[sdiy] Popcorn Noise Generator?

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: Re: [sdiy] Popcorn Noise Generator?
Date: Sun, 10 Jul 2005 18:21:03 -0600
Message-ID: <6.2.1.2.2.20050710181539.03453350 at mail.earthlink.net>

> At 03:36 PM 7/10/05, Magnus Danielson wrote:
> 
> >First create a long PRBS generator, operating at a multiple of the final
> >sampling rate. It is a simple matter of shift-register and XORing with the
> >polynom if the top bit is 1. This generates white noise.
> >
> >Then, integrating the scaled output of the PRBS. This is done by adding or
> >subtracting the scale amplitude due to a 1 or 0 output of the PRBS 
> >respectively
> >to the previous integrator output sample.
> >
> >Now we have our popcorn noise, but in a too high sampling frequency, so we
> >perform a down-sampling by FIR filtering and using every other sample in a few
> >rounds.
> 
> So from what I have been reading, popcorn noise has a 1/f^2 power spectrum 
> above a cutoff fc.

Clarification, the cutoff fc that I am describing is a result from method of
synthesis, where the integrator cap levels off due to amplifier gain limit or
shunting impedance, which ever comes first. However, this is also found in the
presentation you found, I don't recall having that in any of my references.
However, it is not all that strange either.

> So a first order lowpass operating on white noise might be a good way to go.

The trouble is, the fc you want is very (no _VERY VERY VERY_) low, you migth as
well go for the integrator solution directly.

> popcorn power ~ 1/(1 + (f/fc)^2)
> 
> <http://home.zcu.cz/fel/kae/aes2/sumy/noise_sim_tp.pdf#search='popcorn%20noise%20spectrum'> 

I enjoyed this sentence:

"The source of this noise is not fully understood, although it has been shown
to be related to the presence of heavy-metal ion contamination."

Strike out "ion" and it is really amusing! ;O)

Interesting paper. Thanks for the link.

Cheers,
Magnus