
> >> The spec requires that the generated noise which is blended with
> >> translated coeffs have "zero-mean, unity variance".
> > is "zero-mean, unity variance" everything the spec requires or is there
> > more?
> That is all it requires.
> > Because any signal (like the output of any PRNG) can be scaled and translated
> > to have that, there would be no need for normal distributed PRNG.
> > [...]
> This does scale and translate output from one of the already existing
> PRNG's...sort of. :)
> The zero mean part is easy enough. If there is a simpler (and faster)
> way to generate random noise with unity variance distribution, please
> tell me.
well lets assume you have a PRNG that returns 32bit values between
1-(1<<31) and (1<<31)-1 then its variance is
0x155555551555555580000000 / 0xFFFFFFFF
4611686016279904256 / 3
(2^62 - 2^31) / 3
if you want to have unit variance then you have to scale the output of the
PRNG by the inverse of the sqare root of above
/ sqrt((2^62 - 2^31) / 3)
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Many that live deserve death. And some that die deserve life. Can you give
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even the very wise cannot see all ends. -- Gandalf
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