Message-ID: <01C227EF.24643850.g4jnt@thersgb.net>
From: "Andy talbot" <g4jnt@thersgb.net>
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: Amtor FEC on LF
Date: Wed, 10 Jul 2002 08:52:35 +0100
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is over 100 km away, so it does not
> cause the same trouble you might have in areas such as the U.K. but helps
> to fill the band at least with some signals  .... ;-)
>

> But scaled down Amtor (divide by 10 all round) might be useful
>Stewart G3YSX


It is far better to tailor a data modulation and coding sceme to the medium on 
which it is to be used rather than just cobble together an adaptation of 
another scheme.

AMTOR is designed for HF which has the characteristics of short bursts of 
 interference mixed up with  fading and multipath - so has a short packet 
length (3 characters) and low baud rate to fit in between link disturbances. 
 High relative bandwidth is necessary at HF to counter multipath.

137k, on the other hand is characterised by a much more constant noise 
background and does not behave like HF divided by ten -  if fading is present, 
it covers a much longer period of tens of minutes or hours    These 
characteristics suggest a coding scheme that requires less error correction and 
can make use of narrower relative bandwidth to reduce noise.    We already have 
JASON for low speed work, albeit with no error correction, and PSK31 is ideal 
for keyboard typing speeds and includes a QPSK FEC mode, although the linear Tx 
requirement makes it difficult to use.    Link failures at LF will not be cured 
by ARQ (as in AMTOR) to the same extent as at HF due to the non burst nature of 
the noise.

Andy  G4JNT


-----Original Message-----
From:	Stewart Bryant [SMTP:stewart.bryant@virgin.net]
Sent:	2002/07/09 17:50
To:	rsgb_lf_group@blacksheep.org
Subject:	Re: LF: Amtor FEC on LF