[Lf] [Fwd: LF: Amtor FEC on LF]

[Andre Kesteloot]

Roger Thompson wrote:

> Andy and all,
>
> I've been researching how a peak clipper might be implemented in an analog
> LF receiver chain before the narrowband filters, similar to your suggestion
> about taking noise pulses out using DSP.  Such a limiting technique is
> mentioned in the book "Communications Receivers," 2nd edition, by Rohde,
> Whitaker and Bucher, but there is little about a practical design.  My
> feeling is that a limiter could work much like transmit RF clippers, where
> post-clipping selectivity reduces the distortion products that might rise
> from diode clippers and the like.  I'm a little unsure about the remaining
> amplitude-clipped impulses, though, as the rise time of these still may
> overexcite later selective stages.  Has anyone information about how such a
> clipper or limiter could be built that would improve the signal to noise of,
> say, QRSS signals?
>
> Roger
> AD5T
>
> -----Original Message-----
> From: majordom at post.thorcom.com [mailto:majordom at post.thorcom.com]On
> Behalf Of Andy talbot
> Sent: Wednesday, July 10, 2002 7:59 AM
> To: 'rsgb_lf_groupblacksheeporg'
> Subject: RE: LF: Amtor FEC on LF
> Importance: High
>
> OK then, so we need a waveform that is immune to high level broadband
> spikes.
>  Narrow filtering will remove the spike energy, but in turn will spread it
> out
> over the period of the filter response so won't help greatly with
> arbitrarily
> low bandwidth signalling.  So some data repetition or convolution is called
> for
> to get the basic link operational before we start adding error correction by
> repeats.  ARQ can only make a mediocre link good; not a poor or non-existant
> one into a mediocre link.
>
> What hapenned to WOLF ?   That had very heavy convolution and repetition and
> would solve the problem very effectively by coding.
>
> It ought to be easy to take out the noise pulses in DSP.   If these really
> are
> sharp spikes, then an algorithm similar to that used for cleaning up old
> vinyl
> recordings (frequently set as a university third year project a couple of
> decades ago) would clip the spikes before any narrow band filtering and
> demodulation spread them out.   Examine at the signal in the time domain
> (the
> raw samples from the A/D), and look for a sharp rise in energy,  ie. a peak
> in
> sucessive samples.  When a peak above a certain threshold is detected,
> either
> clip it, or replace by an interpolated version of adjacent ones.  It will
> now
> help to be receiving and digitising in as wide a bandwidth as possible, so
> the
> spike affects relatively less sample periods.
>
> Andy  G4JNT