From: Steve Dove <dsp@hifidelity.com>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 12 Jan 2005 21:14:08 -0000
Organization: d s p
In-Reply-To: <HKEKINMDBIDLPDGEIOBMMEAGCEAA.g4wgt@tiscali.co.uk>
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Subject: Re: LF: Audio Filters
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Hi Gary,

There are a couple of these floating around here that spend their time rescuing 
boat-anchors and such.  They really were the best pre-DSP post-receiver filter 
boxes.  The auto-notch works a treat.

In the absence of real IF filters,  there might be advantages to using one to 
crank down the bandwidth prior to Argo or whatever, but such audio filtering lays 
one open to 'surprises'.  Such as (over here) CFH or something else hideously 
strong within the remaining filtered-out and so unheard IF bandwidth starting up 
and knocking back the AGC, leaving one to wonder where everything went.

There really isn't a substitute for IF filtering;  most of the time there are narrow 
CW filters on the 'Grabulators' here (including a couple only 125Hz wide).  That 
said, the receiver for the 137.778kHz window is set at 137.000kHz USB, and is 
wide open (i.e. 2kHz+ bandwidth).  This allows the same receiver audio output 
to feed multiple 'puters and instances of Argo and SpectrumLab for the above 
mentioned window, DCF39 monitoring, and WD2XES's merry 'mode du jour' 
jaunts hither and yon, etc.  The plots seem slightly better behaved with storm 
crashes than those from narrow bandwidth receivers  -  likely a filter group-delay 
thing.  But this wouldn't be possible if there were any danger of a big hairy signal 
popping up in that 2kHz swath.

Where the Datong (or one of the after-market DSP filters) really comes in useful 
at LF is in NDB chasing.  Nominal keying modulation frequency of these (here at 
least) is supposedly 1020Hz.  On an integer kiloHertz channel, there is usually a 
small forest of carriers on or close(ish) to the nominal frequency; about 20Hz 
higher are the modulation sidebands of the beacons 1kHz *lower* in frequency;  
about 20Hz below the carriers are the LSB modulations from beacons a 
kiloHertz higher.  (Confused yet?)   The Datong's or DSP's filter cranked really 
down tight to 30 - 40Hz bandwidth allows separation of these to a degree 
sufficient to allow reading IDs that would otherwise be totally impossible. 

In short, as good as the box is at what it does, it has little applicability to the 
present style of LF reception, QRSS and the like, and in fact could be a liability, 
but does come into its own when 'band cruising' or beacon hunting.

        Cheers,

                Steve

http://www.w3eee.com


 "Gary - G4WGT" <g4wgt@tiscali.co.uk> wrote:
>Is it useful to use a audio filter like the DATONG FL3 MULTI-MODE AUDIO
>FILTER WITH NOTCH FILTER to help with QRSS receive on LF, does 
anyone use
>one or similar?