Re: Noise source processing

[David Honig <honig@sprynet.com>]

At 02:24 PM 8/6/98 -0400, Dave Emery wrote:
>
>	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.

Given that the cheapo radio was 10 cm from a 21" monitor, I don't think
you need to invoke noise-driven resonances in the humble radio.

I was/am interested in robust methods that don't depend on 
exquisite analog equalization, etc.  

Zero crossings are analogs (pun intended) of 
RFC1750's #2 method (via Shannon):

map 01 -> 1 
map 10 -> 0
00, 11 are no-ops.  

Note that for a digital signal you are detecting level-changes.  (Vdd/2
changes?)


Waiting for zero crossing will give you perfect 0:1 ratio,
but you may have to wait arbitrarily long (but will almost
certainly not, in the formal sense ---there's a decaying exponential
in there).