1998-08-07 - Re: Noise source processing

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From: David Honig <honig@sprynet.com>
To: die@die.com
Message Hash: 5be75f7e11553525a810f5d7eae732b55a67bf274e57285587ef3654013ef166
Message ID: <>
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UTC Datetime: 1998-08-07 00:31:16 UTC
Raw Date: Thu, 6 Aug 1998 17:31:16 -0700 (PDT)

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From: David Honig <honig@sprynet.com>
Date: Thu, 6 Aug 1998 17:31:16 -0700 (PDT)
To: die@die.com
Subject: Re: Noise source processing
In-Reply-To: <>
Message-ID: <>
MIME-Version: 1.0
Content-Type: text/plain

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

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).