Content-Transfer-Encoding: 8bit
Message-ID: <65AECDF1F89AD411900400508BFC869F0D74D7@pdw-mail-1.dera.gov.uk>
From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: Transatlantic
Date: Wed, 3 Jan 2001 15:16:30 -0000
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

If the error correction circuitry only trys for a best guess this will
always be the result - random noise in, random characters out.   If the
error checking routine measures the amount of error, for example with
ET1 in Coherent how many of the bits are in error,  then outputs an
error symbol if more than a  particular threshold is reached, there is
less likelihood of noise generating random messages.
The error detection method in Amtor Broadcast mode (no handshaking) does
this.  An incorrectly received character prints out as an error symbol
rather than a, probably more serious, wrong character.

Another analogy to the light description,  is to listen to noise than
has been bandpass filtered to just a few Hz wide - it sounds remarkably
like a tone modulated at the rate of the bandwidth.  The WJ8711 HF
receiver has a 56Hz wide filter included - noise sounds so remarkably
like a weak RTTY signal that we've tried to decode it before now !!

Andy  G4JNT


> I'm sure there are better mathematical terms for this, but 
> the essence is:  
> if the input to COHERENT is truly random, the output is a "weighted 
> randomness."  It's just as truly random as the input.  All 
> legal symbols will 
> eventually appear, and there will be no real order to them.
> 
> However, the chance of any one character appearing at a given 
> instant is no 
> longer a simple 1-chance-in-x-to-the-power-of-y probability.  
> It's more like 
> the randomness of energy from an incandescent lamp.  Energy levels of 
> individual photons in the flux are definitely random, yet the 
> spectral curve 
> peaks at some color temperature around which most of the 
> energy will be found.
> 
> A suitable analogy for the Grab feature might be an optical 
> filter that 
> happens to have the same response curve as the incandescent 
> light source 
> being shone through it.  The more such filters one stacks in 
> front of the 
> light (that is, the greater Grab depth), the narrower the range of 
> wavelengths which will be visible to the eye.  Ultimately, 
> the light would 
> appear nearly monochromatic, just as the decoder would 
> clearly begin to 
> output one character more often than all others; but it would 
> never truly 
> become--pardon the expression--coherent.
> 
> 73,
> John KD4IDY
> 


-- 
The Information contained in this E-Mail and any subsequent correspondence
is private and is intended solely for the intended recipient(s).
For those other than the recipient any disclosure, copying, distribution, 
or any action taken or omitted to be taken in reliance on such information is
prohibited and may be unlawful.