Re: Noise source processing

[Dave Emery <die@pig.die.com>]

On Thu, Aug 06, 1998 at 10:31:20AM -0700, David Honig wrote:
> At 10:31 AM 8/6/98 -0400, mgraffam@mhv.net wrote:
> >On Thu, 6 Aug 1998, David Honig wrote:
> >
> >> I have digitized FM radio hiss using a $10 transistor radio feeding
> >> into the LINE in of my soundcard.  The spectrum looks poisson
> >> with prominant, periodically spaced noise spikes.  It does not pass


	This is expected of hard limiters below threshold  (such as in a
reasonable FM IF system fed Johnson noise).  The spikes occur on phase
discontinuities in the limiter output - what you are seeing is the input
Johnson noise intermittantly inducing phase jumps in the output of the
ringing IF filters shock excited by the broadband input noise.  The IF
filters tend to ring somewhat coherently until hit by another burst of
noise which sets them off on another phase.  Each phase jump shows up as
a spike on the discriminator output.  The narrower the bandwidth of the
filters, the longer the spike and interspike intervals. Thus one needs
an IF bandwidth considerably wider than the sample rate.

	Your parity scheme is pretty good, but simple zero crosses (or
crossings of the long term mean value of the waveform) aren't bad
either.   Many years ago I suggested, half seriously, that simply taking
a suitable cheap Radio Shack audio transformer and stepping the speaker
output of a cheap FM radio tuned between stations to around 15 volts and
shoving it directly into the CD or DSR input of a standard PC serial
port would work as a quicky hardware noise source (this was before the
near universality of soundcards on PCs).   The serial port pins 
incorperate a threshold comparitor in the RS-232/423 receiver chips
which would act to convert the noise to a string of 1s and 0s  that
could be sampled by reading the DSR value occasionally. Or one could use
the DSR interrupt and record the time the zero crossing occurred and use
this time stamp as a random (poisson distributed) value.

> 
> I always thought listening to static was a suspicious activity.
> 
	My wife thinks so...



-- 
	Dave Emery N1PRE,  die@die.com  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18